Offstream movements of fish during drought in a regulated lowland river

Access to offstream habitats is vital for many freshwater fish, but details of their lateral movements are scarce. We describe the movements of fish between the channel of the River Murray and six perennially inundated wetlands in South Australia from August to November 2006. At this time there were unprecedented low flows in the river owing to the combined effects of river regulation, drought and over-allocation to upstream users. Some 210 000 fish from 18 species (14 native, 4 alien) were recorded, including two uncommon native species listed by conservation agencies. Movements of juveniles and adults varied among wetlands despite the shared river reach and the proximity of the wetlands to each other, but showed no consistent directionality. This may reflect the prevailing low-flow conditions, the virtually permanent connections between the wetlands and channel maintained by weirs, levees and barrages, and the dominance of ‘generalist’ species. We speculate that movements facilitate efficient resource utilization and nutrient exchange between homogenized river and wetland habitats in the absence of the flood-pulse. We anticipate directional movements will become apparent when flows are increased, so that our data could provide a comparative baseline for future studies. As modifications to natural flow paths may impede access to/from wetlands by fish and other aquatic fauna, provisions for access should be incorporated into flow-control structures, used locally to manipulate wetland hydrology. Copyright © 2010 John Wiley & Sons, Ltd.