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

Abstract

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.

Ocean observatory data are useful for regional habitat modeling of species with different vertical habitat preferences

“…We speculate that coastal ocean areas impacted by estuarine plumes where upwelling occurs and productivity is high could serve as high quality spawning grounds that place eggs in close proximity to optimal feeding habitats for larvae which are at a lower trophic level of ~3 (Grimes & Kingsford 1996). These same areas also have physical transport mechanisms likely to deliver larvae south and west to important estuarine nurseries (Epifanio & Garvine 2001, Lentz 2008, Tilburg et al 2009, Zhang et al 2009, Gong et al 2010. Spawning habitat selection and suitability should be largely defined by conditions promoting the development, survival and successful transport of early life stages to juvenile nurseries rather than by the immediate requirements of adults.…”

An evaluation of the synchronization in the dynamics of blue crab (Callinectes sapidus) populations in the western Atlantic

“…Coastal Ocean Estuary or transported southward in the coastal current (Tilburg et al 2009;Epifanio et al 1989;Little & Epifanio 1991;Garvine et al 1997). Collectively, these studies indicate that there is likely not much import of blue crab larvae from other estuaries into Delaware Bay because of the oceanographic structure of the region.…”

“…The GSI was strongly and negatively correlated with North Carolina and Florida (Table 3. Delaware Bay. This work has centered on blue crab larval retention with the general conclusion that larvae released from Delaware Bay are either retained or transported southward and there is unlikely any transfer of larvae that originated in the southern estuaries into Delaware Bay (Epifanio et al 1989;Garvine et al 1997;Tilburg et al 2009). Delaware Bay had the worst model fit in all abundance DFAs which could potentially be a result of not receiving larvae from other estuaries and therefore not following the trends of other estuaries as closely.…”

“…The studies from Delaware Bay indicate that there are not many foreign blue crabs migrating into Delaware Bay, but the possibility of larvae from Delaware Bay being transported into other estuaries is viable (Garvine et al 1997;Tilburg et al 2009). Beginning to look at the mixing potential of blue crab larvae from other estuaries would be interesting and may help us to better understand their population dynamics.…”

The ontogeny of larval swimming behavior in the crab Hemigrapsus sanguineus: Implications for larval transport

“…In addition to confirm sinking rates and geotaxis results of earlier work (Park et al, 2004), we tested for endogenous activity rhythms in newly hatched zoeae, measured visual spectral sensitivity electrophysiologically at an intermediate larval stage, and characterized phototactic responses throughout larval development. Consistent with the export-and-return model of larval transport, we hypothesized that early-stage larvae would have endogenous tidal activity rhythms and behavioral responses to gravity and light that would result in a consistent shallow distribution and export from Delaware Bay, whereas later stages would exhibit behavioral responses to these environmental factors that generate vertical migrations and deeper distributions in the coastal ocean consistent with predator avoidance and a return to coastal habitats at the end of larval development (e.g., Epifanio and Tilburg, 2008;Tilburg et al, 2009). …”