Ecological Impacts/Benefits of Dredging for Shellfish

Last updated: 3/10/10

This literature review includes resources which discuss the ecological impacts or benefits of dredging for shellfish.  Citations are organized by type of shellfish dredging the article addresses.  Abstracts are included when available.

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Auster, P. J. and R. W. Langton 1999. The Effects of Fishing on Fish Habitat. pp. 150-187. In Banaka, L. (editor). Fish Habitat: Essential Fish Habitat and Rehabilitation. Amer. Fisheries Soc., Bethesda, MD. Available online at:

Chícharo. L. et al. (2002). Ecological characterization of dredged and non-dredged bivalve fishing areas off south Portugal. Journal of the Marine Biological Association of the UK 82:1:41-50.

Macro and meiobenthic communities of two fishing areas (Vilamoura and Lagos) in the western part of south Portugal (Algarve coast) were analysed. Both locations had been under severe dredge-fishing impact until four years previously. Vilamoura has since continued to be dredged, while fishing activity in Lagos was stopped in 1995 as a response to overfishing. For each location, three replicate areas were analysed at depths of 7–9 m. In each of these areas, 18 quadrats for macrofauna and 12 cores for meiofauna were randomly sampled by SCUBA divers during September 1999. The Shannon–Weiner diversity index was higher for meiofauna in the fished area, whereas macrofauna diversity was higher in the recently non-fished area. Bray–Curtis dissimilarity between the two areas was 87·82%. Major differences were found between Ampeliscidea, Amphiura mediterranea, Spisula solida, Haustoriidae, Nemertinea and Diogenes pugilator populations at the two sites. There was higher abundance but lower biomass of potential macrofaunal scavengers in the fished area, and carnivore biomass was also higher in this area. Deposit-feeders dominated meiofauna abundance in both study areas. The community structure of the continuously fished area was dominated by small, opportunistic, short-lived species while the community structure of the recently non-fished area was dominated by more fragile and long-living sessile organisms.

Coen, Loren D. 1995. A Review of the Potential Impacts of Mechanical Harvesting on Subtidal and Intertidal Shellfish Resources. South Carolina Department of Natural Resources, Marine Resources Research Institute. 46 p. Available online at:

This summary was developed to address recent concerns expressed by the U.S. Army Corps of Engineers (USACOE) and other agencies regarding use of hydraulic/mechanical shellfish harvesters in South Carolina. The document reviews relevant issues and existing information on the use and potential impacts of subtidal and intertidal mechanical shellfish harvesters, with emphasis on subtidal escalator harvesters. Information included in this report summarizes all pertinent literature (both "gray" and "primary") that could be located which provides direct or indirect information on concerns voiced by state, federal and private citizen groups, as well as an extensive bibliography of the above-mentioned literature (see Appendices). Specific recommendations regarding proposed research directions that address potential impacts are also provided.

Collie JS, Escanero GA, Valentine PC. 1997. Effects of bottom fishing on the benthic megafauna of Georges Bank. Marine Ecology Progress Series 155:159-72. Available online at:

This study addresses ongoing concerns over the effects of mobile fishing gear on benthic communities. Using side-scan sonar, bottom photographs and fishing records, we identified a set of disturbed and undisturbed sites on the gravel pavement area of northern Georges Bank in the Northwest Atlantic. Replicate samples of the megafauna were collected with a 1 m Naturalists' dredge on 2 cruises in 1994. Compared with the disturbed sites, the undisturbed sites had higher numbers of organisms, biomass, species richness and species diversity; evenness was higher at the disturbed sites. Undisturbed sites were characterized by an abundance of bushy epifaunal taxa (bryozoans, hydroids, worm tubes) that provide a complex habitat for shrimps, polychaetes, brittle stars, mussels and small fish. Disturbed sites were dominated by larger, hard-shelled molluscs, and scavenging crabs and echinoderms. Many of the megafaunal species in our samples have also been identified in stomach contents of demersal fish on Georges Bank; the abundances of at least some of these species were reduced at the disturbed sites.

DeAlteris J, Skrobe L, Lipsky C. 1999. The significance of seafloor disturbance by mobile fishing gear relative to natural processes: a case study in Narragansett Bay, Rhode Island. Am. Fish Soc. Symp. 22:224-37.

Gaspar, MG, Chicharo, LM. (2007.) Modifying dredges to reduce by-catch and impacts on the benthos. In Kennelly, SJ (Ed.) By-catch reduction in the world's fisheries. Springer. Pp 95-140.  

Hamilton A. 2000. Gear Impacts on Essential Fish Habitat in the Southeastern Region. National Marine Fisheries Service, Southeast Fisheries Science Center, Mississippi Laboratories, Pascagoula Facility, Pascagoula, Mississippi. Available online at:

McAllister, D., & Spiller, G. (1994). Trawling and dredging impacts on fish habitat and bycatch. In Wells P. G., Ricketts P. J. (Eds.), Coastal zone Canada '94: Cooperation in the coastal zone. Pp. 1709-1718.

Trawling and dredging for fishes, shrimp and shellfish have major impacts on habitat and, through bycatches, fish populations of fishing banks. Tracks of trawlers and dredges swept tracks of over 4.3 million kilometres in 1985. The gear, drawn by powerful vessel engines, shears off bottom vegetation and protruding invertebrate animal life including sea anemones, sponges, sea squirts, crinoids and many others. These miniature forests provide shelter for small species and young of large species from predators and harbour food for fish. Removal of this shelter exposes fish to predation and reduces food supply. Habitat impacts and bycatches affect stocks of commercial fishes, the natural biodiversity and the ecological services provided. The industrialization of fishing moves the distribution of benefits from individual fishers and fishing communities to larger ports and distant stockholders. It may also extend the periods of time that fishermen are separated from their families.

Messieh, S. N.; Rowell, T. W.; Peer, D. L.; Cranford, P. J. 1991. The effects of trawling, dredging and ocean dumping on the eastern Canadian continental shelf seabed. Continental Shelf Research 11(8-10): 1237-1263.

This paper presents an overview of current knowledge on the effects of trawling, dredging and ocean dumping on the eastern Canadian continental shelf seabed. The impact of trawling and dredging for fish and shellfish on marine habitats has recently attracted international attention among fisheries and environmental scientists. In Atlantic Canada, trawling and dredging are the principal methods of harvesting groundfish and scallops and ocean clams, respectively. It is estimated that fish trawlers and scallop dredges have swept tracks, cris-crossing the Canadian continental shelf, approximately 4.3 million km in length in 1985. In the past few years several studies were carried out by scientists from Canada, the United States and Europe to assess the impacts of trawling and dredging but results were inconclusive. Some studies showed physical damage as well as biological effects, whereas others indicated that the adverse effects were not considered to be serious. Fishermen are not the only potential users of the resources of the continental shelf. There is an increasing demand for good-quality sand and gravel aggregate and the ocean seabed is being seen as a possible source. The eastern Canadian continental shelf also exhibits hydrocarbon potential and operational and accidental discharges are an environmental concern. Increased marine transportation and expansion of the fishing fleet have resulted in a greater need for harbour dredging. Dredging and dredge spoil disposal were controlled by the Ocean Dumping Control Act and now the Canadian Environmental Protection Act which places restrictions on the composition of material that can be disposed of in the sea. Nevertheless some harbours contain contaminant concentrations exceeding the maximum allowable limits. It is concluded that the impacts of human activities on the continental shelf seabed environment are inevitable and the long-term effects, while difficult to determine, must be assessed. The sub-lethal effects of increased suspended sediment loads on benthic organisms and potential changes to benthic community structure are major concerns and should be the focus of further research.

Murawski, S., Serchuk, F., & International Counc. for the Exploration of the Sea, Copenhagen (Denmark). Shellfish Comm. (1989). Environmental effects of offshore dredge fisheries for bivalves. COPENHAGEN (DENMARK): ICES.

During 1986 and 1987, we conducted submersible observations and associated experiments studying offshore dredge fisheries for scallops and clams in the Mid-Atlantic region off the northeast USA. Objectives of the project were to (1) evaluate the effects of commercial fishing operations on incidental mortality (gear-induced damage) of sea scallops (Placopecten magellanicus ), ocean quahogs (Artica islandica ) and surf clams (Spisula solidissima ); (2) assess the acute mortality rates of these species when dredged by commercial vessels and subsequently discarded as undersized; and (3) observe the general environmental effects of the offshore dredge fisheries for these shellfish. We conclude that, in the Mid-Atlantic region, harvest efficiency of commercial dredges is generally high, there is variable damage among species encountered by the dredges but not retained, and there are variable survival rates of small clams and scallops returned to the sea bed as undersized.

Thrush, Simon F. and Paul K. Dayton. 2002. Disturbance to Marine Benthic Habitats by Trawling and Dredging: Implications for Marine Biodiversity. Annual Review of Ecology and Systematics 33: 449-473. Available online at

The direct effects of marine habitat disturbance by commercial fishing have been well documented. However, the potential ramifications to the ecological function of seafloor communities and ecosystems have yet to be considered. Soft-sediment organisms create much of their habitat's structure and also have crucial roles in many population, community, and ecosystem processes. Many of these roles are filled by species that are sensitive to habitat disturbance. Functional extinction refers to the situation in which species become so rare that they do not fulfill the ecosystem roles that have evolved in the system. This loss to the ecosystem occurs when there are restrictions in the size, density, and distribution of organisms that threaten the biodiversity, resilience, or provision of ecosystem services. Once the functionally important components of an ecosystem are missing, it is extremely difficult to identify and understand ecological thresholds. The extent and intensity of human disturbance to oceanic ecosystems is a significant threat to both structural and functional biodiversity and in many cases this has virtually eliminated natural systems that might serve as baselines to evaluate these impacts.

Watling L, Norse EA. 1998. Disturbance of the seafloor by mobile fishing gear: a comparison to forest clear cutting. Conservation Biology 12:1180-1197. Available online at:

Bottom trawling and use of other mobile fishing gear have effects on the seabed that resemble forest clearcutting, a terrestrial disturbance recognized as a major threat to biological diversity and economic sustainability. Structures in marine benthic communities are generally much smaller than those in forests, but structural complexity is no less important to their biodiversity. Use of mobile fishing gear crushes, buries, and exposes marine animals and structures on and in the substratum, sharply reducing structural diversity. Its severity is roughly comparable to other natural and anthropogenic marine disturbances. It also alters biogeochemical cycles, perhaps even globally. Recovery after disturbance is often slow because recruitment is patchy and growth to maturity takes years, decades, or more for some structure-forming species. Trawling and dredging are especially problematic where the return interval--the time from one dredging or trawling event to the next--is shorter than the time it takes for the ecosystem to recover; extensive areas can be trawled 100-700% per year or more. The effects of mobile fishing gear on biodiversity are most severe where natural disturbance is least prevalent, particularly on the outer continental shelf and slope, where storm-wave damage is negligible and biological processes, including growth, tend to be slow. Recent advances in fishing technology (e.g., rockhopper gear, global positioning systems, fish finders) have all but eliminated what were de facto refuges from trawling. The frequency of trawling (in percentage of the continental shelf trawled per year) is orders of magnitude higher than other severe seabed disturbances, annually covering an area equivalent to perhaps half of the world's continental shelf, or 150 times the land area that is clearcut yearly. Mobile fishing gear can have large and long-lasting effects on benthic communities, including young stages of commercially important fishes, although some species benefit when structural complexity is reduced. These findings are crucial for implementation of "Essential Fish Habitat" provisions of the U.S. Magnuson-Stevens Fishery Conservation and Management Act which aim to protect nursery and feeding habitat for commercial fishes. Using a precautionary approach to management, modifying fishing methods, and creating refuges free of mobile fishing gear are ways to reduce effects on biological diversity and commercial fish habitat.


Falcao, M., Gaspar, M., Caetano, M., Santos, M., & Vale, C. (2003). Short-term environmental impact of clam dredging in coastal waters (south of Portugal): Chemical disturbance and subsequent recovery of seabed. Marine Environmental Research, 56(5), 649-664.

The physical and chemical changes in sediment and near bottom water caused by clam dredging were examined during July and September 1999, at two locations Vilamoura (VL) and Armona (AR), south coast of Portugal. Sediment cores and near bottom water were collected simultaneously before dredging (control samples) and within short time intervals (min-h) after dredging. After dredging operations, microphytobenthos coming from the path were accumulated in the re-worked sediment (ridge). Chlorophyll a in superficial sediment increased from 1.2 mu g g super(-1) before dredging to 1.7 mu g g super(-1) after dredging and these higher values remained for a few hours. However, the expected increase of chlorophyll a in near bottom water due to re-suspension was not observed. After sediment disturbance an instantaneous sorption of phosphorus onto iron oxides occurred in the upper sediment layers (from 2 to 3 mu mol g super(-1) before dredging to 4-5 mu mol g super(-1) after dredging). A microcosm experiment showed that after sediment disturbance HPO sub(4) super(2-) dissolved in pore water decreased from 40 to 10 mu M being simultaneously sorbed onto iron oxides formed in the top layer of sediment. The ammonium, nitrates, organic nitrogen, phosphate and silicate dissolved in pore water decreased immediately after dredging activity and simultaneously an increase in near bottom water was sporadically observed. Generally, the re-establishment of seabed was reached within a short time (min- h), at both stations (VL and AR).

Gilkinson, K.D. et al. 2003. Immediate and longer-term impacts of hydraulic clam dredging on an offshore sandy seabed: effects on physical habitat and processes of recovery. Continental Shelf Research 23(14-15): 1315-1336.

A hydraulic clam dredging experiment was conducted on a deep (70–80 m) offshore sandy bank on the Scotian Shelf in order to examine the immediate impacts of hydraulic dredging on physical habitat and to follow processes of recovery over a 3-year period. Seabed structural complexity in this low-relief habitat consists of small-scale sedimentary features including pits and bivalve burrow openings as well as polychaete tubes and empty mollusc shells. The most obvious effect of dredging was a dramatic change in seabed topography due to the numerous deep (20 cm), wide (4 m) curvilinear furrows that were cut by the dredges. The loss of burrows, tubes, and shells through destruction or burial, and local sedimentation created a smooth surface. Both spatial scale and sampling resolution were critical in identifying longer-term impacts. Dredge furrows were no longer visible in video 1 year after dredging due to their low relief; however, they persisted, while undergoing changes, as evidenced in sidescan sonograms. The margins of furrows were gradually degraded, likely through the combined actions of slumping, sediment transport and bioturbation. Over time, dredge furrows act as traps for empty shells. Differences in patterns of acoustic reflectance between dredge furrows and the surrounding seabed indicate long-lasting effects on sediment structure. Densities of large burrows were reduced by up to 90% after dredging with no signs of recovery after 3 years due to the high mortalities of their architect, the propellerclam, Cyrtodaria siliqua. Dredging effects were detectable against a background of natural temporal (annual) and large-scale spatial variability in physical habitat.

Hall SJ, Basford DJ, Robertson MR. 1990. The impacts of hydraulic dredging for razor clams Ensis sp. on an infaunal community. Neth. J. Sea Res. 27:119-25.

The impact of fishing for razor clams (Ensis sp.) by hydraulic dredging on the associated infaunal community has been examined in a manipulative field experiment executed in autumn in a Scottish sea loch at 7 m depth. Infaunal samples from replicate fished and unfished plots were examined after 1 and 40 days. Major effects on the total number of individuals were observed immediately after fishing and sign test revealed a reduction in the abundance of a significant proportion of species in fished areas. However, after 40 (mostly stormy) days no effects of fishing could be detected and no visible signs of fishing remained on the sea bed. We hypothesized that active migration into the water column and passive suspension during wind- and tide-induced sediment transport dilute localized effects and conclude that, given the restricted depth at which fishing is possible at present, hydraulic dredging is unlikely to have persistent effects on most of the infaunal community in most habitats. The effects on long-lived bivalve species could, however, be more serious.

Haskin, H., & Wagner, E. (1988). Assessment of mortalities in surf clams (spisula solidissima ) due to dredging, sorting and discard. Journal of Shellfish Research, 7(1), 120-121.

A significant portion of the clams available in the Mid-Atlantic region are less than the current minimum legal size limit. Current harvesting practices cause significant mortalities to clams dredged, sorted, and returned to the sea. To assess mortality associated with sorting and discard, clams were dredged and run through a mechanical sorter. Post sort "catch" (larger clams) and "discards" (smaller clams) were transplanted to marked plots at nearby areas. Samples were sorted to determine percent mortality. Results of this study show that: 1) with careful handling, minimal mortality to clams captured in a hydraulic clam dredge will average about 17-18%; 2) sorting the dredged catch by steel rollers (current practice) adds another 18-19% kill; 3) additional stress e.g. holding on deck, shovelling overboard etc. can add another 17-18% mortality. Predators increased in abundance and diversity in planting areas. A single tow evaluation of bottom sorting in this study confirmed high mortality rates reported by others in clams left behind in the dredge path (62% this study).

Meyer, T., Cooper, R., & Pecci, K. (1981). The performance and environmental effects of a hydraulic clam dredge. Marine Fisheries Review, 43(9), 14-22.

The efficiency of a 1.2 m hydraulic clam dredge in a surf clam, Spisula solidissima (Dillwyn), population was demonstrated by diver scientists to be sensitive to factors such as: Speed of towing, scope of tow line and water hose, and distance between cutting blade and water manifold. When these operational specifications were near optimum, the dredge removed 91% of the available clams; when below optimum, efficiency was 80%. When dredge performance was low; larger clams, which burrowed deeper into the sediment, suffered mortalities as high as 92%; when high, mortalities decreased to 30%. In high clam density areas, the dredge filled with clams after approximately 10 m of towing. Once filled, the dredge action was analogous to a snowplow as it pushed and blew clams and sediment to the sides. Initially, the dredge track was conspicuous with a smooth track shoulder, sharply angled walls, and a flat floor. The track rapidly deteriorated through slumping and biological activity until by 24 hours it appeared more like a series of shallow depressions. Predators were more abundant inside the dredge track than outside and were divided into two categories: (1) Ones which fed on the remains of damaged clams, and (2) those which preyed on undamaged clams. After 24 hours, predator density had returned to pre-dredging levels.

Palermo, M. R., Homziak, J., Teeter, A. M. (1990). Evaluation of clamshell dredging and barge overflow, Military Ocean Terminal Sunny Point, North Carolina Vicksburg, Miss. : U.S. Army Engineer Waterways Experiment Station. Technical report D-90-6. 70 p.

Since resource agencies were concerned with operational procedures for clamshell dredges from the standpoint of potential resuspension of sediment during the dredging process and overflow of barges to increase load, a field study was conducted to give site-specific information on the clamshell operation. Barge loading characteristics for both overflow and nonoverflow conditions and potential gain in load due to overflow were determined for three barge loads. At this site, suspended sediment levels observed during dredging and overflow probably did not produce any significant adverse environmental effect upon eggs, larvae, juveniles and adult forms of estuarine-dependent fish and shellfish species.

Rambaldi, E., Priore, G., Prioli, G., Mietti, N., Pagliani, T., & Bianchini, M. (1999). Trials on clam (chamelea gallina) beds of an innovative hydraulic dredge with vibrating and sorting bottom. Journal of Shellfish Research, 18(2), 726.  

To solve some of the productive and environmental problems related to the use of the hydraulic dredge in bivalve mollusc fishing, an experimental gear with vibrating bottom grid and other technical changes has been tested on clam (Chamelea gallina) beds. Comparative fishing surveys have pointed out a significantly different selectivity of the vibrating dredge, with respect to a standard gear: in fact, undersize clams are sieved out during the fishing process, and almost no juveniles were caught. Speaking of the product quality, laboratory analyses show that the internal sediment is significantly lower in the catch from the modified dredge, thanks to a sort of "alarming" device. Nevertheless, the number of damaged clams suggests a greater mechanical stress of the vibrating grid. As for the environmental effects, the vibrating bottom is selective for the associated fauna too, as it is shown by the mean weight of all the by catch species, which is higher in the experimental gear. Moreover, the riddling goes on continuously, allowing the immediate release of the sorted out organisms, which are repositioned in the area origin, thus avoiding a "contagious" distribution. In conclusion, these preliminary indications suggest a positive evaluation of the modified dredge, especially when considering its innovative design, still with wide margins for improvement.


Dolmer, P. (2002). Mussel dredging: Impact on epifauna in Limfjorden, Denmark. Journal of Shellfish Research, 21(2), 529-537.

Species composition and population density of epibenthos are described in two areas in Limfjorden, Denmark. Both areas covered both a mussel fishing ground and an area that has been permanently closed for mussel dredging since 1988. Furthermore, mussels were dredged in a part of the mussel fishing grounds in both areas four months before the investigations. The rest of the fishing grounds had not been exploited for at least four years. This study describes the short-term impact (4 mo) and long-term impact (>4 y) of mussel dredging using the permanently closed areas as controls. The data were analyzed by multivariate statistics. In both short-term study areas significant effects of dredging were observed. A number of taxa (sponges, echinoderms, anthozoans, molluscs, crustaceans, and ascidians) had a reduced density or were not observed in fished areas four months after the fishing was ended. In one of the two long-term study areas, significant differences in species composition and density were observed between fished and closed areas, indicating that the fishery may have a long-term impact on the epibenthic community, whereas in the other long-term area no difference was observed between fished and control areas. Significant reductions in the amount of shell debris and gravel were observed in the dredged areas. The impact of the loss of these benthic structural components on ecosystem processes and functions is discussed.

Dolmer, P., Kristensen, T., Christiansen, M., Petersen, M., Kristensen, P., & Hoffmann, E. (1999). Short-term impact of blue mussel dredging (mytilus edulis L.) on a benthic community. Journal of Shellfish Research, 18(2), 714. Available online at:

The short-term effect of mussel dredging in a brackish Danish sound was studied. A diver identified a commercial dredging track and an analysis of the species composition inside the track and at an adjacent control area showed that dredging changed the community structure by reducing the density of small polychaetes. In order to investigate the extent and the duration of the dredging impact experimental dredging was conducted. The experimental dredging removed 50% of the mussels in the two dredged areas. Immediately after dredging, a significantly lower number of species was measured inside the mussel beds in dredged areas compared to control and boundary areas. This effect lasted for at least 40 days. The analysis of the species composition showed that the dredged area had a significantly lower density, particularly of small polychaetes compared to the boundary area. An increased number of species was recorded outside the mussel beds just after dredging, but this effect lasted for less than 7 days. After dredging, brown shrimps, C. crangon invaded the dredged areas. This species is an important predator of smaller invertebrates, and it is suspected that it was feeding on small vulnerable polychaetes exposed at the sediment surface after dredging. The dredging process was observed to form 2-5 cm deep furrows in the seabed, but the sediment texture and the organic content of the sediment were not affected. The biomass accumulation of individual blue mussels was significantly lower in the dredged area compared to the boundary area. This indicates that the disturbance of the mussel bed structure reduced growth and that the lowering of intraspecific food competition caused by a reduced density of mussels did not increase the accumulation of biomass in the mussels that remained in the dredged area.


Banta, S., Powell, E., & Ashton-Alcox, K. (2003). Evaluation of dredging effort by the Delaware Bay oyster fishery in New Jersey waters. North American Journal of Fisheries Management, 23(3), 732-741.

As part of a study to assess the effect of commercial dredging for eastern oysters Crassostrea virginica on their beds in Delaware Bay, we evaluated the total dredging effort for the Delaware Bay oyster industry in New Jersey waters for 1999-2000 and examined some of the behavioral and gear-related factors that determined this total effort. In a standard 8-h fishing day, a one-dredge boat traverses about 3.8 ha (38,000 m[super]2) of oyster bed. A two-dredge boat traverses nearly twice that area, about 6.4 ha. Oyster boats typically fish in a single area for most of the day, yet catch per unit effort (CPUE) does not decline during the course of the day. Catch per unit effort is stable because of low dredge efficiency. Although an oyster dredge is capable of routinely achieving efficiencies of 10-60%, dredge efficiency during fishing is usually only 4-7%. Low dredge efficiency means that swept-area coverage (area of the bottom traversed by a dredge) is high for the number of oysters taken. The most heavily fished Delaware Bay oyster beds were completely dredged one to eight times in a single year. Oyster catch is typically measured in bushels (1 bushel = 35 L). The CPUE, calculated as the number of bushels of oysters caught per hectare swept by the dredge, averaged 10-12 in 1999-2000. The CPUE did not vary significantly between one- and two-dredge boats but varied by more than a factor of five among oyster beds. Nevertheless, CPUE was not correlated with the number of bushels landed from each bed. Hence, factors other than the rate of capture, such as market quality, determined the spatial distribution of fishing effort.

Carbines, G., & Cole, R. (2009). Using a remote drift underwater video (DUV) to examine dredge impacts on demersal fishes and benthic habitat complexity in Foveaux Strait, southern New Zealand. Fisheries Research (Amsterdam), 96(2-3), 230-237.

Foveaux Strait is a shallow body of water at the southern tip of New Zealand. It supports nationally significant dredge oyster Ostrea chilensis and blue cod Parapercis colias fisheries. Fish counts and benthic habitat descriptions from drift underwater video (DUV) transects conducted in two seasons over an area of recovering biogenic reef and an adjacent recently dredged area are presented. Over all, seven demersal fish species (5.10 per 100m super(2), 75% P. colias) were recorded on the recovering area surveyed, whereas only three species (0.47 per 100m super(2), 91% spiky dogfish Squalus acanthias) were recorded on the recently dredged area. There were few seasonal differences except for S. acanthias. Descriptions of benthic habitat derived from video stills showed topographic complexity was greater on the recovering area; general epifauna cover, sponge cover and macro-algal cover were also greater on the recovering area, but with seasonal interactions. In contrast, the numbers of tunicates and ophiuroids were higher on the dredged area. Sponge cover (absent from the dredged area) was also correlated with the abundance of leather jackets Parika scaber and scarlet wrasse Pseudolabrus miles as well as all color phases of P. colias. Topographic complexity, general epifauna cover, and macro-algae cover were also positively correlated with the abundance of adult P. colias and P. scaber. The drifting video methodology was able to estimate densities of demersal fish and make broad-brush measures of benthic habitat capable of demonstrating the importance of benthic habitat complexity to demersal fish in Foveaux Strait. The potential mitigation of reduced benthic habitat complexity from oyster fishing is then discussed.

Carbines, G, Weimin, J, Beent Jes, MP. (2004). The impact of oyster dredging on the growth of blue cod, Parapercis colias, in Foveaux Strait, New Zealand. Aquatic conservation 14(5): 491-504.

1. Little is known about the potential impact of habitat modification by bottom fishing gear on the growth of demersal fishes. An analysis is presented for the growth of blue cod in Foveaux Strait, southern New Zealand, based on otoliths of fish captured from two sites in Foveaux Strait in 1999. 2. Each site contained two distinct areas of contrasting benthic habitat complexity, one area of relatively 'complex' recovering biogenic reef and another area of relatively 'simple' sand and gravel, both previously modified by oyster dredging. 3. Data were fitted to von Bertalanffy growth models for each sex of blue cod from the four areas sampled. No significant difference in growth models was observed for either male or female blue cod compared between the two types of habitat complexity at the eastern site. However, growth differed significantly for both sexes of blue cod from the two habitat types at the western site. Pairwise t-tests further showed that growth differences only appeared biologically significant for the youngest blue cod sampled (3 years). These fish were, on average, 20% larger in complex biogenic reefs than in simple areas dredged by the oyster fishery. 4. These results suggest that on-going disturbance and simplification of seabed habitat by the oyster fishery may impede the growth of juvenile blue cod. Areas of recovering biogenic reef may, therefore, provide important habitat for the recruitment and early development of blue cod in Foveaux Strait. Remedial actions may be required to protect some areas of recovering biogenic reef from further damage, and to allow dredged areas sufficient time to recover if the blue cod fishery and related resources are to be managed effectively.

CHAI, A., HOMER, M., TSAI, C., & GOULLETQUER, P. (1992). Evaluation of oyster sampling efficiency of patent tongs and an oyster dredge. North American Journal of Fisheries Management, 12(4), 825-832. Available online at:

Sampling efficiency of two oyster fishing gears, patent tongs and an oyster dredge, were compared in reference to diver-harvested quadrats in Chesapeake Bay, which supports important harvests of eastern oyster Crassostrea virginica. Mean densities of spat (35 mm), small oysters (>35 mm to 75 mm), marketable oysters (>75 mm), and all oysters (three size-groups combined) estimated from patent tong samples were not significantly different from those derived from diver-harvested quadrat samples. In contrast, the densities estimated from dredge samples were low, only 2-32% of the diver estimates. Accordingly, patent tongs are recommended as the sampling gear for estimating eastern oyster stock abundance in the Maryland portion of Chesapeake Bay.

Cranfield, H., Carbines, G., Michael, K., Dunn, A., Stotter, D., & Smith, D. (2001). Promising signs of regeneration of blue cod and oyster habitat changed by dredging in Foveaux Strait, southern New Zealand. New Zealand Journal of Marine and Freshwater Research, 35(5), 897-908.

Epifaunal reefs in Foveaux Strait are oyster (Ostrea chilensis Philippi, 1845) habitat. One hundred and thirty years of oyster dredging has diminished the complexity and distribution of these reefs. Commercial densities of blue cod (Parapercis colias) were discovered on epifaunal reef habitat in 1989 and became the focus of a major blue cod fishery. We document habitat changes that followed the closing of the oyster fishery in 1993 and interactions between the blue cod and oyster fisheries after the oyster fishery was reopened in 1996. Evidence from blue cod fishers and oyster surveys suggests that the benthic habitat of some oyster beds regenerated in the absence of dredging and that the relative density of blue cod, and then oysters, rebuilt to commercial levels. Benthic habitat was modified once more when oyster dredging restarted and the relative density of blue cod on oyster beds fell again. The observations suggest that rotational fishing of oysters could mitigate the effects of dredging on habitat and that marine protected areas could expedite habitat recovery. Increasing habitat complexity and blue cod density on a reef of oyster shells formed by an oyster fisher suggests that habitat enhancement might remedy effects of dredging. The questions raised by the observations could be answered by management experiments on the scale of the fisheries.

Cranfield, H., Manighetti, B., Michael, K., & Hill, A. (2003). Effects of oyster dredging on the distribution of bryozoan biogenic reefs and associated sediments in Foveaux Strait, southern New Zealand. Continental Shelf Research, 23(14-15), 1337-1357.

Foveaux Strait has been commercially fished for oysters for over 100 years, focusing principally upon bryozoan biogenic reefs that once covered large areas of the strait. Bryozoan biogenic reefs consisted of linear swards of bryozoan patch reefs, paralleling the peak tidal current, and were formed by the frame-building bryozoan Cinctipora elegans and associated epifauna. Two side-scan surveys one in the late 1970s and the other in the late 1990s and estimates of distribution of oyster density and distribution of fishing effort, show how fishing has effected the distribution of habitat, oysters and sediments over the last 30 years. Fishers dredged the biogenic reefs for their oysters, damaging the framework structure, removing epifauna and exposing associated sediments, which were then reworked and transported down-current in the strong tidal flow. Large volumes of biogenic sediments released in this process formed thick sheets and dune bedforms. When dredging ceased locally (after removal of biogenic reef habitat and oysters), sediment supply downstream dwindled and ceased, bedforms diminished and the seafloor ultimately reverted to relict pebble gravel. Sediment derived from the dredging of biogenic reefs in northern Foveaux Strait was mostly transported and deposited in deeper water to the east, however, sediment derived from biogenic reefs in southern Foveaux Strait was transported in the opposite direction, giving rise to a group of large dunes in the southwest that in 1999 contained ~58 x 10 super(6) m super(3) of coarse biogenic sediment. By 1998 none of the original bryozoan biogenic reefs remained. Sediment distribution in Foveaux Strait is interpreted in the light of these processes, and compared with earlier data to provide snapshots of human-induced modification of the benthic environment. Evidence for regeneration of simple biogenic reefs is also explored in areas no longer fished, and the conditions in which this has occurred are discussed. Communities dominated by byssally attached Modiolus appear to provide the early framework and shelter for development of new patch reefs. However the recent discovery of regenerating colonies of the reef-building bryozoan Cinctipora in eastern Foveaux Strait suggests that bryozoan patch reefs may also be capable of re-establishing where conditions are suitable. The authors favour helical circulation cells in the strongly linear tidal flow of Foveaux Strait as a mechanism for controlling, not only the linear aspect of the reef frameworks, but also for concentrating nutrients, propagules and the settlement of larvae. Observations of regenerating epifaunal habitat on the fishery-modified seafloor suggest that careful management could reverse much of the deleterious effect of fishing.

Cranfield, H., Michael, K., & Doonan, I. (1999). Changes in the distribution of epifaunal reefs and oysters during 130 years of dredging for oysters in Foveaux Strait, southern New Zealand. Aquatic Conservation: Marine and Freshwater Ecosystems, 9(5), 461-483.

Foveaux Strait, a narrow seaway that is exposed to heavy wave action and strong tidal currents, has been the subject of an oyster fishery for over 130 years. Before the oyster fishery commenced the seafloor was extensively covered by epifaunal reefs that were tidally-oriented, linear aggregations of patch reefs. Patch reefs are formed by the bryozoan Cinctipora elegans cemented by encrusting bryozoa, ascidians, sponges, and polychaetes. The molluscan epifauna is dominated by the oyster, Tiostrea chilensis and bysally attached bivalves. Mortality of oysters is probably lower and recruitment and growth may be higher within the reef habitat. Fishers found commercial densities of oysters occurred only on epifaunal reefs. Fishers exploited local groups of reefs. These groups form the patchily distributed oyster beds characteristic of this fishery. Dredging for oysters progressively modified reefs until oysters were the only epifauna remaining. Dredges caught oysters more efficiently after the catch bag no longer became saturated with other epifauna. This heightened efficiency allowed fishers to rapidly reduce oyster density to commercial extinction. Oyster density has not rebuilt on oyster beds abandoned by fishers. The rate of modification of epifaunal reefs was slower during the early years of the fishery but has accelerated, especially over the last 37 years. Frequency of disturbance increased as the numbers of vessels fishing grew and fishers developed speedier dredging methods. Intensity of disturbance also increased as heavier dredges were introduced and allowed focused fishing of reefs. Oysters became reduced to low densities in the eastern and central areas that fishers then abandoned. The commercially exploited area subsequently expanded to the limits of Foveaux Strait. With accelerated modification of oyster habitat, disease mortality has become more important. Attempting to rebuild the fishery by oyster enhancement may be more successful conjoined with habitat restoration.

Ismail, N. (1985). The effects of hydraulic dredging to control oyster drills on benthic macrofauna of oyster grounds in Delaware Bay, New Jersey. Internationale Revue Der Gesamten Hydrobiologie.Berlin, 70(3), 379-395.

This study describes the extent and nature of the effects of hydraulic dredging to control oyster drills (Urosalpinx cinerea and Eupleura caudata , family Muricidae, order Neogastropoda) on benthic macrofauna and sediments of the oyster grounds in Delaware Bay, New Jersey. The immediate effects of hydraulic dredging were reductions in numbers of species as well as in total numbers of animals on the three oyster grounds selected. However, oyster drills were most, affected. Benthic populations have recovered three to ten months after dredging.

Langan, R. (1997). The effect of dredge harvesting on eastern oysters and the associated benthic community. In Dorsey E. M., Pederson J. (Eds.), Effects of Fishing Gear on the Sea Floor of New England. pp. 108-110.

A study was conducted in 1994 to determine the effects of dredge harvesting on oyster populations and the benthic community associated with the oyster bed. The study area was located in the Piscataqua River, which divides the states of Maine and New Hampshire. An oyster bed approximately 18 acres in size is located in the river channel and is divided nearly equally by state jurisdictional lines (Figure 1). Differences in regulations between the two states provided a unique study opportunity. At the time of the study, the State of Maine classified the area as "restricted for depuration" and allowed commercial harvesting, whereas New Hampshire had placed a "prohibited" classification on the area many years prior to the study and did not allow commercial harvesting. The Maine side of the bed had been harvested using a small oyster dredge two days per week on average for five years prior to the study, while the New Hampshire side had not been harvested by any method for many years. The dredge used in the area is 30 inches wide and weighs approximately 60 lbs. The bottomtending portion of the dredge has blunt, two-inch teeth spaced approximately three inches apart and a chain mesh bag. The different state regulations with regard to harvesting allowed a well-controlled study of the effect of dredge harvesting both on the oysters and on the associated benthic invertebrates.

Lenihan, H., & Peterson, C. (2004). Conserving oyster reef habitat by switching from dredging and tonging to diver-harvesting. Fishery Bulletin, 102(2), 298-305. Available online at:

A major cause of the steep declines of American oyster (Crassostrea virginica) fisheries is the loss of oyster habitat through the use of dredges that have mined the reef substrata during a century of intense harvest. Experiments comparing the efficiency and habitat impacts of three alternative gears for harvesting oysters revealed differences among gear types that might be used to help improve the sustainability of commercial oyster fisheries. Hand harvesting by divers produced 25-32% more oysters per unit of time of fishing than traditional dredging and tonging, although the dive operation required two fishermen, rather than one. Per capita returns for dive operations may nonetheless be competitive with returns for other gears even in the short term if one person culling on deck can serve two or three divers. Dredging reduced the height of reef habitat by 34%, significantly more than the 23% reduction caused by tonging, both of which were greater than the 6% reduction induced by diver hand-harvesting. Thus, conservation of the essential habitat and sustainability of the subtidal oyster fishery can be enhanced by switching to diver hand-harvesting. Management schemes must intervene to drive the change in harvest methods because fishermen will face relatively high costs in making the switch and will not necessarily realize the long-term ecological benefits.

Mann, R., Southworth, M., Harding, J., & Wesson, J. (2004). A comparison of dredge and patent tongs for estimation of oyster populations. Journal of Shellfish Research, 23(2), 387-390.

Exploited oyster stocks on public grounds in Virginia waters are subject to regular surveys effected using a traditional oyster dredge and, more recently, patent tongs. Dredges provide semiquantitative data, have been used with consistency over extended periods (decades), and provide data on population trends. Surveys with patent tongs provide absolute quantification (number of individuals per unit area) of oyster stocks but are more labor intensive. Absolute quantification of dredge data is difficult in that dredges accumulate organisms as they move over the bottom, may not sample with constancy throughout a single dredge haul, and may fill before completion of the haul thereby providing biased sampling. Selectivity of dredges versus patent tongs with respect to oyster demographics has not been rigorously examined. The objective of this study is to compare demographic oyster data collected at the same sites in the same years from both gear types. Data for the study were taken from 1993 to 2001 surveys conducted in the James River, Virginia, by the Virginia Institute of Marine Science and the Virginia Marine Resources Commission wherein the same stations were sampled by both techniques. Dredge surveys give data in oysters per bushel and assume no selective retention of live oysters with respect to shell substrate by the dredge. Patent tong surveys provide data as per tong estimates of oysters by size class and shell by volume. The hydraulically operated, 1-m square tong used in VMRC/VIMS surveys is designed to sample on and below the reef surface and include elements of buried shell that are probably not well sampled by a dredge, although the sampling ensures collection of all oysters within the tong mouth. Oysters collected by both gear types were classified as small (25-75 mm) or market (>75 mm SL) for comparisons across methods. Shell volumes collected in patent tong surveys were standardized to bushel increments assuming 35.28 L of shell per bushel. The summary plots of mean values from 1993 to 2001 and 1998 to 2001 illustrate differences related to sampling gear. More shell per unit oyster (lower bushel counts) are observed in a patent tong sample. The appropriate model for attempting to fit a predictive line is open to debate, and will be influenced by patent tong penetration as determined by the degree of consolidation of the underlying substrate. The available data do not strongly support the ability to predict a relationship between dredge and patent tong population estimates at this time.

Powell, E., & Ashton-Alcox, K. (2004). A comparison between a suction dredge and a traditional oyster dredge in the transplantation of oysters in Delaware Bay. Journal of Shellfish Research, 23(3), 803-823.

One mechanism to enhance oyster production is the timely transplant of oysters from nursery beds to beds used for commercial harvesting. Transplanting in Delaware Bay is normally done with a traditional oyster dredge. Such dredges can concentrate market-size oysters, a desirable characteristic for some transplant goals. Unfortunately, catch rates are slow. The suction dredge is much faster, but suction dredges likely do not concentrate large oysters and, by removing most surficial material, may reduce bottom shell coverage and decrease bottom complexity. We investigated the relative benefits of using a traditional oyster dredge and a suction dredge in a transplant program. In this study, traditional oyster dredges used for transplant operations had dredge efficiencies of approximately 5%, about 100 bushels of material being loaded per hectare swept. The tendency for the dredge to catch larger particles preferentially was negated by the tendency to operate the dredge at below-optimal efficiencies. Nevertheless, deck loads contained a factor of 2 to 3 more oysters per bushel than present on the bottom. The suction dredge operated very differently, although deck loads contained 1 to 3 times as many oysters as were present on the bottom. Catch efficiencies were high, between 19% and 58%. Swept area per bushel loaded was much lower, about 600 bushels being loaded per hectare swept. Catch efficiencies were highest for small particles. Dredge efficiency rose markedly after transplanting, from 6% to 28% on the plots worked by the traditional oyster dredge and from 11% to 56% on the plots worked by the suction dredge. Nevertheless, neither method proved deleterious to bottom complexity, cultch availability, oyster growth and mortality, or population health. In a sustainable transplant, the number of small oysters and amount of cultch moved should be minimized. This goal was not achieved. The suction dredge, by selective removal of smaller particles enriched in juveniles and cultch, risks a long-term decline in live oyster abundance and shell coverage. The traditional oyster dredge has the inherent capability of concentrating larger animals, but, as used in the transplant process, much of the selective advantage disappears. A behavioral shift to exploit the desirable selective advantage of the traditional oyster dredge may improve the efficiency of the transplant program.

Powell, E., Ashton-Alcox, K., Banta, S., & Bonner, A. (2001). Impact of repeated dredging on a Delaware Bay oyster reef. Journal of Shellfish Research, 20(3), 961-975.

The impact of commercial dredging on an oyster reef was evaluated at four sites chosen on New Beds, one of the most important commercial oyster beds in Delaware Bay. Dredging occurred on two of these sites in late October 1999, early and late November 1999, April 2000, and July 2000. Dredging was conducted according to standard industry procedures. Each day, dredging was continuous during approximately an 8-h period. Both one-dredge and two-dredge boats were used. Market-size oysters were culled and sacked in the standard manner. Total dredge coverage for the study was about 240,000 m super(2) on each experimental site. The most heavily dredged areas were completely covered by the dredge 4 to 6 times during the study. Two 8-h dredging events within a 10-day period produced barely detectable changes in the oyster population. Minor chipping and abrasion of the shell increased in frequency, but no other discernible impacts were found. Over the 10-mo study that included five dredging events, many of the taphonomic indicators of dredge damage showed time-dependent trends that differed between control and experimental sites. However, these effects were limited mostly to minor chipping and indications of abrasive wear, rather than the more serious aspects of shell damage defined as major chipping, breakage, cracking, and shell perforation. A variety of population health indicators were assayed during the study, including the ratio of live oysters to boxes, condition index, Perkinsus marinus infection intensity, and oyster size-frequency distribution. These indicators should have monitored growth, disease pressure, and mortality. Essentially no significant effects could be discerned for any of these measures. Over a very long time, dredging may significantly influence oyster bed physiography and community structure. However, once the bed has become a fished bed, this study suggests that moderate dredging that results in a yearly swept area of no more than four times the area of the bed is unlikely to result in significant further impact on the oyster populations living there.

Rothschild, B.J., Ault, J.S., Goulletquer, P., Heral, M., 1994. Decline of the Chesapeake Bay oyster population: a century of habitat destruction and overfishing. Marine Ecology Progress Series 111, 29– 39. 

The oyster population in the Maryland portion of Chesapeake Bay, USA, has declined by more than 50-fold since the early part of this century. The paper presents evidence that the mechanical destruction of habitat and stock overfishing have been important factors in the decline, even though it is commonly thought that 'water quality' and, more recently, oyster diseases are critical. Quantitative analyses show that the long-term decline of oysters largely results from habitat loss associated with intense fishing pressure early in this century, and stock overfishing from early in the century through recent times. Furthermore, the major ecological effects on Chesapeake Bay occurred well before World War II, before industrialization and the reported prevalence of disease. To effect the recovery of the ailing Chesapeake Bay oyster stock, a 4-point management strategy is proposed.

Visel, T. (1988). Mitigation of dredging impacts to oyster populations. Journal of Shellfish Research 7(2): 267-270. Available online at:

Maintenance and extensive navigational dredging in coastal areas along the Northeast and Mid-Atlantic coasts have altered the population dynamics of oysters, Crassostrea virginica . In most instances, oyster production has been reduced by removing shell bases and reefs upon which spat could set. One type of mitigation of dredging impacts may be made through a variety of reshelling programs. In July 1986, 8,000 bushels of clam shell were planted over the shell base which obtained at set of 0-year oysters. A harvest of several thousand bushels of seed oysters was anticipated in 1987. Mitigation agreements, which are small in scale and do not interfere with other coastal activities, can be expanded to improve oyster resources.

Oyster Shells 

Conner, William G. and Joseph L. Simon. (1979). The effects of oyster shell dredging on an estuarine benthic community. Estuarine and Coastal Marine Science 9(6): 749-758.

This paper describes the extent and nature of the effects on the benthos of physical disruptions associated with dredging fossil oyster shell. Two dredged areas and one undisturbed control area in Tampa Bay, Florida, were quantitatively sampled before dredging and for one year after dredging. The immediate effects of dredging on the soft-bottom community were reductions in numbers of species (40% loss), densities of macroinfauna (65% loss), and total biomass of invertebrates (90% loss). During months 6–12 after dredging, the analysis used (Mann-Whitney U Test, α=0·05) showed no difference between dredged and control areas in number of species, densities, or biomass (except E1). Community overlap (Czeckanowski's coefficient) between dredged and control areas was reduced directly after dredging, but after 6 months the pre-dredging level of similarity was regained.

Goeke, G. D., & Army Engineer Dist., New Orleans, LA (USA). (1987). Oyster shell dredging in Atchafalaya bay and adjacent waters, Louisiana. draft environmental impact statement and appendixes.

Oyster shells have been removed by means of hydraulic cutter-head dredges from the waters of coastal Louisiana since 1917. The shells have been harvested primarily for use in construction activities, although a variety of other uses are common. There has been considerable controversy over the impacts of shell dredging. This Draft Environmental Impact Statement has been prepared to assess the impacts of oyster shell dredging in East Cote Blanche Bay, Atchafalaya Bay, and Four League Bay, Louisiana as permitted under 5-year permits issued in 1982 that will expire in December 1987.

Judy, C. (1989). Environmental assessment of oyster shell dredging in the upper Chesapeake Bay. Journal of Shellfish Research, 8(2), 480. 

The Maryland Department of Natural Resources conducts an annual oyster repletion program which depends on the planting of oyster shell cultch to provide habitat for oyster settlement. The shells are obtained by hydraulically dredging large, buried shell deposits at sites in the upper Chesapeake Bay. Oyster shell dredging has been conducted since 1960. In 1986 an environmental assessment of the effects of oyster shell dredging was initiated. It investigated changes in bottom topography, water quality, benthic community structure, and fish usage of the dredging areas. Dredge areas were compared to undredged areas.


Aschan, M., & International Counc. for the Exploration of the Sea, Copenhagen (Denmark). Shellfish Comm. (1988). The effect of Iceland scallop (chlamys islandica ) dredging at Jan Mayen and in the Spitsbergen area. COPENHAGEN (DENMARK): ICES.

In this paper the effect of dredging on the macrobenthos of Chlamys islandica fields will be presented. The study was conducted from the research vessel F/F Johan Ruud in the summer 1987 and 1988 in an area south of Jan Mayen at 60-120 m depth and at the northern and north-western side of Spitsbergen at 25-80 m depth. Data on the faunal composition was collected through use of dredging, photography and underwater video recording.

Bishop, MJ et al. (2005). Effects of harvesting methods on sustainability of a bay scallop fishery: dredging uproots seagrass and displaces recruits. Fishery Bulletin 103: 712-719. Available online at:

Bradshaw, C., Veale, L., Hill, A., & Brand, A. (1999). The effect of scallop dredging on Irish Sea benthos: Experiments using a closed area. Journal of Shellfish Research, 18(2), 709.

A 2 km super(2) area off the southwest coast of the Isle of Man (Irish Sea) has been closed to commercial fishing with mobile gear since March 1989. This area was heavily fished for Pecten maximus prior to closure, and the seabed immediately surrounding the closed area is still one of the most heavily dredged in the Irish Sea. Two methods have been used to study the effect of scallop dredging on the benthos in this closed area and adjacent fished areas. Firstly, divers have carried out visual transect surveys of the epibenthos regularly since closure. Secondly, biannual fine-meshed dredge and grab sampling of experimental plots inside and outside the closed area since 1995 has enabled comparisons of the benthic infauna and epifauna of experimentally dredged plots, undredged control plots and plots exposed to commercial dredging. Since 1989, there have been consistent significant increases in the mean numbers of many species in the closed area, including Pecten maximus and Luidia ciliaris, and upward trends in numbers of hermit crabs, spider crabs and brittlestars have also been recorded. Conversely, the common starfish, Asterias rubens, appears to be decreasing in abundance. Communities of experimentally disturbed plots have become less similar to adjacent undisturbed control areas and more similar to commercially dredged areas. At each sampling date, similarity between samples was greater outside the closed area than inside. These results present strong evidence that scallop dredging alters benthic communities and suggest that the closure of areas to commercial dredging may allow the development of more heterogeneous communities and allow the populations of some species to increase. A common problem with studying fishing disturbance is the lack of good control sites and this work also demonstrates the value of closed areas to scientific studies of bottom fishing.

Caddy JF. 1973. Underwater observations on tracks of dredges and trawls and some effects of dredging on a scallop ground. Journal of the Fisheries Research Board of Canada 30:173-80.

Currie DR, Parry GD. 1996. Effects of scallop dredging on a soft sediment community: a large-scale experimental study. Marine Ecology Progress Series 134:131-50. Available online at:

Changes to benthic infauna caused by scallop dredging at a site in Port Phillip Bay, southeastern Australia, were examined experimentally using a BACI (before, after, control, impact) design. The experimental dredging was undertaken by commercial fishermen and was typical of normal commercial operations in its spatial extent, intensity and duration. Changes to benthic community structure following dredging were monitored using grab samples taken on 3 occasions pre-dredging and 6 occasions post-dredging. The significance of changes was assessed using ANOVA for the more abundant species and, for pooled groups of species, Bray-Curtis community dissimilarities and multidimensional scaling (MDS). The abundance of 7 of the 10 most common species changed significantly (ANOVA p < 0.10) after dredging; 6 species decreased in abundance while 1 species increased. The size and persistence of dredging impacts varied between species, but most species decreased in abundance by 20 to 30%. Dredging impacts became undetectable for most species following their next recruitment. Most species recruited within 6 mo of the dredging impact, but a small number of species still had not recruited after 14 mo. These latter species appeared to cause a persistent change in community structure which was still detectable after 14 mo using Bray-Curtis dissimilarities. MDS ordination indicated that changes to community structure caused by dredging were smaller than those that occur between seasons and years.

Currie DR, Parry GD. 1999. Impacts and efficiency of scallop dredging on different soft substrates. Canadian Journal of Fisheries and Aquatic Sciences, 56:539- 50. Available online at:

Impacts of scallop dredges and their efficiency were examined experimentally in three areas with different soft substrates in Port Phillip Bay, southeastern Australia. Physical and biological changes were measured on large (600 × 600 m) experimental plots that were dredged with an intensity and duration similar to normal fishing operations. Dredges were most efficient on soft, flat, muddy sediments (51-56% of commercial-sized scallops caught) and least efficient on firm, sandy sediments with more topographic variation (38-44%). Dredging flattened all plots, but changes to topography were most apparent on plots dominated initially by callianassid mounds. Dredges caught predominantly the scallop Pecten fumatus, and damage to bycatch species was slight, except for high mortality rates (>50%) of spider crabs and the probable mortality of many discarded ascidians. Changes to benthic community structure caused by scallop dredging were small compared with differences between study areas, and even marked reductions in the size and longevity of scallops over the last two decades may not be due entirely to dredging. The recent cancellation of all scallop dredging licences offers a unique opportunity to determine the contribution of scallop dredging to ecological changes in the bay over the past 30 years.

DuPaul, W., Rudders, D., & Smolowitz, R. (2006). Industry trials of a sea scallop dredge modified to minimize the catch of sea turtles. Journal of Shellfish Research, 25(1), 283.

In response to increasing numbers of sea turtle interactions observed by the sea scallop industry and subsequently corroborated by NMFS observers, a series of 15 experimental cruises were carried out during the summer and early fall of 2003 and summer of 2004 on the continental shelf waters of the mid-Atlantic Bight. The objective of the cruises was to examine the efficacy of a modified commercial sea scallop dredge designed to reduce the bycatch of sea turtles in the sea scallop fishery. The modification consisted of a chain mat spanning the opening of the dredge mouth. The performance of the experimental gear was assessed by comparing a modified dredge fished simultaneously with an unmodified dredge. Results indicate that the modification was successful in eliminating the bycatch of turtles with relatively small reductions in the catch of the target species. A total of 3,078 tows in 277 days at sea was observed during the trials with eight sea turtles captured in the unmodified dredge and none captured in the modified dredge. Of the tows that were sampled by the observers, the modified dredge captured significantly (P < 0.001) less scallops relative to the unmodified dredge. On a percentage basis, the modified dredge captured 6.8% less scallops than the unmodified dredge. It is anticipated, however, that the difference in sea scallop catches will decrease over time as industry becomes more familiar with the use of the chain configuration. These cruises demonstrated that a simple modification to the standard sea scallop dredge can be effective in eliminating the incidence of sea turtle bycatch without substantial concomitant reductions in the capture of the target species.

Eleftheriou A, Robertson MR. 1992. The effects of experimental scallop dredging on the fauna and physical environment of a shallow sandy community. Netherlands Journal of Sea Research 30:289- 99.

An experimental dredging operation was carried out in a small sandy bay in Scotland, with the aim of quantitatively assessing the effects of scallop dredging on the benthic fauna and the physical environment. An area within the 10-m depth contour was selected; a 1.2-m modified scallop dredge was operated at frequencies of 2, 4, 12 and 25 dredges, carried out over a period of nine days. The effects on the bottom topography, the physical characteristics of the sediment and the fauna were investigated by grab and core sampling, and direct observations were carried out by a diving team. Observed changes in bottom topography were not translated into changes in the disposition of the sediments, their grade distribution and the organic carbon and chlorophyll content, all of which showed no effects. The infaunal community, which consisted of bivalve molluscs and peracarid crustaceans, both taxa adapted morphologically and behaviourally to a dynamic environment, did not show any significant changes in abundance or biomass. Sessile forms such as polychaetes showed a noticeable decrease, and the burrowing spatangid Echinocardium was substantially reduced from the dredged area. Corresponding changes in the biomass of the different taxa were also evident but not significant. However, the most important effect of this experiment was on the epifaunal and large infaunal organisms recorded by the divers. Large numbers of molluscs (Ensis), echinoderms (Asterias) and crustaceans (Cancer) were killed or damaged by the dredging operations. Very large concentrations of the burrowing sand eel Ammodytes were also destroyed. The overall conclusion to be drawn from this experimental dredging operation is that its effect was limited tot he selective elimination of a fraction of the fragile and sedentary components of the infauna, and the destruction of the large epifaunal and infaunal organisms.

Grant, J. (2000). Modelling approaches to dredging impacts and their role in scallop population dynamics. Division of Commercial Fisheries, Alaska Department of Fish and Game. Available online at:

The dramatic decline in many coastal and shelf fisheries worldwide has focused attention on fishing methods and their potential to impact benthic habitats. These concerns extend to the mortality of target species that do not make up part of the catch but suffer indirect or incidental mortality. There is similar concern about bycatch which may include commercially valuable species. For example, in the context of invertebrate fisheries in Alaskan waters, there are analogous issues that involve dredging for scallops Patinopecten caurinus and bycatch of king crabs and Tanner crabs. The potential habitat alteration caused by mobile fishing gear has been the subject of a variety of recent studies. It is apparent that the impacts are dependent on sediment type, life history stage, and functional group. Although some effects, such as the disruption of colonial epifauna, are obvious, questions remain about the implications of fishing practices for the population dynamics of the target species. Despite the goal of optimizing fisheries yields, there are surprisingly few studies which attempt to quantify how gear affects target species. This is a particularly relevant topic for scallop fisheries since the gear is bottom directed (in contrast to some trawls), and the target species is somewhat "delicate" compared to infaunal bivalves, which can burrow or tightly close. For the scallop example, there are more studies of dredge effects on benthic communities than on scallop populations.

Hall-Spencer JM, Moore PG. 2000. Scallop dredging has profound, long-term impacts on maerl habitats. ICES Journal of Marine Science 57:1407- 15. Available online at:

Maerl beds are mixed sediments built by a surface layer of slow-growing, unattached coralline algae that are of international conservation significance because they create areas of high biodiversity. They are patchily distributed throughout Europe (to similar to 30 m depth around the British Isles and to similar to 120 m depth in the Mediterranean) and many are affected by towed demersal fishing. We report the effects of Newhaven scallop dredges on a previously unfished maerl bed compared with the effects on similar grounds that have been fished commercially in the Clyde Sea area, Scotland. Sediment cores were taken to assess the population density of live maerl thalli prior to scallop dredging on marked test and control plots. These plots were then monitored biannually over a four-year period. Live maerl thalli were sparsely distributed at the impacted site, and experimental dredging had no discernible effect on their numbers. The previously unfished ground had dense populations of live maerl and scallops (both Aequipecten opercularis and Pecten maximus). While counts of live maerl remained high on the control plot, scallop dredging led to a >70% reduction with no sign of recovery over the subsequent four years. The vulnerability of maerl and associated benthos (e.g., the delicate bivalve, Limaria hians) is discussed in relation to towed demersal fishing practices. Copyright 2000 International Council for the Exploration of the Sea

Jenkins SR, Beukers-Stewart BD, Brand AR. 2001. Impact of scallop dredging on benthic megafauna :a comparison of damage levels in captured and non-captured organisms. Marine Ecology Progress Series 215:297-301. Available online at:

The impact of scallop dredging on benthic megafauna was assessed by direct observation of damage, both in the bycatch and in organisms encountering dredges but not captured. Damage was assessed using a simple 4-point scale adapted for different taxonomic groups. Experimental dredging was undertaken on a scallop fishing ground in the North Irish Sea, off the Isle of Man. Divers were deployed immediately after dredges had passed, to record levels of damage to megafauna left in the dredge tracks. Mean damage levels, and the proportions of the 4 damage scores in the bycatch and on the seabed, were the same in most species. Some common species did show differences. The edible crab Cancer pagurus was more severely damaged when not captured, while the starfish Asterias rubens and whelk Neptunea antiqua received greater damage within the bycatch. Capture efficiency for the megafauna was low, ranging from 2 to 25% among species. The results indicate that the majority of damage to large benthic invertebrates during scallop dredging occurs unobserved on the seabed, rather than in the bycatch.

Thrush SF, Hewitt JE, Cummings VJ, Dayton PK. 1995. The impact of habitat disturbance by scallop dredging on marine benthic communities: What can be predicted from the results of experiments? Marine Ecology Progress Series 129:141-50.

Field experiments were conducted on 2 subtidal sandflats to identify the short-term impacts of commercial scallop dredging on macrobenthic communities. The 2 sites (1400 m2) were situated 14 km apart, both at about 24 m depth, with similar exposure aspects and were characterised by infaunal communities dominated by small and short-lived species. Prior to dredging, preliminary sampling failed to reveal significant differences in the density of common macrofauna within each site, although community composition was distinctly different between sites. The experiment was initiated by using a commercial scallop dredge to dredge half of each study site. Macrofauna samples were collected in both the dredged and adjacent reference plot at each site immediately after dredging and again 3 mo later. The density of common macrofaunal populations at each site decreased as a result of dredging, with some populations still significantly different from the adjacent reference plot after 3 mo. Significant compositional differences in the assemblage structure between dredged and reference plots were also recorded at each site over the course of the experiment. The findings of this experiment are considered a conservative assessment of bottom disturbance by fishing because of the area of seabed used, the types of community present and the intensity of disturbance used in the experiment. The findings of this and similar short-term experiments are discussed in light of the need to predict and assess possible large-scale changes to benthic communities as a result of habitat disturbance by fishing.


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