Feeding relationships of galaxiids, bullies, eels and trout in a New Zealand river

Abstract

The feeding relationships of the common river galaxias, Galaxias vulgaris Stokell, upland bully, Gobiomorphus breviceps (Stokell), long-finned eel, Anguilla diefenbachii Gray, short-finned eel, A. australis schmidtii Phillipps, and brown trout, Salmo trutta L., were examined in the Glentui River, Canterbury, New Zealand, in February, May and November 1971. With few exceptions, the same food organisms were utilized by all species, but the relative proportion of each food type in the diet varied between species. Kendall rank correlation coefficients indicated that the diets of native fish were dissimilar, with the exception of galaxiids and bullies in May, but in six out of nine comparisons involving the introduced trout, the coefficients indicated varying degrees of similarity with the diets of the native species. Since their feeding mechanisms and feeding localities are different, similarities in the diets of eels and trout, trout and bullies, and bullies and galaxiids can be regarded as giving rise to indirect competition. However, since trout (C 200 mm long) and galaxiids occupy the same microhabitat and feed in the same manner, similarity in their diets can be regarded as giving rise to direct competition. Such direct competition for food, combined with interspecific aggression and similar microhabitat requirements, may help to explain the reduction in abundance of other galaxiids in areas in which trout have been introduced.