Impacts of Managed Flows on Fish Spawning and Recruitment: Report prepared for the Murray-Darling Basin Commission

Abstract

This study tested aspects of the 'window-of-opportunity' hypothesis (a variation on the 'match/mismatch' hypothesis), which attempts to relate the timing of production of prey to the timing of production of fish larvae and subsequent recruitment of fish. The 'window-of-opportunity' hypothesis states that fish which spawn over a 'protracted' period have a recruitment advantage, in any one year, over fish which spawn over a 'brief' period. This is because the probability that a proportion of the larvae of protracted spawning species will encounter a period when conditions are optimal for recruitment is greater than for larvae of brief spawning species. This study tested the 'window-of-opportunity' hypothesis in the context of fishes in lowland rivers of the Murray–Darling Basin, with the primary aim of comparing the estimated hatch date distribution of early stage larvae with that of juveniles at the end of the spawning period (recruits) for a protracted spawning species: Australian smelt. This species was chosen because previous work has indicated that it can spawn for many months each year, and, given that it is widespread in the Murray–Darling Basin, it is likely to occur in a range of river types. We also sought to compare and contrast the relative timing of spawning and recruitment of other native and alien fishes in rivers which have substantially regulated flows (flow-managed rivers) and in rivers which have mostly unregulated flows (non flow-managed rivers), with the premise being that river regulation alters the rearing environment for young fishes and thus may result in different patterns of spawning and recruitment between these two types of rivers.