Disentangling the Role of Environmental Effects on Fishery-Independent Survey Estimates and Stock-Recruit Relationships

We created two statistical models to address the possibility that inadvertent over- or under-sampling of mature female blue crab spawning stock biomass (SSB) by fishery-independent surveys in Pamlico Sound, North Carolina, USA during natural environmental variation can bias stock abundance estimates and mask underlying trends in population size.  The first statistical model adjusted for temporally dynamic abiotic variables that affected catchability, which provided an adjusted annual mean of fishery-independent SSB that was a more reliable index of the year-to-year variability of blue crab spawning stock biomass than unadjusted indices.  This environmentally-adjusted SSB index demonstrated a decline in blue crab SSB over the past 20 years, especially post-1999.  The second statistical model incorporated temporally static abiotic variables to model the spatial distribution of SSB.  The highest predictions of SSB consistently occurred in the same areas of Pamlico Sound over a 20-year period, suggesting the presence of essential habitat areas that could be considered as potential no-take sanctuary sites to protect the population of mature female blue crab during the spawning season.  We then assessed the role of tropical storms on the strength of stock-recruit relationships using the adjusted SSB indices.  There was a positive and highly significant (78% of variation explained) relationship between SSB and subsequent postlarval recruitment of blue crabs once we accounted for storm events that provide a mechanism for postlarval transport to the estuary.  Information gained from these approaches will help improve management efforts designed to sustain blue crab spawning stock biomass over time.  In addition, the approach taken in this study can be applied to a wide range of fishery-independent surveys whose target species distribution also varies in space and time in response to environmental variables, thereby affecting their availability to survey gear.