Risk Assessment for Stocking Selected Recreational Fish Species into Lake Colac
Lake Colac is a large, shallow, eutrophic, brackish-freshwater lake, primarily fed by localized rainfall and was formed by early volcanic activity. Lake Colac is approximately 2670ha in area with a mean water depth of 2.4m (Khalifé 2006). The sediment of Lake Colac comprises mainly black-grey clay, with silt, clayey sand and sandy clay, plus small occurrences of fine to coarse sand and gravel (Khalifé 2006). Two major tributaries, Deans Creek and Barongarook Creek, plus the Barwon Water sewage treatment plant, are the main sources of inflow to the lake. Each of these flows into Lake Colac on the southern shore of the lake (Figure 1). Surface outflow is restricted to wet seasons when Lough Calvert connects Lake Colac with the Barwon River (Figure 1). Flood frequency has a 25 year recurrence interval and last occurred in 1998. Water level may also be regulated artificially at lower lake levels.
Lake Colac is classified as "Poorly Degraded", primarily due to its water quality attributes, under Sate Environment Protection Plan (SEPP) objectives. Furthermore, the sediment of Lake Colac is classified as moderately to grossly polluted. The major sources of pollutants to the lake include runoff and/or discharge from urban, industrial and agricultural landuses. For example, the mean ammonia concentration in water discharged to Lake Colac from Barwon Water's sewage treatment plant is 15800μg/l (Khalifé 2006). Lake Colac also experiences high nitrogen and phosphorus concentrations and is subject to periodic blue-green algal blooms, the most recent recorded in 2005 (Khalifé 2006).
Despite the poor environmental status of Lake Colac, recreational fishing has historically been a major activity undertaken in the lake and the recent end to prolonged drought in western Victoria has provided the opportunity to explore recreational fish stocking opportunities for Lake Colac. To this end the Association of Geelong and District Angling Clubs has contracted Aquaprime Consulting to undertake a risk assessment for the stocking of a selected recreational fish species for Lake Colac. The risk assessment is preceded by a shortlisting process of possible candidate species from which a preferred candidate was selected, and for which the risk assessment process was undertaken.
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