The mitigation of the negative impacts on biodiversity and fisheries values of the refurbishment of Waroona Dam, south-western Australia
AbstractPrior to the commencement of the refurbishment of Waroona Dam by the Water Corporation of Western Australia, a management plan to mitigate the negative effects of draining on the biodiversity and recreational fishing values within Lake Navarino (the reservoir created by Waroona Dam) was instigated. The plan, the first of its kind in Australia, was devised and implemented by collaboration between the Water Corporation, the Department of Fisheries, Murdoch University, Alcoa and volunteers to meet the following objectives; 1. De-stock Lake Navarino of large numbers of target species of aquatic fauna prior to draining (marron - Cherax tenuimanus, gilgie - Cherax quinquecarinatus, western minnow - Galaxias occidentalis, western pygmy perch - Edelia vittata, rainbow trout - Oncorhynchus mykiss and brown trout - Salmo trutta); 2. Establish breeding populations of marron, gilgies and endemic fishes in secure, purpose built facilities at the Pemberton Freshwater Research Centre (PFRC – Department of Fisheries) and Alcoa’s Willowdale marron farm (AWF) to produce animals for re-stocking into Lake Navarino following the completion of the refurbishment works; 3. Transport aquatic animals surplus to breeding requirements and trout to adjacent waters to boost recreational fishing opportunities; 4. Control feral species, principally redfin perch (Perca fluviatilis), within the residual coffer dams during refurbishment of Waroona Dam; 5. Install appropriate artificial habitats within the basin of Lake Navarino; and 6. Re-stock Lake Navarino with the ex-broodstock and progeny of marron, gilgies and native fishes, and eventually rainbow trout. The de-stocking of Lake Navarino was successful in providing adequate numbers of three species to establish breeding populations. As considerable numbers of targeted aquatic animals were removed prior to drainage, the scale of post-drain faunal mortality was reduced. The success of the de-stocking program was the direct result of the valuable participation of volunteers that facilitated a large amount of de-stocking effort in a short period of time. The high level of volunteer involvement highlights the community’s interest and support for such integrated natural resource projects. Overall, the captive breeding programs performed better than expected. At the PFRC, high berry-up (i.e. spawning) rates of marron, high production of juveniles, and high survival rates of juveniles and adults were reported. This is the first demonstrated re-use of marron broodstock, likely a result of proven, intensive husbandry practices. Further, the project involved utilising very large nursery ponds for the mass production of juveniles to an advanced stage. The use of existing ponds at AWF was largely aimed at providing a secure, cost-effective back-up facility at which considerable numbers of all target species could be maintained throughout the project as part of risk minimisation. That is, should an unforeseen catastrophic loss of stock have occurred (e.g. outbreak of disease) at the purpose-built PFRC ponds, considerable numbers of Lake Navarino brood-stock and juveniles would still be available to re-stock the reservoir from an alternate facility. The control of feral aquatic animals involved netting, draining and concussive (blasting) methods. The control techniques reduced the number of redfin perch by over 99%; however, 100% elimination was not achieved. This was likely due to small numbers of redfin perch avoiding the techniques employed (for example in the tributaries of Lake Navarino) or the larger than predicted coffer dams (residual pools maintained during the refurbishment of the Lake Navarino basin) reducing the effectiveness of those techniques. Nonetheless, the considerable reduction in the numbers of redfin perch, a major predator of juvenile marron and native fish, limited the predatory impact of this species on the captively-bred, newly stocked marron thus aiding their re-establishment in Lake Navarino. The addition of artificial aquatic habitat will also increase the survival rate of stocked marron. Along with the large extension of rock habitat on the inside of the refurbished wall of Waroona Dam as part of the refurbishment, five lines of rocks of approximately 30-100 metres in length were established in Lake Navarino. The rocks formed a large number of irregularly sized holes and crevasses suitable for marron and other aquatic species. The artificial habitats were also placed away from major ski-areas and were well below the minimum water depth required for skiing. The design and placement of the habitats achieved the objective of increasing aquatic habitat while meeting Water Corporation’s engineering requirements and not compromising other recreational values of Lake Navarino. Over 63,000 aquatic animals were re-stocked into Lake Navarino (the majority being marron during 2004) and far exceeded the predicted numbers to be re-stocked at the commencement of the project. The captively bred marron and gilgies ranged in both size and age and the captive breeding programs were designed to maintain as wide a genetic variation as possible, a major difference to the breeding approaches adopted for commercial aquaculture. This breeding aim was necessary in order to produce stocks of aquatic species with similar levels of genetic variation that were present in those populations prior to the project. Further, the reintroduction of multiple age classes (i.e. the offspring from two captive breeding events in 2002, 2003 and the original broodstock) aimed to re-establish populations similar to those present before the refurbishment of the dam commenced. The program will allow legal sized marron to be present in Lake Navarino when the first post-project marron season opens (i.e. in January 2005). Although western minnows did not breed in captivity, close monitoring revealed its continued presence in the major tributary, Drakes Brook (highlighting its importance), and thus this species would have not have been completely eliminated by the draining event. High survival of the broodstock in captivity allowed restocking of a considerable proportion of those de-stocked, with the remainder with-held for further breeding attempts. Overall, this unique project was extremely successful in mitigating the negative effects of draining for the refurbishment of a large sized irrigation dam on recreational fishing and biodiversity values. The success of the project was achieved via the participation of a diverse range of organisations their staff, and volunteers at all stages of the project. Although successful, a range of lessons were learned and a flow-chart of suggested management actions is included in this report to allow the Water Corporation, or other similar natural resource managers, to apply these techniques to other similar projects in the future. The long-term effectiveness of the current project will also be assessed via ongoing monitoring of the endemic species in Lake Navarino, in particular by using marron as the major indicator species.