A New Estimate of Zooplankton Retention by Gill Rakers and Its Ecological Significance

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Abstract
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