Non Technical Summary

Cold water pollution has been recognised in the Murray Darling Basin Commission's (MDBC) Native Fish Strategy (2002) as a major threat to the viability of native freshwater fish in the Basin. Cold water pollution occurs below large dams that have a bottom valve offtake. Cold water from the bottom of the dam is released into the river downstream, cooling the river water often to as much as 10 - 12oC below its natural range.

Using funds from the Water Management Fund, an experimental facility was built on the banks of the Macquarie River just below Burrendong Dam in 1998 which allowed fish to be exposed to either cold water during releases from the dam, or to warm water at ambient temperature from a reservoir below the spillway. Experiments to test the effects of cold water pollution used juveniles of two species of native fish, silver perch and Murray cod. Three experiments were done on four aspects of fish biology - growth, survival, re-distribution and activity.

Juvenile silver perch grew substantially more, in both weight and length, in warm water than in cold water. Few silver perch survived in the cold channels, with numbers declining rapidly within the first 16 days of the experiment. In thermal gradient experiments offering a choice of warm or cold water, silver perch exhibited a strong preference for warm water. Juvenile Murray cod showed no major difference in growth between warm and cold channels and survival was high in both warm and cold channels. Murray cod showed a similar response to silver perch in thermal gradients, exhibiting a strong preference for warm water.

Consequences of cold water pollution include reduced recruitment to adult populations, forced re-distribution of fish populations to less favourable habitats and general declines in fish populations because of decreased survival or reduced reproductive output. Murray cod and silver perch showed some differences in their respective responses to cold water pollution suggesting that some species may be more detrimentally affected than others. Further experiments should be done on other native species (such as catfish and gudgeons).

Fish communities were sampled downstream of Burrendong Dam on the Macquarie River (which is affected by cold water pollution) and in the nearby Bogan River, (which is not affected). Samples were collected during the irrigation season, when cold water released from Burrendong Dam chills the Macquarie River downstream, and during the non-irrigation season, when both rivers follow natural seasonal temperatures. The abundance of fish did not change between non-irrigation and irrigation seasons in either river, but the fish communities in the Macquarie River were different from those in the Bogan River. These differences may be the result of the different thermal and flow regimes between the two rivers. There is strong evidence that the fish community in the Macquarie River is seriously degraded, and is affected by multiple disturbances. Consideration should be given to modifying the outlet works of Burrendong Dam to raise the temperature of water in the river downstream of the dam during irrigation releases as a first step towards improving conditions for native fish in this river. Consideration should be given to modifying the outlet works of other dams where cold water pollution has been demonstrated.

The results of this study identified several knowledge gaps, and it is recommended that further studies be done on the impacts of cold water pollution on other native fish species and on the interrelationships between fish, water temperature and aquatic habitats at a range of spatial and temporal scales. This would help identify the scales at which temperature effects occur and determine whether cold water pollution has a blanket infiltration throughout the river, or if there are regions of warmer water that are used by fish as thermal refuges. The effects of cold water on macroinvertebrates, algae and other trophic levels should also be investigated to provide a broader understanding of how temperature regimes influence the functioning of riverine ecosystems.