Density and Distribution

The density of crabs in the Rhode River, a sub-estuary of Chesapeake Bay, ranged between $\approx$ 0.01 and 0.1 crabs per m$^2$ during 1991 to 1993, but has been as high as 0.5 crabs per m$^2$ on other years (Clark et al. 1999). The vast majority of such crabs are juveniles (Orth and van Montfrans 1987). In the model, the maximum density (#/m$^2$) of 7 to 12 instar crabs varied between $\approx 0.3$ to 0.25 and decreased to $\approx 0$ shortly before 7th instar crabs were instantiated into the estuary (Fig. B13). The average density of 13 to 17 instar crabs in the model was $\approx 0.027$ while 18+ density was $\approx 0.017$ (#/m$^2$). Recent studies also suggest that areas of low salinity and unstructured habitats can contain high densities of juvenile crabs (Lipcius et al. 2005; Posey et al. 2005).

Crab density and distribution in the model emerges primarily based on crab cannibalism and avoidance of hypoxia. The distribution of different instar crabs across the estuary (Fig. B14) showed that the movement algorithm (Appendix A.5.4) did result in a higher density of mature female crabs towards the mouth of the estuary (Appendix A.5.4). Small crabs were located in shallower waters on the edge of the estuary while large crabs were located at intermediate depths during summer and moved back into the deeper parts of the estuary during fall.