Swimming performance and dispersal potential of larval Australian freshwater fish in a regulated riverscape

Riverscape ecology aims to incorporate the biotic and abiotic patterns and processes at spatial and temporal scales appropriate to organisms of interest. Set within a riverscape ecology framework, and using life history traits as a basis for understanding, this thesis explores how fish larvae in a lowland river are affected by regulated flows.

This study investigated the potential of anabranches, or secondary channels, in the riverscape, to provide habitat for larval fish under regulated flows. The fish larvae of a highly flow-modified river (main stem), a highly flow-modified anabranch and an anabranch which has a less modified flow regime, were sampled during the normal breeding season in south-eastern Australia. The main stem supported a relatively high abundance of larvae, but the anabranch with a less modified flow regime, supported more species overall than the other river types. Anabranches may provide habitat heterogeneity and act as a refuge for larvae under regulated conditions.

To help understand limitations on habitat use in, and ability to move through,
riverscapes by the early life history stages of fish the swimming capabilities of the larvae of six species of native fish were investigated in a laboratory flume. Swimming performances for species broadly corresponded with life history strategies: equilibrium species were the best swimmers, periodic species the poorest and opportunistic species in between. Swimming capability increased with ontogeny, and developmental stage better explained swimming ability than did length, size or age.

To investigate the implications of differential swimming performance in the wild, in situ dispersal experiments in a reach of the anabranch with a less modified flow regime were conducted with the larvae of two species of native fish - with different life history strategies and swimming abilities - and with sunflower seeds, as passive particles. Seventy-one per cent of released Murray cod larvae passed through the 500 m reach, whereas 16% of released golden perch larvae and 3-4% of passive particles did so. Furthermore, Murray cod did not drift in the same pattern as sunflower seeds, but golden perch larvae did, suggesting that the latter was more passive in its dispersal than the former.

Models were developed to determine how the distribution of current speeds
throughout the experimental reach of the anabranch changes with discharge levels below bankful and how this in turn might affect the available hydraulic habitat available for fish larvae. As discharge increased, so did overall current speeds, resulting in a more homogeneous flow environment and decreasing available hydraulic habitat for all species examined. However, the relative impact of changes in discharge and current speeds is species-specific, depending on whether dispersal is active or passive or non-existent.

I conclude that under regulated, high flows, during the spawning season: (1) the
larvae of equilibrium life history strategy fishes may have a reduced ability to
actively disperse through the riverscape into suitable nursery habitat due to an
increase in current velocity above critical swimming thresholds; (2) there may be an increase in prevalence of the larvae of periodic life history strategy fishes passively drifting through the riverscape into unsuitable habitat, because there are fewer slow/no flow habitats; and (3) there may be an increase in occurrence of the larvae of opportunistic life history strategy fishes being washed out of suitable habitat for similar reasons. Under regulated high flows, a less modified anabranch may provide alternative, suitable nursery habitat within the main stem/anabranch riverscape.