1. The mid-summer declines of Daphnia species in three small lakes were investigated to examine the relative roles of reduced natality and increased mortality. Reduced natality (assessed by quantifying clutch size, lipld index, and available food) could not account for the decline in daphnid abundance in any of the populations examined.2. The role of increased mortality imposed by zooplanktivorous fish was assessed by estimating the sizes of the mid-water refuge areas where daphnids could escape fish predation. The boundaries of the refuge areas were estimated from field measurements and literature values and were based on gradients of temperature, dissolved oxygen and light. Observed decreases in refuge thicknesses correlated well with the mid-summer declines of large-bodied Daphnia species in all three lakes. Intermediatesized daphnids were less affected as the refuges thinned and small-bodied species increased in abundance.3. The importance of refuge thickness in modifying zooplanktivoreinduced mortality was further tested in large enclosures where refuge thickness was experimentally modified. In the presence of zooplanktivorous fish, large-bodied Daphnia, which used the refuge, persisted when the refuge was thick but disappeared when it thinned. Daphnia galeuta mendotae, which did not occupy the refuge zone, was rapidly eliminated regardless of refuge thickness. We conclude that refuge availability plays a major role in Daphnia population dynamics.

A computer model was developed to predict the feeding selectivity of planktivorous white crappie (Pomoxis annularis) from a known distribution of zooplankton. The model was based on the assumption that each predation event could be subdivided into a series of independent steps: prey location, pursuit, attack, and retention. The probability that white crappie successfully completed each step was determined for potential zooplankton prey species in a series of laboratory experiments. The four steps were then incorporated into a stochastic model where the probability of a particular prey type being consumed is equal to the product of the probabilities of the individual steps. The model was field tested by sampling fish, zooplankton, and physical parameters from discrete depth strata in a small reservoir on nine dates from October 1978 through November 1979. The model proved to be very accurate at predicting the species and size distribution of the ingested prey across the range of light intensities, turbidities, temperatures, and zooplankton densities encountered. Prey consumption could not be characterized as simply size selective; rather, it reflected the selectivity expressed at each step in the feeding cycle.

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The probability that prey will be retained by gill rakers of white crappies Pomoxis annularis (9–15 cm total length) was determined by comparisons of size‐frequency distributions of prey in a laboratory pool and in stomach contents. White crappies trained to feed on large‐bodied Daphnia magna were released individually into a pool containing that species and an assemblage of small‐bodied species. Most attacks were directed at Daphnia magna, but several prey were ingested for every attack, and nonselective capture was assumed for the small‐bodied among them. Estimated retention probabilities for species of Ceriodaphnia, Bosmina, and Cyclops were less than 10% for mean prey sizes smaller than 0.35 mm and increased linearly with mean prey size to 100% for prey larger than 0.55 mm. In contrast, retention probabilities calculated by traditional means from distances between gill rakers predicted that all prey larger than 0.23 mm would be retained. The new retention estimates, based on actual results of the retention process, predict that many small‐bodied zooplankton present in lakes are immune from predation by white crappies because they cannot be retained by the gill rakers. Received April 1, 1982 Accepted May 25, 1983

White crappie (Pomoxis annularis) are a valuable sport fish, but they have received little ecological study. They are piscivorous when large (greater than 16‐17 cm TL) but are almost completely planktivorous when small. We studied the location and periodicity of crappie feeding in a series of midwater trawl studies in a Kansas reservoir and found crappie between 8 and 17 cm long to be wholly pelagic. Crappie were distributed throughout most depths during the cool times of the year but were grouped more vertically during the warmer months‐in part because of the anaerobic hypolimnion extending up to the 5‐m depth. Crappie rarely fed at night and fed entirely on zooplankton during the day. Daphnia, even when relatively sparse, made up much of the diet whereas diaptomid copepods were relatively rare. Feeding rates of crappie ranged from 20 to 200 prey per hour when preying on zooplankton during the morning. These rates were estimated to provide sufficient energy for small crappie (5‐20 g) but only marginal energy for larger fish. The larger crappie seemed to reduce energy expenditure by feeding in the cooler metalimnion during the day. However, all crappie spent the summer evening hours in the warmer epilimnion. The reason for this seemingly inefficient evening distribution is unknown.