Effects of Habitat Structure and the Presence of Brown Trout on the Population Density of Galaxias truttaceus in Tasmania, Australia

To examine the effect of introduced brown trout Salmo trutta on populations of native Galaxias truttaceus (Galaxiidae), known locally as spotted galaxias, population abundance models based on the habitat use patterns of G. truttaceus were used to compare streams with and without brown trout. In selected streams in southeast Tasmania, habitat use by G. truttaceus was examined with respect to four principal components extracted from eight habitat variables. Different size‐classes of G. truttaceus displayed varying nonrandom patterns of habitat use, shifting from shallow, open habitats to deep, cover‐rich habitats with increasing size. All size‐classes preferred slow‐flowing sections to fast‐flowing sections. Population abundance models were constructed for three size‐classes of G. truttaceus , and given the hydrologically variable nature of the streams studied, all of the models were reasonably successful in explaining variation. The application of the models to streams containing brown trout indicated that the presence of brown trout was more important than habitat characteristics in determining the abundance of G. truttaceus . In streams with brown trout, the density of each size‐class of G. truttaceus was substantially less than that expected on the basis of habitat characteristics. The study provides strong evidence that brown trout adversely affect populations of G. truttaceus , because habitat differences were quantitatively accounted for when streams with and without brown trout were compared.