From the OFFICE OF THE FISHERIES REVENUE ALLOCATION COMMITTEE

Wednesday 10 October 2007

Evaluating the potential of the Nagambie Lakes for golden perch and Murray cod.

The Nagambie Lakes, just an hour’s drive north of Melbourne, has the potential to be a favourite haunt of recreational anglers targeting native fish.

To maximise this potential Fisheries Victoria and the Nagambie Angling Club tried to establish a self-sustaining (reproducing) native fishery. They did this by releasing around 440,000 small fish, mostly golden perch, into the lake system from 1979 to 1994. But the fish would not cooperate and little evidence of them still remain!

Formed to regulate irrigation supplies for Shepparton, Central Goulburn, Rochester and the irrigation system of the Waranga Basin, this lake has a range of deep channel and shallow backwater habitats, which were thought to be suitable for native fish.

To work out what the Nagambie Lake system was lacking from a fish’s perspective, researchers from the Arthur Rylah Institute for Environmental Research decided to go right to the source and ask the fish!

Thirty-two Murray cod and fifteen golden perch were enlisted, fitted with radio tags and released into the middle of the Lake system at Picnic Point, Kirwins Bridge and Bazley Island.

With the aid of five radio tracking logger stations set up through out the lake, the scientists were able to see what the fish were doing and where they were going during the ten-month study.

The response of the Murray cod to being released into the Nagambie Lakes was to stay put!

The hatchery reared Murray cod tended to be fairly sedentary and perhaps failed to come to terms with a natural lake environment. Many died during the course of the study. While predators (most likely pelicans), probably ate many, the researchers felt that the Murray cod’s hatchery upbringing compromised their ability to escape from predators and even to hunt for food. This finding indicates that the stocking of larger, hatchery raised fish may not always be advantageous.

The golden perch, sourced from the lower Goulburn River below Goulburn Weir, adapted much better to the lake environment than did the Murray cod.

Golden perch were highly mobile throughout the course of the study, most undertaking long upstream and downstream migrations. Only one fish remained in the location where it was released. Many fish swam more than 3 km to explore downstream gates and several left the lake system escaping through control gates on irrigation channels.

Interestingly, there was one control gate through which golden perch would not venture even though many fish approached it on many occasions. It is possible that gate configuration and associated features, such as turbulence and noise, can greatly influence the decision of golden perch looking to exit large impoundments via irrigation release points.

Both the golden perch and Murray cod released into the lakes were strongly associated with the main river channel and were found 90% and 70% of the time respectively in the deeper water. In this deeper water, both golden perch and Murray cod were always found close by some form of in-stream structure, most often large woody debris. Neither the Murray cod nor golden perch released into the Nagambie Lakes used backwater habitats to any great extent, possibly because these habitats were too shallow due to the accumulation of sediment.

In addition to finding out what the fish were doing, the researchers also looked at Environmental conditions within the Nagambie Lake system.

A series of continuous temperature loggers were installed at a number of places around the lake they collected other information on a monthly basis, which further described the quality of the lake for golden perch and Murray cod.

Both Murray cod and golden perch spawn when water temperatures are around 23°C. The researchers found that water temperatures suitable for spawning were achieved in the backwater habitats during summer but not in the deeper water channel habitats where the released fish spent most of their time.

The researchers also noticed that while the water temperatures began to increase from mid-September, releases of irrigation water over the summer months caused water temperatures across the lakes system to drop. During the study, in January 2003, water temperatures across system decreased by 6 to 9°C over eight days.

Unseasonal temperature fluctuations like these are likely to further reduce the ability of golden perch and Murray cod to successfully reproduce within the Nagambie Lakes.

The scientists also described the quantity and variety of food species that were available for golden perch and Murray cod within the lake system. Crustaceans such as yabbies, freshwater prawn and shrimp and small fish such as flat-headed gudgeon, carp gudgeon and smelt presented a smorsgeboard of choice for both golden perch and Murray Cod.

The food available to golden perch and Murray cod within the Nagambie Lakes system was less abundant than in the lower Goulburn River.

By asking the fish, scientists were able to identify the following factors that probably prevented Murray cod and golden perch from successfully establishing selfsustaining populations in the Nagambie Lakes system:

• Fish were moving upstream into the river and
downstream via irrigation channel outlets;
• The limited extent of deep-water habitat with instream
structures such as large woody debris;
• Cold incoming water causing rapid temperature
fluctuations in spring and summer; and
• A lack of connectivity between the backwater and
the deep-water channel habitats, thereby reducing
access to potential spawning and nursery habitat.

Recreational Fishing Licence revenue funded this research project. For further information about this project please contact Justin O’Mahony, at Arthur Rylah Institute for
Environmental Research on (03) 9450 8600.

Contact the FRAC Secretariat during business hours on 9658 4369.