An ecological approach to re-establishing Australian freshwater cod populations: an application to trout cod in the Murrumbidgee catchment
Trout cod were formerly abundant and widespread throughout the southern Murray-Darling Basin but by the 1980’s they had declined to a single natural population in the middle reaches of the Murray River. There were also two known translocated populations, one in Cataract dam in NSW and one in Seven Creeks in Victoria. The species was listed as endangered in NSW and the ACT and as threatened in Victoria, and captive breeding programs were initiated to reintroduce captive bred fingerlings into sites within their former range. The NSW captive breeding program is based at the Narrandera Fisheries Centre, with stockings of fingerlings being made on an annual basis into the Murrumbidgee river.
During this project, two field experiments were used to determine if dispersal of post-juvenile Trout cod, Maccullochella macquariensis, is responsible for the apparent lack of success of stocking this species into rivers of the Murray-Darling basin. These experiments also served as a trial for releasing Trout cod as sub-adult fish rather than as fingerlings.
The first experiment used tracking of radio-tagged fish to monitor dispersal in the lowland reaches of the Murrumbidgee river near Narrandera. The trial consisted of 27 trout cod stocked as two year-old fish compared to 31 fish that had been originally stocked as fingerlings (but which had since grown to the same size and an assumed similar age to the sub-adult hatchery fish). Most of the ‘hatchery’ group quickly dispersed substantial distances downstream (5-55 km) with only 33% of the group remaining close to the release site. Further, all but a few either died or lost their tags over a 13 month period. Conversely, only one of the ‘wild caught’ group died or lost its tag during this time, and most remained very close to the release site. However, 7 wild-caught individuals did undertake long distance, unsynchronised movements, but all eventually established home ranges where they stayed.
The second experiment also used radio tracking of hatchery-reared sub-adult Trout cod released into two sites in the ACT where the species had not been recorded for several decades. Underwater video cameras were set up to monitor the behaviour of the newly released fish at the Cotter River site, but this proved ineffective because of poor water clarity. Again, survival of these fish was very low with 100% mortality occurring after 7 months. Predation by water rats and cormorants was implicated in these rapid declines.
Poor survival of hatchery-reared, two-year old Trout cod was consistently recorded during both experiments at one lowland (Narrandera) and two upland (ACT) sites. This result demonstrates that the re-establishment of cod populations based on the release of larger juvenile fish is not a straightforward alternative to the existing practice of stocking fingerlings into rivers.