The Maryland blue crab trawl survey is a DNR sampling program conducted from May through October. The trawl survey began in 1977 with data collected from six river systems in Maryland's Chesapeake Bay region. The summer trawl survey produces information on trends in blue crab abundance, carapace width and weight.
Samples are collected once a month; May through October. There are a total of 37 sites in 6 different river systems; Chester River, Patuxent River, Choptank River, Eastern Bay, Tangier Sound and Pocomoke Sound. Each of the 37 sites are sampled monthly.
In 2002 auxiliary sites in 3 additional rivers were added. These include 8 sites in the Little Choptank River and 4 sites each in Fishing Bay and the Nanticoke River. Although information from these sites is not included in our indices of abundance, these sites serve to validate observations and improve the coverage of the survey.
A 16' semi-balloon otter trawl is used for sampling. The net is towed once at each site for six minutes at a speed of three knots.
After the sample is removed from the trawl the crabs are sorted out of the sample and counted. The crab's carapace is measured to the nearest millimeter to determine carapace width (CW) and the weight is measured to the nearest gram (missing claws are noted). The sex, maturity and molt stage are recorded for each crab.
Data and Analysis
All crabs caught during the trawl survey are grouped into three categories:
Recruit = All crabs smaller than 60mm (2½ in)
* This includes males and females
Growth = Immature females bigger than 60mm
Males between 60-119mm (2½ - 4¾ in)
Mature = All mature females
Males bigger than 120 mm (4¾ in)
Monthly catch per unit effort (CPUE) is considered to be an index of abundance, and is calculated as crabs per tow for each size category. Monthly indices are compared to the previous year and a base average calculated from abundance indices from 1990 to 1999. For annual indices of growth crab abundance the growth crabs sampled in July and August are used. For annual indices of mature female abundance the mature females sampled between August and October are used.
The 2011 monthly abundance indices for recruit crabs showed an increase in October compared to 2010. In 2011 abundance lagged 2010 until October but was greater than the 1990-1999 ten year average. In both years and the ten year average, monthly abundance is greater in the early summer only to decline in the mid-summer and to rebound in the fall with the arrival of new recruits to the fishery. (Figure 1)
The 2011 abundance of growth crabs was consistent with 2010, peaking in July. In both years and the ten year average, monthly abundance of growth crabs follows a similar pattern with the highest abundance occurring in the mid-summer.(Figure 2)
The 2011 monthly abundance indices for mature crabs were highest in September. Abundance in 2011 increased steadily from May through September before falling in October. (Figure 3)
Yearly abundance of growth crabs is determined by the average of the abundance in July and August. July and August are used because by July small growth crabs are widely distributed around the Bay and after August the growth crabs molt to legal size and are susceptible to the commercial and recreational fisheries. Abundance has shown no trends over the length of the study. High abundance occurred in 1977, 1996, and 2010 with the lowest abundance seen in 1980. Growth crab abundance has showed a steady climb to the fourth highest value on record since a recent low in 2007. (Figure 4)
Yearly abundance for mature females is determined by averaging the monthly abundance from August, September and October. The fall is the period of highest abundance in Maryland, after the females have matured and while they are migrating south to over winter. Abundance of mature females does not show a trend, with high abundance seen in 1984, 1996, and 2011. Female abundance in 2011 increased to the third highest abundance on record for the MDDNR summer trawl survey. (Figure 5)
Average Carapace Width
The yearly average carapace width is calculated for legal males and mature females using the size data collected from May to October. The average size of legal males showed an insignificant increase in size for 2011 after a decrease in 2010. The average size of mature females has been variable since 1996, with a slight insignificant increase in 2011. (Figure 6)
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