Altering Clam Recruitment in Shallow Waters

The rules governing clam recruitment (Appendix A.4.3) include a factor, given by $\gamma_{cD} = 1 - \exp(-D^2)$, limiting recruitment in shallower parts of the estuary. This factor enters into the overall recruitment level computed using Equation (A.24). To address the concern that clam recruitment is actually higher in shallower parts of the estuary, we set $\gamma_{cD} = 1$ independent of depth. With this change, clam biomass increased to 384 (g/m$^2$) (originally 334 g/m$^2$ Table 1) while density decreased slightly to 827 (#/m$^2$) (originally 864 #/m$^2$). Overall crab density for 7-12, 13-17 and 18+ instars increased slightly (0.1266, 0.0217, and 0.0204 #/m$^2$ respectively) relative to the original parameterization (0.1106, 0.0256, and 0.0145 #/m$^2$ respectively). The increased 7-12 crab density lowered background biomass to 127 (g/m$^2$) from the original 146 (g/m$^2$). There were no indications that crab gut fullness or egg production were impacted by these changes.

In summary, allowing greater clam recruitment on the shallower parts of the estuary increased overall crab densities and rates of recruitment. Thus, the effect of the original parameterization is that a greater proportion of clams in the estuary are being exposed to hypoxia than occurs in nature, leading to lower clam biomass, thereby increasing the possibility of food limitation, and decreasing the importance of cannibalism.