Blue Crab Summer Trawl Survey

Introduction

Blue crabs in a basket with the one on top being measured. 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.

Methods

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

Photo identifying crab sexes - male, immature female, mature female. 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.

Results

Recruit:

The 2023 monthly abundance indices for recruit crabs lagged 2022 but exceeded the 2000-2009 ten year average. 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)

summer trawl figure 1

Monthly Abundance

Growth:

The 2023 monthly abundance indices for growth crabs were consistent with 2022, 2021, and the 2000-2009 ten-year average in the mid-summer only to decline into the fall. 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)

summer trawl figure 2

Mature:

The 2023 monthly abundance indices for mature crabs followed the traditional pattern, observed in 2022, 2021, and the 2000-2009 ten-year average, with the highest abundance occurring in the late summer only to decline in the fall. (Figure 3)

summer trawl figure 3

Yearly Abundance

Growth:

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 for 2023 declined below the survey average. (Figure 4)

summer trawl figure 4

Mature Females:
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, 2011, and 2019. Mature female abundance in 2023 declined below the survey average since a recent high in 2019. (Figure 5)

summer trawl 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 a minimal increase in size for 2023. The average size of mature females has been variable since 1996, with a decrease in 2023. (Figure 6)

summer trawl figure 6