Fish larvae and the management of regulated rivers

Alterations to the natural hydrologic regime in regulated rivers can disrupt cues that initiate the maturation and spawning of riverine fish, or they can change the conditions which are suitable for the recruitment of larvae into juvenile populations. Observations of fish larvae have the potential to provide insights into the effects of flow regulation, showing whether it has had a greater impact on fish by preventing spawning or by reducing or eliminating recruitment. We investigated historical and current records of native fish in the highly regulated Campaspe River and the moderately regulated Broken River, Murray‐Darling Basin, Australia, and compared these with the results from sampling of fish larvae over three consecutive years to assess the likely impact that river regulation has had on fish populations in lowland Australian rivers. Of the 12 native species of fish that have been recorded historically from the Campaspe River, eight still occur, generally in low abundance, but only three of these were recorded as larvae in this experiment. From recent records, ten native fish species are extant in the Broken River from a suite of 15 that have been recorded there; of these, nine were collected as larvae. The presence of Murray cod larvae in this river was a significant finding. Thus, the less regulated Broken River is in a much healthier state than the Campaspe River. The results of sampling in both rivers indicated that most species of fish spawned each year, despite high inter‐annual variation in antecedent hydrological conditions. This suggests that for the species present in these two rivers, the hydrology (pattern of daily discharge) during the winter and spring preceding breeding was unlikely to be a cue for final maturation and spawning. These findings are only preliminary, but they may show that river regulation has had more of an impact on post‐spawning recruitment than on prevention of spawning. This has important implicatons for the remediation of the effects of river regulation, with targeting of recruitment processes and the factors influencing these, a priority. Copyright © 2000 John Wiley & Sons, Ltd.