The intrapopulation variability in the size and age structure of the spawning stock and migration of the threatened Macquarie perch Macquaria australasica in Lake Dartmouth was investigated between 2008 and 2016. Sampling centred on the core reproductive period (October-December) when mature fish migrate from the lake into riverine habitat to spawn. Spawning fish were predominantly large, spanning a broad age structure, with a high proportion of fish (25%) aged 15-30 years. The overall median size of spawning fish did not change for males or females during the 9 year study period. The size of the smaller mature male fish did change in some years suggesting a small proportion of male M. australasica matured at age 1+ and 2+. Acoustic telemetry employed over 3 years showed that M. australasica were most likely to be in the spawning reach from October to mid-December, migrated to this reach annually and moved large distances throughout the lake all year, with no evidence for any spatial structuring. Mature fish sometimes occupied the spawning reach for several months after the core reproductive period, which increased their vulnerability to recreational fishing. Males tagged in the lake were seldom recorded in the spawning reach, presumably because a high proportion had not yet entered the spawning stock despite their size suggesting maturity. Maintaining a broad age and size-structure of the spawning stock of long-lived iteroparous fish species is crucial for recruitment stability and population persistence. Overexploitation of the spawning stock has probably contributed to previous population declines in the lake as well as the collapse of other M. australasica populations in south-eastern Australia.
Keywords: Murray-Darling Basin; acoustic telemetry; age structure; population structure; threatened species.
© 2018 The Fisheries Society of the British Isles.