Observations on interactions between indigenous Galaxiidae and introduced Salmonidae in the Lake Eucumbene catchment, New South Wales

Abstract

The fish fauna of all major streams within the Lake Eucumbene catchment in south-eastern New South Wales was sampled by electrofishing or poisoning with rotenone. Galaxias coxii was found in only four, and G. olidus in only one other of the 27 streams sampled whereas the introduced salmonids (Salmo trutta or S. gairdneri, or both) occurred in all but the stream containing G. olidus. Values for galaxiid biomass were low in comparison with those for trout where both occurred together. Sampling in 1971 and 1974 in the one stream observed in 1971 to contain only G. olidus spanned an invasion by S. gairdneri and by 1974 the galaxiid species had completely disappeared below a natural barrier to trout, but above this barrier the biomass and population structure of G. olidus had not changed greatly compared to the 1971 values. This indicated that the presence of S. gairdneri was the sole environmental factor causing galaxiid disappearance below the barrier. Historical data for the catchment area suggested that the introduction and subsequent success of trout are primarily responsible for the present, much fragmented galaxiid distribution.