The Feeding Relationships of Salmon, Trout, Minnows, Stone Loach and Three-Spined Stickle-Backs in the River Endrick, Scotland

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Abstract
The food of Salmo salar, S. trutta, Phoxinus phoxinus, Nemacheilus barbatula, and Gasterosteus aculeatus, was assessed by examining the stomach contents of 1435 specimens of these fish. Most fish were taken from a single tributary stream and the composition of its bottom fauna was estimated by taking monthyl samples from the dominant substrate. Though the diet of each of the 5 species of fish was characteristic in some way, all were found to feed on the common aquatic invertebrates present (e.g. Baetis, Gammarus and Orthocladius). Surface organisms formed a large part of the food of trout but were less important to salmon and minnows, and only rarely taken by stone loach and three-spined sticklebacks. The availability of certain invertebrates to fish was assessed by comparing the composition by bulk of the bottom fauna with that of the aquatic invertebrates eaten by each fish species. The common invertebrates present exhibited availability factors which varied from one species to another, and which also depended upon which fish species was involved. Competition among these fish is discussed it appears that while each species has a characteristic diet, all have food items in common and competition among them is likely, its degree probably being determined by the abundance of the food items being competed for.