Ecologist and former parks and conservation director Bryan Pratt said the lake was a food-rich environment for growing Murray cod and golden perch, which had thrived because of released fingerlings and a growing catch-and-release policy.

''It is also a lesson for the rest of Australia and the world - we're growing fish in an urban environment, we're growing some of the biggest freshwater fish in the world.

''That's quite an achievement. The idea is we look to fish as the environmental marker. If fish grow big and fat and strong in the local lakes, then it suggests we're doing the right thing in the catchments and lakes themselves.''

The National Capital Authority and Canberra Fisherman's Club released more than 37,000 Murray cod fingerlings into Lake Burley Griffin yesterday.

Dr Pratt said 30kg and 40kg cod were being hooked in the lake. One of his students hooked and released a 50kg monster near Black Mountain Peninsula. Fish 90cm long were becoming common throughout Canberra's five local lakes.

Capital Region Fishing Alliance delegate Steve Samuels said extra cod fingerlings were released to allow for losses from birds and other fish, especially redfin, but in time cod, which could put on a kilo a year, would turn the tables on the pest species and eat them.

More than 10,000 Murray cod were released in the Queanbeyan River yesterday. Capital Region Fishing Alliance president Shane Jasprizza said stocking tied in with restoring habitat and would help both anglers and the environment in the future.

Dr Pratt said releasing more native species would not control carp numbers. The only long-term prospect of achieving that was either developing a target pathogen or manipulating the genetic profile of fish to produce a daughter-less carp.

Cod and golden perch would rise and fall with carp and redfin numbers, in a relationship not unlike Africa's lions and wildebeest.

''Cod will be useful on preying on carp and redfin, it is unlikely they will be the source of biological control. You'd want an enormous amount to have a significant effect on redfin and carp,'' Dr Pratt said.

The authority and Canberra Fisherman's Club funded yesterday's release which follows 100,000 golden perch released last year.

The Capital Region Fishing Alliance has netted $20,000 from the NSW Government's habitat action grants that it will spend replanting and re-snagging a section of the Queanbeyan River within the town limits later this year.