Summary

Fifty-five freshwater fish species inhabit the NSW rivers and at least 36 of them have a requirement to migrate at some stage of their life. Adult native fish have been previously observed undertaking large-scale upstream migrations of up to 1,400km whilst juveniles also perform mass upstream migrations. The presence of dams and weirs often prevent successful migrations by blocking pathways to spawning habitats, and also feeding areas and have been widely implicated in the declines of many freshwater fish species.

In addition to their effects on migration, dams and weirs are known to significantly alter the natural diets of predatory fish species because large numbers of migratory fish often accumulate downstream of barriers whilst migrating, leading to increased competition. In addition, post-construction habitat changes may also alter the abundance and distribution of prey. Although demonstrated for predatory fish in Europe and North America, there are no previous studies documenting similar processes in Australian freshwater systems, even though such interactions may be expected.

This paper reports on a study undertaken at Yanco Weir on the Murrumbidgee River that sought to identify changes in fish diet composition arising from the presence of an instream barrier. To this end, the stomach contents of over 560 Murray cod, golden perch and trout cod were collected, from upstream and downstream reaches, to determine any differences in dietary composition. In addition, the occurrence of potential prey items was also recorded to identify any potential changes in prey availability.

Substantial differences in fish assemblage structure were observed between reaches, largely because more common carp, trout cod, golden perch, Australian smelt and goldfish were sampled from downstream reaches. Interestingly, fish were subsequently identified from a greater proportion of stomachs from the downstream reach, suggesting that predatory species were exploiting this abundant source of food. In addition, it was also found that diet composition, overlap and the occurrence of empty stomachs substantially differed between the two reaches. In particular, all species exhibited greater dietary overlap with each other, and a higher proportion of empty stomachs, where fish were accumulating downstream of Yanco Weir. These observations suggested that competition for resources might be greater from downstream zones where such a large number of fish were present.

It was suggested that similar trophic effects might be observed at other sites where the movements of migratory fish are obstructed. Therefore, to effectively reduce these effects on diet composition, it was recommended that fishways be constructed on various dams and weirs to restore free passage and reduce the potential for high-density accumulations to occur.