Modeling effects of increased larval mortality on bay anchovy population dynamics in the mesohaline Chesapeake Bay: evidence for compensatory reserve

We applied an individual-based population model to examine the potential compensatory scope of the bay anchovy Anchoa mitchilli in Chesapeake Bay. Model simulations were analyzed to estimate: (1) how losses of individuals in different life stages affect year-class production; (2) the changes needed in individual processes and, simultaneously in multiple processes, to offset a 50% increase in larval-stage mortality; and (3) population responses to increasing larval-stage mortality under conditions of presumed high compensatory potential. We hypothesize that, in response to lower densities, the bay anchovy population could compensate for increased larval mortality through increased growth rates, increased allocation of energy to reproduction, or reduced prédation mortality as predators target other species. Simulation results indicate that late-larval and juvenile bay anchovy may be able to consume a significant fraction of their Zooplankton prey, suggesting that anchovy is at abundances that can cause density-dependent growth in the Chesapeake Bay. However, densitydependent effects on prey resources alone had a limited buffering effect against a 50% reduction in larval-stage survival. The potential effect of losses of larvae on future production of a year class depended upon when during the larval stage individuals are removed from the population. Modeled alone, large changes in spawning intensity (no. of batches and eggs per batch), egg survival, or mortality of juveniles and adults were required to offset increased larval mortality. When all processes were varied simultaneously, much smaller changes were required. Under a high compensation scenario, there was a strong dome-shaped response in adult production potential to increased larval mortality, such that highest adult production occurred when survival rate of larvae was reduced by as much as 60%. While the information presently available to examine density-dependent population responses in bay anchovy is limiting, the modeled results indicate that the bay anchovy population in Chesapeake Bay potentially can regulate its abundance through simultaneous shifts in processes believed to be sensitive to population density.