Re-introduction of trout cod into the Murray- Darling River Basin, Australia

Summary

Trout cod (Maccullochella macquariensis), once widespread across the southerly tributaries of Australia's Murray-Darling Basin, now survives as a single natural breeding population in the Murray River between Yarrawonga and Barmah plus two earlier translocated populations, having declined from habitat degradation, overfishing, and introduced species. New South Wales and Victoria state fisheries agencies began captive breeding and re-introduction programs in the mid-1980s using only wild broodstock from the Murray River, regularly refreshed with new wild fish rather than hatchery-bred stock, achieving the first successful captive spawning via hormone injection in 1986. The first re-introduction released 1,000 hatchery-reared juveniles into the upper Murray River in January 1987; since then, 984,600 fingerlings have been stocked at rates of 13,000 to 151,000 annually across 32 sites in eight river catchments, supplemented by 11,700 intensively on-grown yearlings and two-year-olds. Chemical marking of hatchery fish since the early 1990s, combined with length-frequency and otolith aging, has confirmed natural recruitment in four rivers, the lower Murrumbidgee, Ryans Creek, Goulburn, and Cotter Rivers, and suspected recruitment in three more, though at least three Victorian stocking sites have failed outright with no recent captures. Natural recruitment took roughly a decade to appear, longer than the five years initially expected once stocked fish reached maturity, and telemetry studies found older on-grown fish generally survive poorly after release. The project rated itself Successful, citing the scale of releases and the confirmed survival and recruitment of trout cod at many stocked sites.