Foraging Theory and Piscivorous Fish: Are Forage Fish Just Big Zooplankton?

Insights into the dynamics of fish growth can be gained by using foraging theory to link bioenergetics models of fish and their prey. These links are critical for modeling fish daily ration and growth, prey mortality, selection among prey, and competition among predators. However, the foraging theory that is relatively well developed for planktivores does not always apply to piscivores without important modifications. Visual encounter is often limited by visual acuity for most planktivores, but probably limited by prey contrast for piscivores, so that piscivore reactive distance is much less dependent on prey size. Whereas handling time per prey may limit the capture rate for some planktivores, it is irrelevant for most piscivores, which eat relatively small numbers of large prey and are more likely to have daily ration limited by rates of digestion or prey encounter. Time for gastric evacuation or digestion should not be a part of handling time, because search can occur simultaneously with digestion. If handling time is not important for piscivores, then Hollingˈs type I functional response may be more appropriate than his type 2 response. An alternative form of the functional response is presented for predators that feed on prey of uniform size and stop foraging each day when some maximum number of prey are ingested. This functional response rises to an asymptote like a type 2 response, but it is based on a very different mechanism. Simulations with a bioenergetics model show that the variance in daily growth among individuals is likely to be greater for piscivores, which feed on a small number of large prey, than for planktivores, which feed on a large number of small prey.