PREY PATCHINESS AND LARVAL FISH GROWTH AND SURVIVAL - INFERENCES FROMAN INDIVIDUAL-BASED MODEL

We used an individual-based simulation model to evaluate how prey patc hiness and fish swimming behavior affect larval fish survival and mort ality source (predation or starvation). Simulations revealed that coho rt survival increased linearly with greater average patch residence ti mes and that patch residence times for individual fish with different fates (survived, starved, eaten) diverged substantially during the fir st few days of feeding. Further, by examining the interaction of patch spatial distribution (uniform random versus clumped) with three possi ble swimming behaviors, we found that swimming behavior, via its effec t on prey encounter and feeding rates, affected both cohort survival r ates 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 pa tch spatial arrangement, however, patch intensity (division of food be tween 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 su fficiently rapid growth for survival. As patch intensity increased, su rvival increased to a maximum and then declined, with peak survival oc curring at higher patch intensity as average food concentration declin ed. Finally, the degree of patchiness also determined the intensity of selection on growth rates. In patchier environments, there was strong er selection for fast growth rates leading to eight-fold differences i n average cohort growth rates after only three days of growth. In gene ral, 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. (C) 1997 Elsevier Science B.V.