Overfishing has been a major cause. Murray cod have been grossly overfished by recreational anglers over the last few decades, and up until very recently, commercial fishermen too. The situation is still very serious. Fishing regulations for wild Murray cod are still too weak and allow recreational fishermen too kill far too many Murray cod. The minimum legal of 50cm in Victoria and NSW does not allow Murray cod to breed properly before being killed by anglers. Research has shown that strong numbers of juvenile Murray cod from natural spawning Lake Mulwala and the Murray River basically cease to exist when they reach the minimum legal size, and that Murray cod larger than the minimum size are almost non-existent. It is a very serious situation. Fishing regulations need tightening dramatically and Fisheries Departments need to stop being irresponsible and worrying about introduced trout (that have also caused great damage to native fish in mountain rivers) and start managing wild populations of our greatest native fish properly. River regulation has also been a major cause. Dams and weirs stop natural river flows and most flood events. Many parts of the Murray-Darling now have their annual flow patterns reversed. Water releases from reservoirs are un-naturally cold and stay so for long distances downstream. Excessive water extraction leaves some reaches barely flowing. All of this has played havoc with the breeding of native fish and their survival of their larvae. Often larval survival is negligible. Irresponsible de-snagging (snags being large sunken trees) on a huge scale in past decades has had a massive impact on Murray cod. Snags are critical habitat for Murray cod, critical feeding and breeding sites for them, and also critical for river ecosystems. The Murray-Darling system has been enormously damaged by the stupid and thoughtless removal of most of its snags. Huge sections of the Murray River are almost devoid of Murray cod habitat due to de-snagging. An important issue too is smaller woody snags for juvenile Murray cod. Widespread damage or removal of native riparian (riverbank) vegetation has removed the source of this important habitat for juvenile Murray cod in many area. Introduced fish have also been a major problem. Before introduced carp became common, introduced redfin perch were common in the Murray-Darling rivers and were a huge problem. They are very predatory and basically devoured almost all native fish larvae and juvenile native fish and most food resources. They caused a crash in native fish numbers in the decades when they were common. Redfin became rare in Murray-Darling rivers when introduced carp tragically invaded the system, however carp were no better. Carp caused enormous damage, permanently destroying most aquatic weed in rivers and permanently muddying waters. They also spawn billions of carp larvae in spring at the same general time Murray cod spawn. Research reveals these carp larvae have a similar diet to Murray cod larvae. So now Murray cod larvae must compete with billions of carp larvae for food. Through this and the habitat destruction they cause, carp have had a very negative effect on Murray cod numbers. These are the main reasons, but a thorough discussion of Murray cod and the causes of their decline from the start of European settlement can be found at http://en.wikipedia.org/wiki/Murray_cod. I think the most important things to remember is that historically (i.e. second half of the 1800s) were grossly, grossly overfished by commercial fishermen and this was the first thing that really brought Murray cod to their knees. River regulation then became a serious problem, and, well, overfishing by recreational fishermen continues to be a major problem. As well as returning more environmental flows to the rivers, and controlling carp numbers, the most important thing now is to tighten fishing regulations and stop recreational fishermen overfishing Murray cod. It should always be remembered that Murray cod are big, slow-growing, long-lived fish with delayed sexual maturity and relatively low egg counts. Fish with these characteristics are vulnerable to overfishing, and more importantly, simply can't cope with overfishing. Angler take of these fish needs to be reduced to almost nil, as unpopular as that might be.

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