Can stocking play a role in recovering fish populations after fish kills? Lessons following blackwater events in the Edward-Wakool river system

Localized catastrophic events can dramatically affect fish populations. Management interventions are often undertaken to re-establish populationsthat have experienced such events. Evaluations of the effectiveness of these interventions are required to inform future management actions, andknowledge of the recovery pathways contributing to successful restoration are an essential component. Multiple hypoxic blackwater events in 2010–2011 substantially reduced fish communities in the Edward-Wakool river system in the southern Murray-Darling Basin, New South Wales, Australia.These events led to extensive fish kills across large sections of the entire system following a period of prolonged drought. To expedite recovery efforts,golden perch Macquaria ambigua and Murray cod Maccullochella peelii fingerlings were stocked at five locations over 3 years. All fish stocked werechemically marked with calcein to enable retrospective evaluation of wild or hatchery origin. Targeted collections were undertaken 3 years poststockingto investigate the relative contribution of stocking efforts and recovery via natural recruitment in the system. Of the golden perch retainedfor annual ageing (n = 93) only nine were of an age that could have coincided with stocking activities. Of those, six were stocked. The dominant yearclassof golden perch were spawned in 2009; before the stocking programme began and prior to blackwater events. All Murray cod retained (n = 136)were of an age that coincided with stocking activities, although only eight were stocked. Among the Murray cod captured, the dominant year-classwas spawned in 2011, after the blackwater events occurred. The results from this study provide evidence that natural spawning and recruitment, andpossibly immigration, were the main drivers of golden perch and Murray cod recovery following catastrophic fish kills. Interpreted in the context ofother recent examples, the collective results indicate minor benefit of stocking to existing connected populations already naturally recruiting inriverine systems.