Changes in the reactive distance of bluegill (Lepomis macrochirus) to various sizes of Daphnia pulex were measured at light intensities ranging from 0.70 to 215.3 lx (0.065–20.0 ft-c) and at turbidities ranging from 1 to 30 Jackson Turbidity Units (JTU). Both reduced illumination and increased turbidity caused substantial reduction in the reactive distance of bluegill for all prey sizes, and particularly for large prey. This result should be considered in efforts to determine fish feeding rates in lakes, and may be particularly relevant to vertically migrating zooplankton, or those inhabiting more turbid waters.

Abstract.Although it is known that visual predation by planktivorous fish tends to be size selective, the mechanism by which fish select their prey has not previously been described.Experiments in which bluegill sunfish (Lepomis macrochirus) were given a binary choice be tween prey of different sizes presented at different distances showed the fish selected the prey that appeared largest, either because of its actual size or its proximity to the fish. This paper incorporates this mechanism of prey selection by apparent size into a model of bluegill predation. According to the model, bluegill, in choosing the apparently largest prey under all conditions, alter their diet composition depending upon the abundance of prey. When prey are abundant, bluegill predominantly select prey of the largest size class available because these have the greatest probability of appearing largest; as large prey become scarce and smaller prey have a greater chance of appearing large, the fish tend to eat more prey from smaller size classes. When the model is tested against data from published fish-feeding ex periments, the predicted size ratios of prey eaten correlate accurately with the observed ratios and numbers of prey eaten.

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Translocation is a tool commonly used for the conservation of threatened and endangered fish species. Despite extensive use, the biological implications of translocation remain poorly understood. Of particular interest is the effect of translocation on genetic variability. Maintenance of genetic variability in these “refuge” populations is assumed to be important for both short‐ and long‐term success. We examined allozyme variability at 16 loci for western mosquitofish (Gambusia affinis) populations with known histories of introduction. Refuge populations had significantly lower levels of heterozygosity. Refuge populations also had considerably lower levels of allelic diversity than parental populations. All losses were of relatively rare alleles (frequency less than 0.1 in parental population). These losses were probably due to an undocumented bottleneck early in the introduction history. These results were surprising because the initial transplant involved 900 fish and because mosquitofish have numerous reproductive traits that should minimize the effects of bottlenecks on genetic diversity. A literature review revealed that genetic variability is often reduced in refuge populations and that such reductions typically involve the loss of alleles. We suggest that translocated populations be examined periodically for losses of genetic variability.

Sunfish will select Daphnia preferentially over copepods even though the apparent size of the copepods may be considerably larger. This selectivity is probably the result of the fishes having learned differences in the evasion capabilities of the two different prey types, and having translated this into a differential selectivity. Sunfish have the ability to correlate information about the evasion capabilities of prey with visual appearances of the prey, and to select less evasive prey. Such information may have considerable significance in defining the observed selectivity of these fish in nature, and such behavioral responses should be considered in future modeling efforts.Key words: predation, Lepomis, selectivity, behavior