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Field data and laboratory feeding experiments support the hypothesis that predation can be an important factor in the adaptive significance of vertical migration among zooplankton. In both Gatun Lake in Panama and Fuller Pond in Connecticut the diel vertical migration patterns of prey populations assume distributions which result in lessened predation by the dominant lake planktivores. It is concluded that such patterns of vertical migration will result when prey populations are under intense, selective pressures from visually dependent predators.

We examine the effects of fish predation on a population of Bosmina longirostris (O. F. Muller), a smaller, limnetic cladoceran, in Gatun Lake, Panama. We test the relative importance of two different modes of predator selection: body—size selection, in which fish choose the largest of otherwise identically appearing individuals, and visibility selection, in which fish choose individuals having the greatest amount of body pigmentation. In B. longirostris, this pigmentation is most heavily concentrated in the large, black, compound eye of these otherwise transparent animals. We find that these predators are feeding according to visibility selection and that body—size selection is of negligible importance under these circumstances. We then discuss the general applicability of these findings, and the evolutionary consequences for prey populations under this type of selective pressure.

Bias refers to the accuracy of a particular estimator. We evaluate bias, using analytic and simulation technics, for six measures of overlap: the likelihood ratio measure, the chi—square measure, the measure based on the Freeman—Tukey statistic, Morisita's adjusted index, Morista's original index, and Horn's information index. We present an exact formula for a seventh, the percentage similarity measure. We consider bias due to resource a sample size, total number of different resources, and evenness of resource distribution. Results indicate that of the seven measures, changes in evenness of resource of distribution produce significant bias only in the percentage similarity measure and Morista's adjusted index. All measures show increasing bias with increasing number of resources. For estimating unbiased overlap, Morisita's original measure of overlap gives the most accurate results, especially when using small sample sizes. The percentage similarity measure, one of the most commonly used measures among ecologists, is also one of the most biased and for this reason is not preferred.