Simulated Influences of Hatching-Date Dependent Survival on Year Class Composition and Abundance

Juvenile fish survival is often strongly density dependent and results in relatively consistent average recruitment over a wide range of spawning stock biomasses. Hatching date-dependent mortality can limit contributions of individual hatching sub-cohorts (i.e., fish hatched in one period relative to another within the overall hatching distribution) to the year class and influence the potential for compensatory survival. We used trophic-based ecosystem models to evaluate effects of hatching datedependent survival on the potential for compensation and regulation of year class strength and adult biomass by simulating variable early-life mortality. We built one model using data for north Florida lakes to represent a contracted spawning distribution and one model for south Florida lakes to represent a protracted spawning distribution using data for largemouth bass Micropterus salmoides with hatching-date dependent survival. Hatching datedependent survival strongly influenced contributions of individual hatching sub-cohorts to year classes (up to 70+% change in sub-cohort biomass), but total effects on year class strength and adult biomass were small (range -9% to +13% total biomass change). Total survival to age-1 was largely regulated by predation such that increases in individual subcohort survival did not result in large increases in total recruitment. Ecosystem models indicated that spawning distributions affected sub-cohort interactions to influence compensation and regulation, which had implications for understanding adult spawning periodicity and fisheries management.