Effects of Individual Habitat Selection in a Heterogeneous Environment on Fish Cohort Survivorship: A Modelling Analysis

Abstract

1. This work investigates how cohort survivorship predictions are affected by the rules used for moving individuals between habitats in a variety of prey and predator environments. 2. We present an individual-based simulation model of the survival of a juvenile, planktivorous fish cohort over the growing season in a spatially explicit environment. The model represents the environment as a 10 x 10 grid of cells (habitats) that can vary in food density and predator number. 3. Juvenile fish begin with identical characteristics, then grow, move between cells, and die based on their individual experiences. Juveniles use one of four moving-between-cell (cell-departure) rules: random, maximize growth, minimize mortality risk, and minimize the ratio of mortality risk to growth. The model includes size-dependent rules for juvenile consumption, 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 affected by the environment's spatial heterogeneity, the cell-departure rule of juveniles, and the initial cohort number (Experiment 1); which cell-departure rule individual juveniles should use (Experiment 2); and how survivorship predictions differ between this explicit, spatially heterogeneous model and a similar, spatially homogeneous model (Experiment 3). 5. Experiment 1 showed that predator distribution, juvenile number, zooplankton density and cell-departure rule had important effects on cohort survivorship. Experiment 2 showed that no single cell-departure 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 cell-departure rule with the highest single-year survivorship. Experiment 3 showed that density effects on juvenile survivorship can be much greater in a spatially explicit model, with individuals using fitness-based cell-departure rules than in an analogous, spatially homogeneous model. 6. The results of this work indicate that the cell-departure rule used by individuals can have an important effect on cohort survivorship. In addition, none of the state-and time-independent cell-departure rules investigated was an ESS, suggesting that such static rules may not be an appropriate mechanism for modelling individual habitat selection in a dynamic environment.