EFFECTS OF INDIVIDUAL HABITAT SELECTION IO A HETEROGENEOUS ENVIRONMENT ON FISH COHORT SURVIVORSHIP - A MODELING ANALYSIS

1. This work investigates how cohort survivorship predictions are affe cted by the rules used for moving individuals between habitats in a va riety of prey and predator environments. 2. We present an individual-b ased simulation model of the survival of a juvenile, planktivorous fis h cohort over the growing season in a spatially explicit environment. The model represents the environment as a 10 x 10 grid of cells (habit ats) that can wry in food density and predator number. 3. Juvenile fis h begin with identical characteristics, then grow, move between cells, and die based on their individual experiences. Juveniles use one of f our moving-between-cell (cell-departure) rules. random, maximize growt h, minimize mortality risk, and minimize the ratio of mortality risk t o growth. The model includes size-dependent rules for juvenile consump tion, encounters between juveniles and predators, and juvenile death. Predators have three different distributions: uncorrelated, correlated with zooplankton, and correlated with juveniles. 4. Three simulation experiments were conducted to address how cohort survivorship is affec ted by the environment's spatial heterogeneity, the cell-departure rul e of juveniles, and the initial cohort number (Experiment 1); which ce ll-departure rule individual juveniles should use (Experiment 2); and how survivorship predictions differ between this explicit, spatially h eterogeneous model and a similar, spatially homogeneous model (Experim ent 3). 5. Experiment 1 showed that predator distribution, juvenile nu mber, zooplankton density and cell-departure rule had important effect s on cohort survivorship. Experiment 2 showed that no single cell-depa rture rule was consistently the evolutionarily stable strategy (ESS), and that survivorship of cohorts using the ESS cell-departure rule(s) was lower than that of cohorts using the eel-departure rule with the h ighest single-year survivorship. Experiment 3 showed that density effe cts on juvenile survivorship can be much greater in a spatially explic it model, with individuals using fitness-based cell-departure rules th an in an analogous, spatially homogeneous model. 6. The results of thi s work indicate that the cell-departure rule used by individuals can h ave an important effect on cohort survivorship. In addition, none of t he state- and time-independent cell-departure rules investigated was a n ESS, suggesting that such static rules may not be an appropriate mec hanism for modelling individual habitat selection in a dynamic environ ment.