Data from the literature suggest that predatory success declines as habitat complexity increases. To explain this phenomenon, we studied the predator-prey interaction between largemouth bass Micropterus salmoides and bluegills Lepomis macrochirus in four laboratory pools (2.4-3.0 m diameter, 0.7 m deep), each with a different density (0, 50, 250, 1,000 stems/m e) of artificial plant stems. Behavior was quantified for both predator and prey during largemouth bass feeding bouts lasting 60 minutes. Predation success (number of captures) by largemouth bass was similar at 0 and 50 stems/m 2, then declined to near zero at 250 and 1,000 stems/m 2. As stem density increased, predator activity declined due to a decrease in behaviors associated with visual contact with prey. Reduced predation success by largemouth bass in habitats of increased complexity apparently is related to increases in visual barriers provided by plant stems as well as to adaptive changes in bluegill behavior.

In laboratory aquaria, smallmouth bass (Micropterus dolomieui) influenced activity, substrate selection, behavior, and food consumption of crayfish (Orconectes propinquus) of different sizes and sexes. In the presence of a predator, crayfish selected substrates affording most protection, significantly modified the frequency of various behavior patterns, and reduced their overall activity. Active behavior patterns such as walking—climbing and feeding were suppressed while defensive patterns, such as burrowing and chelae display increased. Grazing by crayfish on their detrital food supply was also suppressed by bass. Degree of response of crayfish appeared correlated with liability to predation, i.e., small, vulnerable crayfish were most affected, whereas large, less vulnerable crayfish were least affected. Because crayfish ♂ ♂ have larger chelae than ♀ ♀, they apparently were better able to defend themselves against predators; consequently, less vulnerable ♂ ♂ did not modify their behavior as dramatically as ♀ ♀. In field collections crayfish exposed on the substrate were larger than those buried; fewer ♀ ♀ than ♂ ♂ were exposed. Thus, field observations correlated well with laboratory findings, suggesting that predators affect distribution and behavior of crayfish in natural communities. By affecting several trophic levels within aquatic systems, influence of predators goes beyond simple interactions of predator and prey.

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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 “imp...

The detritivorous fish, gizzard shad (Dorosoma cepedianum), provides nutrients to phytoplankton in reservoirs by ingesting organic detritus associated with sediments and excreting substantial quantities of nutrients such as N and P in soluble forms that are highly available to phytoplankton, We estimated nutrient excretion by gizzard shad in a eutrophic reservoir (Acton Lake, Ohio) during April-October 1994 by measuring N and P excretion of fieldcaught fish (n = 135). Excretion rates were then extrapolated to nutrient release by the gizzard shad population using quadrat rotenone biomass estimates, electrofishing surveys, and historic seasonal trends. N and P excretion were positively correlated with fish wet mass and temperature, but mass-specific excretion declined with increasing fish mass. Lakewidc gizzard shad biomass in July 1994 was 417 kg ha -I, Our estimates of nutrient excretion by the gizzard shad population ranged from 0.487 to 0.769 pmol NH,-N liter-' d-l and 0.022 to 0.057 pmol soluble reactive phosphorus liter -I d .I, with the highest excretion occurring during mid-summer through early fall. The low N : P ratio at which gizzard shad excrete [mean molar N : P = 16.75 (kO.89 SE)] may alter phytoplankton community composition, favoring cyanobacteria. Our results indicate that nutrient excretion by detritivorous fish can be an important source of nutrients to open waters, especially when other sources of nutrients are reduced.