Size-dependent vulnerability of marine fish larvae to predation: an individual-based numerical experiment

Abstract

Cowan, J. H., Jr, Houde, E. D., and Rose, K. A. 1996. Size-dependent vulnerability of marine fish larvae to predation: an individual-based numerical experiment. ‐ ICES J. mar. Sci., 53: 23‐37. Twenty-day simulations of responses by a larval fish cohort were investigated with an individual-based model of predation by ctenophore, medusa, and planktivorous fish predators. Results indicate that the relationship between larval size and vulnerability to predation was generally dome-shaped for invertebrate predators and could be dome-shaped for fish predators if they foraged optimally by size, and depended upon attributes of both predators and larval fish prey. For the predators that did not forage optimally, cohort-specific mortality generally decreased as the mean length of larvae in a cohort increased, but bigger or faster-growing larvae within a cohort were not always most likely to survive. Until larvae grew through a ‘‘window’’ of vulnerability and reached a threshold length when susceptibility to the predators decreased more rapidly with larval length than encounter rate increased, mean length or growth rate of surviving larvae on each day was slightly lower, or not diVerent from those that died in most of the simulations. After the threshold length was reached, predators began to catch smaller larvae, which resulted in larger survivors. The time necessary to grow through the window and reach the threshold length depended on growth rate of the larvae, size of the predators, and the variance structure of these parameters. These results indicate that size and growth rate of fish larvae are partially decoupled by the predation process and, ultimately, act diVerentially to determine cohort survival rate, although both may be most important after larvae have reached the threshold length. In these simulations, the threshold length was reached after a significant portion (56‐99%) of total larval mortality had occurred; time to reach the threshold was generally shorter for the faster growing cohorts. Initially, both fast- and slow-growing individuals within a larval cohort diVered little in size and, therefore, were nearly equally vulnerable to predation. However, reduced risk of predation occurred when all members of a cohort had reached the threshold length, which suggests that mean growth rate of individuals within a cohort, not their size, is probably the more important parameter aVecting cumulative mortality, especially when the rate is high. We propose that characteristics of larval survivors may be more influenced by attributes of the predators to which they were exposed in early life, rather than by their initial status within a cohort with respect to length at hatching and potential growth rate. ? 1996 International Council for the Exploration of the Sea