Critical Habitats and Stock Assessment: Age‐Specific Bias in the Chesapeake Bay Blue Crab Population Survey

Abstract

AbstractPopulation surveys used in stock assessments often ignore habitats that are difficult to sample under the assumption that these habitats harbor an inconsequential fraction of the population. We tested this assumption for the Chesapeake Bay population of blue crab Callinectes sapidus, which numbers in the hundreds of millions, supports one of the world's most productive crustacean fisheries, and is managed based on a rigorous stock assessment. The stock assessments rely on a baywide winter dredge survey (WDS), which produces an absolute estimate of abundance for age‐1 and older crabs. Although the WDS does not sample waters shallower than 1.5 m deep, where some age‐0 crabs overwinter, it has been assumed that abundance in these shallow habitats is not substantial. We sampled vegetated and nonvegetated shallow‐water habitats not sampled by the WDS at four locations in lower Chesapeake Bay in February‐April 2011. Age‐0 density in shallow vegetated habitats was about two orders of magnitude higher than in deeper vegetated habitats sampled by the WDS; density in shallow nonvegetated habitats was about 40 times that of deep nonvegetated habitats. Even after excluding juvenile crabs that would not be retained by WDS gear, juvenile density was still 50 times higher in shallow vegetated and 32 times higher in nonvegetated habitats than in deep nonvegetated habitats. Extrapolating these densities baywide, we estimated approximately 850‐1,500 million age‐0 crabs in Chesapeake Bay, in contrast to the 2011 WDS estimate of about 205 million age‐0 crabs. Thus, the WDS only sampled 14‐24% of age‐0 crabs. If the fraction of age‐0 crabs sampled by the WDS differs interannually, then empirical estimates of abundance and exploitation rate will be biased and potentially lead to risk‐prone management actions. Our findings provide an empirical example whereby inattention to habitat utilization as a function of age or ontogeny may introduce significant error into fishery management.Received May 17, 2013; accepted February 18, 2014