A test for interaction between brown trout (Salmo trutta) and inanga (Galaxias maculatus) in an artificial stream

The interaction between brown trout (Salmo trutta; fork length (FL) range 255-390 mm) and inanga (Galaxias maculatus; FL range 55-115 mm) was tested during summer through autumn in an artificial stream consisting of a single run-riffle-pool sequence with a natural food supply. Each experimental trial lasted for 15 days, and consisted of two brown trout and 50 inanga collected fresh from a nearby stream, with each species given prior residence in four replicate tests, totalling eight trials in all. In addition, two control trials (each 10 days), with 50 inanga in each, were run. Brown trout almost exclusively occupied the pool, whereas inanga occupied all habitat types, although in different proportions, when tested with and without brown trout. The proportion of inanga in the pool was appreciably lower in the experimental trials with brown trout than in the control trials with no brown trout; prior residence had no significant effect on inanga habitat use. Mortality of inanga attributable to predation by brown trout ranged from 0 to 40% with a mean of 14.5+/-4.7%. The results suggest that habitat use and survival of inanga populations in small streams can be adversely affected by brown trout.