Clams - Growth, Spawning, Density and Biomass

The rate at which clams grow is dependent on the environmental variables the clam experiences (Appendix A.4.1). In the model, clams spawn at fixed times provided they are larger than a minimum size threshold. Thus, overall clam recruitment is indirectly dependent on the environmental variables. The particular criteria used included the daily rate of clam growth; time to reach sexual maturity; average clam density in the estuary; average clam biomass and the spatial variation in clam biomass.

Simulations indicated that 20 years was an adequate burn-in time for clams and background prey (Fig. B4). Distinct annual cycles were present even without predation. Clam growth rates agreed with those reported by Nichols and Thompson (1982) and are generally $<$ 0.002 (g/day) over the first 2 years following recruitment (Fig. B5) and showed no difference with depth. Further, model clams lost mass during winter, agreeing with  Honkoop and Beukema (1997). Clams spawned at the beginning of May reached sexual maturity (a size $\geq 1$ cm) anywhere from November to the following March while clams spawned at the beginning of September reached maturity between February and March of the next year. This is in agreement with studies done in Chesapeake Bay (Holland et al. 1987, Fig 7).

There was no difference in the model's spring and fall recruitment densities. For the model, average clam density without predation was 1000 clams/m$^2$. Given the differences in the mesh-sizes of screens used to filter sediments for clams, comparing this density to empirical findings is difficult. Under predation,  Seitz et al. (2003b) reported densities up to 500 per m$^2$ using a 1 mm mesh, while in upper Chesapeake bay, recruitment densities of $> 0.5$ mm clams at particular sampling times were as high as 30,000 per m$^2$ in the absence of predation and 6500 per m$^2$ with predation (Holland et al. 1980).

In the model average clam biomass was $\approx$ 750 (g/m$^2$) and average background biomass was 240 (g/m$^2$). In Chesapeake Bay under predation, total biomass in mud and sand habitats was 119 and 177 (g dry/m$^2$) or $\approx$ 720 and 1100 (g wet/m$^2$) with clams accounting for at least 76% of the total biomass (Hines and Comtois 1985) (i.e., clams $\approx$ 547 or 840 (g wet/m$^2$) and background biomass $\approx$ 173 and 254 (g wet/m$^2$)). The highest clam biomass density in the model occurred at depths between 1 and 4 m, with clam biomass decreasing rapidly for depths less than 1 m or greater than 4 m. The lower biomass in the deeper parts of the estuary is caused by clam die-off due to hypoxia and not limitations in recruitment since maximum clam density in the model occurred in the deepest parts of the estuary. The movies indicate that clam and background biomass decreased towards the mouth of the estuary where hypoxia was generally more severe.