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 popula tions of native Galaxias truttaceus (Galaxiidae), known locally as spo tted galaxias, population abundance models based on the habitat use pa tterns 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. t ruttaceus displayed varying nonrandom patterns of habitat use, shiftin g from shallow, open habitats to deep, cover-rich habitats with increa sing size. All size-classes preferred slow-flowing sections to fast-fl owing 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 succe ssful in explaining variation. The application of the models to stream s containing brown trout indicated that the presence of brown trout wa s more important than habitat characteristics in determining the abund ance of G. truttaceus. In streams with brown trout, the density of eac h size-class of G. truttaceus was substantially less than that expecte d on the basis of habitat characteristics. The study provides strong e vidence that brown trout adversely affect populations of G. truttaceus , because habitat differences were quantitatively accounted for when s treams with and without brown trout were compared.