High concentrations of blue crab (Callinectes sapidus) larvae along the offshore edge of a coastal current: effects of convergent circulation

Using a combination of observations and numerical modeling, we examined the distribution of blue crab (Callinectes sapidus) larvae along the edge of the buoyant plume emanating from the mouth of Delaware Bay along the east coast of the USA. Our observations consisted of larval sampling and hydrographic measurements taken along a series of transects that spanned the plume, the plume edge, and the coastal ocean. The numerical model is a realistic circulation model of Delaware Bay and the adjoining coastal ocean that includes daily wind forcing, daily river discharge, and tides. We tested the hypothesis that the offshore edges of coastal currents are regions of convergence and retention for larvae of estuary dependent species. Collected data showed a marked difference between the distribution of early and late stage larvae. Patches of early stage larvae occurred within lower salinities typical of the estuarine plume and higher salinities associated with the offshore edge of the plume. Late-stage larvae occurred almost entirely in salinities characteristic of the offshore edge. The field study was followed by a modeling component that simulated larval distributions over the spawning season of 2005. Output from the model showed simulated larval distributions that were similar to the observations. This study provides new insight into the distribution of larvae and larval patches in Delaware Bay and any region with extensive buoyancy and tidally driven flow. Results indicate that larvae tend to congregate along the edges of plumes emanating from estuaries due solely to physical mechanisms, regardless of the initial spawning location.