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.
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Vol. 24 • No. 1