The influence of juvenile brown trout (Salmo trutta) on habitat use of inanga (Galaxias maculatus) in a stream simulator

Abstract

Introduced salmonid fishes may have affected native galaxiid fishes in New Zealand by forcing alterations in their use of spatial resources. A stream simulator was used to test the effects of juvenile brown trout (Salmo trutta) (fork length range 66–115 mm) on habitat use of inanga (Galaxias maculatus) (fork length range 51–100 mm) during autumn. The stream simulator consisted of a single run‐riffle‐pool sequence lined with gravel and cobble substrates, and c. 8 m3 of untreated artesian water was recirculated through the stream simulator at a rate of 3 litres s‐1. Stream invertebrates were introduced into the stream simulator to provide food for the fish. Four experiments were run, each consisting of three 1‐day trials using either 10 inanga, 20 inanga, or 10 inanga and brown trout. Counts of inanga visible in each of 12 designated habitat zones were made throughout the day, and proportions of visible inanga were used in repeated measure ANOVAs to test whether the proportion of fish visible varied according to the time of day, the density of inanga, or the presence of trout. Our experiments showed that neither the presence of juvenile brown trout nor the density of inanga had significant influence on inanga habitat use in the stream simulator. We conclude that in New Zealand streams any influence of brown trout on inanga is more likely to result from predation and/or competition for food by larger brown trout.