Prey patchiness and larval fish growth and survival: inferences from an individual-based model

We used an individual-based simulation model to evaluate how prey patchiness and fish swimming behavior affect larval fish survival and mortality source (predation or starvation). Simulations revealed that cohort survival increased linearly with greater average patch residence times and that patch residence times for individual fish with different fates (survived, starved, eaten) diverged substantially during the first few days of feeding. Further, by examining the interaction of patch spatial distribution (uniform random versus clumped) with three possible swimming behaviors, we found that swimming behavior, via its effect on prey encounter and feeding rates, affected both cohort survival rates and whether fish died from predation or starvation, but that the spatial distribution of patches (fine-scale (100s m)) only influenced whether fish died from predation or starvation. Within a particular patch spatial arrangement, however, patch intensity (division of food between patches and non-patches) had a major effect on survival. Except at high food levels, fish did not survive when there were no patches, suggesting that average, well-mixed prey densities will not support sufficiently rapid growth for survival. As patch intensity increased, survival increased to a maximum and then declined, with peak survival occurring at higher patch intensity as average food concentration declined. Finally, the degree of patchiness also determined the intensity of selection on growth rates. In patchier environments, there was stronger selection for fast growth rates leading to eight-fold differences in average cohort growth rates after only three days of growth. In general, survival was not directly related to fast cohort growth the best survival occurred with high average prey densities and weak patchiness. Prey patchiness, by influencing the average as well as the variance in individual growth rates, can have a substantial impact on survival rates of larval fish cohorts.