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Prey specialization by individual trout living in a stream and ponds: some effects of feeding history and parental stock on food choice Bryan, James Ernest

Abstract

This study had two purposes: to determine whether individual trout living in the same area select different kinds of food, and to find out why. Analyses of stomach contents showed that the kinds of prey eaten by trout were seldom distributed at random among the individuals. Repeated observation of food eaten by individuals in a stream and ponds showed that prey types were eaten in proportions which were characteristic for an individual. Three trout species were studied: brook trout, Salvelinus fontinalis (Mitchill); cutthroat trout, Salmo clarki Richardson; and rainbow trout, Salmo gairdneri Richardson. Further analyses were performed to describe the nature of food specialization in trout. In the stream, individuals with either positive or negative specialization measures ate similar assortments of food. However, those with negative measures had eaten slightly more of prey types available for short time periods than had those with positive measures. Although the degree of specialization was higher during shorter intervals, the data suggested that some specialization persisted for half a year. There were no striking correlations between degree of specialization and other individual properties such as: size, growth rate, weight of food, number of kinds of food, previous specialization, or area of recapture. A field experiment was conducted using laboratory-reared rainbow trout held in small ponds. The food of each trout was sampled repeatedly. In analysis of variance, interaction among the individuals and kinds of prey eaten showed that food specialization occurred. Prey abundance was constant during the experiment. Feeding history and parental stock were manipulated to determine whether they could influence food selection in young rainbow trout. After 9 training meals of one food, trout selected that food, the familiar one, when given a choice between it and a novel food. (Most choice situations used high and equal densities of unconcealed foods). Selection of the familiar food occurred with several kinds of non-living food. Trout trained on live prey, however, did not always select the familiar one when both prey were alive, although they did when both prey were dead. Some characteristics of the training effect were investigated. As they became satiated, trout consumed relatively more of the novel food. Duration of food deprivation before a choice test did not change the degree of selection for the familiar food. In addition to eating more of the familiar food, trout struck but rejected relatively more of the novel food. Individual trout trained on two foods ate them in proportions which were characteristic for an individual. After they had learned to select one food, trout "were given further training on one of the following: the familiar food, a novel food, or both. Further training on the familiar food did not change the proportion selected. Trout trained on one food for 12 meals and then on a second food for 12 meals selected the second food when given a choice. Then the initial training was followed by continuous feeding of both familiar and novel food, trout continued to select the familiar food for 14 to 23 meals. All results suggested that effects of such feeding history would not greatly influence food selection in natural situations. Progeny of parental stocks were tested to determine whether parental food can influence food selected by offspring. Eggs from trout which ate different kinds of food were hatched in the laboratory. For their first meal, trout were given choices of the kinds of food eaten by the parental stocks. In three main experiments, the young trout did not select the type of food commonly eaten by their parents.

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