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1 January 2005 ABUNDANCE, DYNAMICS AND MORTALITY RATES OF THE DELAWARE BAY STOCK OF BLUE CRABS, CALLINECTES SAPIDUS
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Abstract

The Delaware Bay stock of blue crabs supports a bistate fishery in New Jersey and Delaware, with annual landings climbing through the 1980s and 1990s to almost 11 × 106 pounds (4,390 metric tons) in 1995 and then declining to a recent average of 7 × 106 pounds (2,796 metric tons) over the last 5 y. In Delaware, this fishery ranks as number one in value. Landings declines in 1996 spurred efforts to conduct a stock assessment, which is now updated annually. This assessment was based on: (1) a biomass-based minimum recruitment threshold from a Ricker stock-recruitment model fit to indices of relative abundance from a research trawl survey and (2) a catch-survey model incorporating observation and process error that produced annual estimates of absolute abundance, biomass, and fishing mortality rates from 1979 through 2002. Adult blue crab abundance estimates showed a positive trend over the period, ranging from 20 × 106 in 1979 up to 146 × 106 in 1993, with recent estimates between 70 × 106 and 97 × 106. Estimated average exploitable stock biomass over the period was 23.43 × 106 pounds (9,357 metric tons). Recruit abundance was highly variable, ranging from 34 × 106 up to 631 × 106. Use of the log survival ratio to estimate Z showed no trend in Z, although estimates were highly variable. Estimation of the exploitable stock size was problematic due to high density-dependent recruit mortality. Because of this fact, we developed upper and lower bounds of the exploitation rate, then estimated upper and lower bounds of F from Baranov catch equation, F = μ/(1 − e−Z)*Z. We also estimated the Collie & Kruse (1998) harvest rate and extended it to estimate F. The upper bound of F ranged from 0.13 up to 0.77 and averaged 0.44. The upper bound on F and the Collie-Kruse F showed a positive linear or curvilinear trend. Annual M estimates from Z - F, conditioned on an original model input value of constant M = 1.0, were erratic and showed no trend but were correlated with recruitment, supporting the hypothesis of compensatory density dependence. The relatively low estimate of F versus M and the overcompensatory and resilient stock-recruitment relationship suggest that overfishing is not occurring on this stock.

DESMOND M. KAHN and THOMAS E. HELSER "ABUNDANCE, DYNAMICS AND MORTALITY RATES OF THE DELAWARE BAY STOCK OF BLUE CRABS, CALLINECTES SAPIDUS," Journal of Shellfish Research 24(1), (1 January 2005). https://doi.org/10.2983/0730-8000(2005)24[269:ADAMRO]2.0.CO;2
Published: 1 January 2005
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