Volume 14, Number 3 • May-June 1996

Counting Crabs

In 1968, when the Baltimore Gas and Electric (BG&E) Company began construction of the Calvert Cliffs Nuclear Power Plant, George Abbe began setting crab pots offshore in the Chesapeake Bay. Abbe, a scientist at the Benedict Estuarine Research Center, also set pots up-Bay at Kenwood Beach and three miles down-Bay at Rocky Point. Working with support from BG&E, he wanted to determine if the discharge of heated water from the nuclear plant would affect crab populations.

Though support from BG&E has slowed, he is still baiting pots in the same locations, tracking the year-to-year fluctuations of crabs per pot (called catch per unit effort), taking weight and size measurements, and distinguishing between the numbers of males and females. His data set - one of the most comprehensive and long-term measures of crab stocks that researchers in the Chesapeake Bay have - has the added advantage of consistency: he has set pots in the same locations and has only varied the number of days fished during the year.

Twenty-eight years of setting pots and more than 113,000 crabs later, here are some of his findings:

From 1968 to 1982, the annual percentage of males in pots decreased significantly; since 1983, annual percentages have fluctuated but there have been no further decreases.
Mean width and weight of males have decreased significantly.
Percentage of legal males (5 inches across) decreased from 56% in 1968 to 19% in 1995.
Catches of female crabs have remained relatively steady, declining only slightly in the in the last few years.

Between 1968 and 1980, Abbe's counts averaged 3.6 legal-size crabs per pot, very close to the 3.7 average between 1986 and 1995. However, between 1981 and 1985, the average shot up to 8.1 crabs per pot, a number that was especially high because of the 1981 average of 13.

How representative are Abbe's findings for the rest of the Chesapeake Bay? He and Cluney Stagg of the Maryland DNR calculated a number of comparisons with other data - for instance, the annual record of commercial landings in Maryland and Virginia, as well as the statistics that Maryland gets on the harvesting efforts of commercial crabbers. The researchers also compared Abbe's data with the winter dredge survey sponsored by the Chesapeake Bay Stock Assessment Committee. In general, says Stagg, the records from survey data, from commercial catch statistics and from Abbe's records all seem to track.

Chesapeake Bay Blue Crab Landings	Blue Crabs Caught by George Abbe
Just as Chesapeake crab harvests rise and fall, so too does crab abundance and catch per unit effort. Researcher George Abbe is now catching fewer large crabs, a possible signal that while harvest pressure is up, stocks of larger males may be falling.

For the last ten years, Abbe's pots have turned up crab numbers similar to those between 1968 and 1980 - but sizes of males are smaller. "If you look at the percentage of legal crabs that we caught over time, the average size of all males, and the average size of legal males, you see downward trends. To me," he adds, "that says we've put so much effort into the fishery that we're pulling out males as soon as they hit the minimum legal size. We're getting males primarily between five and six inches and males over six inches have declined precipitously."

Whether the decline in large males is representative Baywide and what that might imply for the blue crab fishery remains unknown. If the largest male crabs continue to be harvested, could this have an effect on the reproductive capacity of future stocks? So far, there is no consensus for predicting a future decline - surveys of the incoming numbers of young crabs (recruitment) have been steady or rising for the last five years. Meanwhile, Abbe's results are being incorporated into the CBSAC stock assessment effort, and for now at least, Abbe plans to keep catching crabs, and to keep counting them.

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Counting Crabs

Chesapeake Bay Blue Crab Landings

Blue Crabs Caught byGeorge Abbe

Blue Crabs Caught by
George Abbe