MODELING DETERMINISTIC EFFECTS OF AGE STRUCTURE, DENSITY-DEPENDENCE, ENVIRONMENTAL FORCING, AND FISHING ON THE POPULATION-DYNAMICS OF SARDINOPS-SAGAX CAERULEUS IN THE GULF-OF-CALIFORNIA

We used an age-structured deterministic model to investigate how popul ation dynamics of Pacific sardine, Sardinops sagax caeruleus, in the G ulf of California are affected by age structure, density-dependent rec ruitment, environmental forcing, and fishing. Density-dependent recrui tment had a very strong effect; it stabilized the population dynamics and caused mesoscale (4-5 year) cycles in abundance. A sinusoidal func tion was used to vary survival rates with constant period of 60 years and various amplitudes. This representation of environmental forcing c aused long-term cycles in the abundance of sardine, similar to the pat tern observed in real stocks. A linearly increasing fishing schedule o ver a period of 25 years on a stock with environmental forcing caused strong reductions in abundance that extended for up to 20 years, and m esoscale oscillations for up to 40 years after the harvest period. The negative impact was longer if harvest started when survival rates wer e at their maxima, but biomass decreased most when harvest started at the descending node of survival rates. A stability analysis indicated that the sardine stock in the Gulf of California is unstable; however, when age structure is included in the simulations the stock is very r esilient and can recover from low levels of biomass. This theoretical result was probably due partly to the absence of stochastic effects in our model.