Geostatistical analysis of the abundance and winter distribution patterns of the blue crab Callinectes sapidus in Chesapeake Bay

Spatial heterogeneity is a striking life history feature of the blue crab Callinectes sapidus in the Chesapeake Bay. Spatial patterns of sex- and age-specific habitat use have been well documented and affect the fisheries in the bay. However, a quantitative assessment of the spatial distribution of blue crabs during winter, when they are generally buried in the sediments, has been lacking. We applied geostatistical techniques (variogram modeling and kriging) to 13 years of winter dredge survey (WDS) data to map patterns of blue crab winter abundance. These maps were then quantified to derive a time series of baywide abundance and to examine changing patterns of habitat use and aggregation. Geostatistical abundance estimates were generally similar to those calculated from design-based methods but were more highly correlated with fishery catch per unit of effort. Both abundance time series showed a large and significant decline from 1990 to 2002. Changes in spatial distribution were evaluated using trend maps and a density-weighted centroid. Interannual variation in the latitude of the weighted centroid was positively correlated with baywide abundance, suggesting possible density-dependent changes in distribution. The southward shift in winter distribution at low stock size may increase the vulnerability of blue crabs to exploitation in a winter dredge fishery that occurs only in the southern portion of the bay. Such quantitative and spatially explicit information provides a potentially useful base for constructing population models and evaluating alternative management options.