Scientists believe they have discovered a number of environmental conditions affecting Murray cod and golden perch populations in the Nagambie Lakes, north of Melbourne.

A 12-month study, funded by Recreational Fishing Licence revenue, found fluctuating temperatures and food shortages can prevent Murray cod and golden perch establishing self-sustaining communities.

During the study researchers from the Department of Sustainability and Environment monitored the activity of 32 Murray cod (from a hatchery) and 15 golden perch that were released into the lake and fitted with radio transmitters.

DSE Research Scientist, Justin O’Mahoney said, “Temperatures decreased by nine degrees during spring and summer and these unseasonable temperature fluctuations may have reduced the ability of native fish to reproduce.

“Warm water native fish such as Murray cod and golden perch usually breed when temperatures average 20 degrees and above.

“During the study, temperatures warmer than 20 degrees were recorded during summer in
backwater habitats. Colder temperatures persisted in the deeper water channel habitats where the released fish spent most of their time under the cover of large woody debris.

“Factors such as turbulence, noise and water movement through these irrigation outlets appeared to greatly affect the movement of golden perch, and many of them were reluctant to stay in the lake and often approached the gates to exit into other waters.”

The study also found water in the backwaters remained stagnant and received very little flow and this lack of fresh water may have been a contributing factor in making the habitat unfavourable.

The golden perch sourced from the lower Goulburn River, adapted much better to the lake
environment.

Golden perch were highly mobile throughout the course of the study, with most venturing upstream and downstream. Only one fish remained in the location where it was released.

Many fish swam as far as 3 km to explore downstream gates and several left the lake system escaping through control gates on irrigation channels. Some of these fish have been found active several kilometres downstream.

 source:  www.fishvictoria.com