The Maryland blue crab trawl survey is a Department of Natural Resources 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.
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 2000 to 2009. 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 2018 monthly abundance
indices for recruit crabs remained strong in May and June compared to 2017 and
the 2000-2009 ten year average. In 2018,
the abundance indices for recruit crabs surpassed 2017, 2016, and the ten year average
after August. In all 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 2018 abundance of growth crabs was consistent with 2016
up to mid-summer. In 2017 and the
2000-2009 ten year average, monthly abundance of growth crabs followed the
traditional pattern where the highest abundance occurs in the mid-summer only
to decrease in the fall. (Figure 2)
The 2018 monthly abundance indices for mature crabs followed
the traditional pattern, observed in 2017 and the 2000-2009 ten year average,
with the highest abundance occurring in the late summer only to decline in the
fall. In contrast, in 2016, mature crabs increased steadily from May through
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 has occurred in 1977, 1996, and 2010 with the lowest
abundance seen in 1980. Growth crab
abundance for 2018 rebounded from a recent low in 2017. (Figure 4)
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. Mature female abundance in 2018 has shown increases
since a recent low in 2014. (Figure 5)
Average Carapace Width
The yearly average carapace width
is calculated for legal males and mature females using size data collected from
May to October. The average size of
legal males showed an increase in size for 2018 after a decrease in 2017. The average size of mature females has been
variable since 1996, with an increase in 2018. (Figure 6)
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