The feasibility of excluding alien redfin perch from Macquarie perch habitat in the Hawkesbury-Nepean Catchment
Knight J (2010) The feasibility of excluding alien redfin perch from Macquarie perch habitat in the Hawkesbury-Nepean Catchment. Final report to the Hawkesbury-Nepean Catchment Management Authority for Project No. HN 0507 B6D. Industry & Investment NSW – Fisheries Final Report Series No. 121. Port Stephens, NSW, Australia. 53pp.
This report assesses the feasibility of excluding alien (i.e., non-native) redfin perch from a feeder tributary of Warragamba Dam in the Hawkesbury-Nepean catchment that supports threatened Macquarie perch. Objectives included to review the distribution and conservation status of Macquarie perch in the Hawkesbury-Nepean catchment; assess the techniques available for excluding redfin perch from Macquarie perch habitat within the Warragamba Dam study area; determine the potential effects of an exclusion device on the local fish populations; identify the most appropriate location within the study area for an exclusion device; and determine the approvals required to construct a device.
The Macquarie perch is an endangered freshwater fish native to the cooler middle-upper reaches of the Murray-Darling Basin and several eastern coastal streams including the Shoalhaven, Georges and Hawkesbury-Nepean River systems. It has a fragmented distribution within the latter coastal system and often occurs in low numbers. Redfin perch are heavily implicated in its decline through predation on young fish, competition for space and food, and the transfer of disease. Macquarie perch populations considered to be immediately threatened by the recent invasion of redfin perch into the Hawkesbury-Nepean catchment are those currently known to inhabit the feeder tributaries of Warragamba Dam, including Kanangra Creek and the Cox’s, Kowmung, Kedumba and Little Rivers. Intervention is required to prevent redfin perch from invading these tributaries.
Many of the techniques available for excluding redfin perch were not suitable for installation within the study area because of an inability to effectively exclude this species; a lack of knowledge on their effectiveness; their potential effects on non-target species and the environment; the unavailability of electricity required to power behavioural modification devices; and the costs associated with constructing and maintaining some technologies. The most appropriate device was deemed to be a velocity barrier. This technique is considered 100% effective at excluding redfin perch and has been successfully used in Tasmania for this purpose, with design specifications available. It is also relative inexpensive to construct and maintain, and has relatively minor environmental impacts in comparison to some of the other techniques available.
Of the 17 fish species recorded in the Warragamba Dam study area, four gudgeons, Australian smelt and Macquarie perch were considered to be potentially impacted by an exclusion device such as an in-stream barrier. These impacts could be mitigated, however, by installing a velocity barrier designed specifically to exclude redfin perch while allowing other species of fish to pass. The device should be placed downstream of the known limits of the resident Macquarie perch population to reduce the risk of population fragmentation and disruption of instream movements.
Rucksack Ridge Ford on the Kedumba River was considered the most suitable of the eight sites assessed for the installation of an exclusion device. Human-assisted introductions of redfin perch upstream of this site were considered unlikely given the remoteness of the surrounding catchment; fragmentation of the resident Macquarie perch population was unlikely; there was direct vehicle access; and the site had a range of geomorphological and hydrological features conducive to the effective exclusion of redfin perch and to the cost-effective construction, maintenance, and monitoring of an exclusion device. Given its location in the lower reaches of the Kedumba River, this site would also exclude redfin perch from much of the river, thereby maximising the size of the refuge habitat for Macquarie perch and limiting disruption of instream movements.
A Review of Environmental Factors relating to the installation of an exclusion device must be submitted to the NSW Department of Environment, Climate Change and Water and the Sydney Catchment Authority as part of the environmental impact assessment process.