Dietary and spatial overlap between stream populations of a native and two introduced fish species in New Zealand

Abstract

Benthic and drifting invertebrates as well as fish were sampled in a lake-inlet stream over a 24-h period in summer in order to investigate interactions for food and space between a native fish species (Galaxias brevipinnis, Galaxiidae) and juveniles of two exotic fish species (Salmo trutta and Oncorhynchus mykiss, Salmonidae) in New Zealand. Galaxias brevipinnis fed almost exclusively at night on the benthos, whereas both trout species fed mostly during the day on drifting and benthic prey. Larvae of Deleatidium (Ephemeroptera), Hydrobiosis and Oxyethira (Trichoptera), and Austro-simulium and Chironomidae (Diptera) were the major prey in the diets of all three fish species. Small prey made up a greater proportion of the diet of G. brevipinnis than for either trout species. All three species of fish were found primarily in riffles and runs in water depths of 0.1-0.3 rn, with G. brevipinnis frequently being in faster-flowing areas (water velocity 0.8-1.1 m s^-1) than was either trout species (water velocity 0.4-0.7 rn s^-1). These differences in pattern of die1 feeding and micro- habitat use no doubt lessen the interaction between the galaxiid and salrnonid fish species.