Fish Facts

Fish Facts: Cod stocking – a double-edged sword

HISTORIC overfishing, declining water quality and habitat alteration in our inland and coastal river systems are major factors that have contributed to the decline of iconic species such as Murray cod (Maccullochella peelii), trout cod (M. macquariensis) and their coastal cousins the eastern freshwater cod (M. ikei) and the Mary River cod (M. mariensis).

Indeed, the coastal cods were formerly abundant throughout the freshwater reaches of the Clarence, Richmond, Logan, Albert, Brisbane and Mary River systems until the early 20th century. But since then their numbers have declined significantly, initially due to overfishing by early European settlers who used nets, set lines and explosives, but subsequently due to massive siltation of their spawning habitats caused by land-clearing, destruction of riparian vegetation and catchment development. Erection of dams and weirs has also blocked their migrations and isolated populations of fish that otherwise could mix during the spring spawning period, increasing the chances of inbreeding in remnant populations. Because of this, all of the freshwater cods are now listed as endangered on the IUCN Red List.

Restocking of cod is seen by some people to be an answer to the need to halt their decline. However, scientists have found that the situation is not that straightforward. Studies of eastern freshwater cod populations in areas of the Clarence River where restocking has occurred have shown stocking has not been successful in re-establishing self sustaining populations of cod. Indeed, the process of stocking appears to have been detrimental to the genetic diversity of the remaining populations of wild fish. Survival of stocked fish was also low, probably due to poor-quality habitat (east coast cod require near pristine river conditions to survive), while failure to establish self-sustaining populations may also have been due to a lack of suitable spawning habitats not affected by sediment. Also, there is evidence that suggests stocked fish have trouble learning the sometimes complex territorial and spawning behaviours employed by wild fish.

In another study, radio tracking experiments of stocked trout cod in the Murrumbidgee and Cotter Rivers showed that the tagged fish did not move very far, staying within 5 km of their release sites. Mortality of released fingerlings was rapid – one month after release 40 to 70 per cent of stocked trout cod had died, and all of the radio tagged fish had perished within seven months of their release. Predation by cormorants and water rats were observed to be the most common causes of mortality of the stocked fish in this study.

So from these two examples it is clear that stocking freshwater cod has not been a panacea. Indeed, rehabilitation of fish habitat by improving water quality through restoring riparian vegetation and resnagging, halting gully erosion and land clearing on adjacent land, and regaining fish passage past artificial barriers are the real goals that need to be achieved in order to protect the remaining self sustaining wild populations of our iconic freshwater cods.

Clearly, due to the genetic issues and the fact that stocked fish compete for the same food and spawning substrates as wild fish, stocking of natural rivers is a double edged sword that is really a “last resort” that should be employed only if all these other efforts to improve habitat have failed. In those cases where stocking of hatchery reared fish into the wild is considered the only option to ensure persistence of these species, it appears that the genetics of the stocked fish must be carefully appraised, predator avoidance training of hatchery reared fingerlings is important, and a balance must be made between stocking greater numbers of smaller fingerlings (with resultant higher mortality rates), or increasing the size of fish that are stocked in the hope that they are thus better equipped to avoid predators.

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