An individual-based model of bay anchovy population dynamics in the mesohaline region of Chesapeake Bay

Bay anchovy Anchoa mitichilli population dynamics in the mesohaline region of Chesapeake Bay are described and analyzed with an individual-based model. The model begins with spawning by individual females and simulates the daily growth and mortality of each female's progeny as they develop through the egg, yolk-sac larva, (feeding) larval, juvenile, and adult stages in a single, well-mixed compartment. The model runs for 50 yr. Eggs and yolk-sac larvae develop at rates dependent on temperature, and die at fixed daily rates. All feeding individuals (larvae, juveniles, and adults) consume zooplankton and grow according to defined bioenergetics relationships. Encounters, attacks and captures of prey, and the probability that feeding individuals will die are treated stochastically using Monte Carlo techniques. Net immigration of spawners into the modelled box each year is simulated in 2 ways: multiplier of survivors and as a constant number. Model predictions of stage-specific growth and survival rates, and diets were similar for multiplier and constant immigration simulations, and both were similar to observed values. Density-dependent growth of larvae and juveniles led to a negative relationship between mean length and number of recruits and to density-dependent survival of larvae and juveniles. Density-dependent growth was due to anchovy consumption reducing prey densities, and not due to temperature effects on numbers surviving and growth. First-year survivorship was lower in years of high egg production than in years of low egg production. Larval growth and survival rates were positively related to recruitment, but the reverse was true for juveniles. Years of high recruitment were not years of high first-feeder production, but rather were years of high larval survival. The model simulates a single spatial box with a forced net immigration of spawners and assumes density-independent spawning and mortality processes. The model quantifies the basic processes leading to recruitment of bay anchovy and sets the stage to explore potential compensatory responses of anchovy.