Why Do Most Fish Produce so Many Tiny Offspring?

A simulation model containing size-based rules for foraging, growth, and probability of survival was created to track the fates of pelagic larval fish. The relative success of cohorts comprised of equivalent initial biomass but containing different numbers and sizes of first-feeding larvae was compared in environments having different levels of patchiness and densities of food resources. In environments containing randomly distributed prey, the rate of growth and probability of survival were always greater, and duration of the larval stage was shorter for larger larvae due to the size-based rules. Broods comprised of fewer but larger first-feeding larvae resulted in the greatest number of survivors in low-prey-density environments. In prey-rich environments, broods containing the same initial biomass divided into greater numbers of small first-feeding larvae resulted in more survivors. This result occurred despite the fact that, on a per-individual basis, survival rate was much lower initially in larger broods. We used a negative binomial algorithm to generate encounter probabilities with patchily distributed prey on small spatial scales in which the previous day's experiences had no relationship to current probabilities for encounter. In prey-poor environments, the strategy of producing fewer large larvae was superior at all levels of small-scale prey patchiness. In prey-rich environments, broods containing larger numbers of smaller larvae resulted in more survivors in simulations for randomly distributed and moderately clumped prey. With greater clumping of prey, the greatest number of survivors resulted from the strategy of producing fewer but larger larvae. To examine the effect of large-scale prey patchiness, we solved for the percentage of a larval cohort that would have to settle and remain within a prey-rich patch, in order for a strategy of producing many small larvae to yield more survivors than a strategy of producing fewer larger larvae under the same conditions. When prey patches contained 200 prey/L (compared with 50 prey/L outside) and as few as 1% of the brood settled into the prey-rich patches, large broods comprised of 3-mm larvae yielded more survivors than small broods comprised of 10-mm larvae. Our simulations indicate that the superior evolutionary strategy is the investment in larger numbers of smaller eggs when resources are patchy on a relatively large spatial scale.