Selective Predation, Optimal Foraging, and the Predator-Prey Interaction Between Fish and Crayfish

Abstract

Through laboratory experimentation, I quantified the behavioral intraction between an inshore fish predator, the smallmouth bass (Micropterus dolomieui) and an active, defensive prey organism, the crayfish (Orconectes propinquus). Experiments, run in aquaria or large wading pools, involved pacing different sizes and life stages of crayfish with smallmouth bass to document selective predation. To determine whether bass, in preying selectively on crayfish maximized net energy intake, a predictive foraging model, in which handling and pursuit times are balanced against prey digestible organic ([Ht + Pt]/O), was developed and tested. Fieldwork, designed to yield electivity indices such that laboratory findings could be confirmed or denied, involved simultaneously sampling inshore fish predators and crayfish from 3 lakes in northern Wisconsin. In laboratory experiments, when offered a choice of crayfish sizes on sand, smallmouth bass (25 cm, total length) chose the smalles crayfish (4 mm, carapace length) first, and then conumsed animals in acending order of size. When offered the same choices on pebble (16—32 mm), intermediate—sized (16—20 mm) crayfish were eaten first. On large substrates, small size classes are relatively less exposed than large size classes. Therefore, the increased search (i.e., waiting) time to obtain these small size classes appears to decrease their value relative to more available intermediate size classes. Both predicted size from (Ht + Pt)/O and selection sequence from the pebble substrate agreed quite closely with field electivity indices, suggesting that bass are optimal foragers. Within any size class of adults, life stages ordered from low to high susceptibility to predation by smallmouth bass were: ovigerous ♀ ♀, Form I (FI, capable of breeding) ♂ ♂, ♀ ♀ ≈ Form II (FII, incapable of breeding) ♂ ♂, and recent molts. Determined empirically via laboratory experimentation, this order was validated by electivity indices from the field and predicted by the (Ht + Pt)/O optimal forging construct. As a result of their differential susceptibility to predators, some sizes and life stages (juveniles, ♀ ♀, and recent molts) appeared to modify thei field microdistribution to minimize risk to predators. In general, crayfish densities on sand were inversely related to relative densities of inshore fishes; degree of behavioral response of individual life stages appeared correlated with vulnerability. Fish predators not only prey selectively on crayfish but also cause shifts in their microdistribution and behavior. In fish—crayfish interactions, sizes and life stages with the greatest vulnerability due to morphological and physiological traits also possessed the most significant behavioral attributes for reducing risk. In general, selection of particular life stages from crayfish populations makes ecological sense; fish—crayfish interactions as predator—prey systems persist. Their persistence results from the subtle interaction between predator and prey, ultimately producing a stable system in which “important” life stages (ovigerous ♀ ♀ or FI ♂ ♂) are more or less exempt from predatory mortality.