Assessing the Population Status of the Endangered Booroolong Frog (Litoria booroolongensis) in Areas Subject to Non-Native Trout Stocking. Prepared by David Hunter and Rod Pietsch Biodiversity Conservation Section Department of Environment and Climate Change (NSW) PO Box 2115, Queanbeyan NSW 2620 Emails: [email protected] [email protected] Unpublished Report to the NSW Department of Primary Industries July, 2008 Trout Stocking and the Endangered Booroolong Frog Summary The range of the Booroolong frog (Litoria booroolongensis) has greatly contracted over the past 25 years, and as a consequence, this species is currently listed as endangered at both the State and National level. A number of factors have been suggested as contributing to this decline, including; disease caused by infection with the amphibian chytrid fungus (Batrachochytrium dendrobatidis), habitat degradation, and predation by exotic fish species. This study was contracted to assess the likely impact of trout stocking on the Booroolong frog in the South West Slopes region of New South Wales. There were two primary components to this study: The first was to assess whether the Booroolong frog has persisted in areas where trout stocking has been undertaken in recent years. The second was to assess the infection status of Booroolong frog populations for the amphibian chytrid fungus, as the transport and release of fingerling trout has the capacity to spread this potentially virulent amphibian pathogen. We found Booroolong frogs present along sections of stream immediately adjacent to, or within, areas where trout stocking currently occurs. Given the long history of trout stocking in these areas, this result suggests that Booroolong frogs are capable of persisting in the presence of trout stocking. This conclusion does not discount the possibility of more subtle impacts, or that the relative impact of stocking in the future may not increase. I also found low to moderate levels of infection with the amphibian chytrid fungus in Booroolong frog populations subject to trout stocking. While this result demonstrates that trout stocking will not introduce the amphibian chytrid fungus into areas that are free of this pathogen, there is still the possibility that stocking may spread different strains of this pathogen. If trout stocking is to continue in streams supporting populations of the Booroolong frog, then the following recommendations should be considered: 1) Monitor the persistence of Booroolong frog populations along sections of streams where trout stocking is undertaken as well as at paired sites on streams that are not stocked. 2) Undertake research to determine whether fish stocking may introduce potentially harmful strains of the amphibian chytrid fungus into Booroolong frog populations. NSW Dept. of Environment and Climate Change Trout Stocking and the Endangered Booroolong Frog Table of Contents: 1 Background:.......................................................................................................... 1 1.1 Potential Impact of Non-Native Salmonid Stocking on Amphibians................ 1 1.2 Decline and Conservation of the Booroolong Frog.......................................... 2 1.3 General Scope and Aims of this Study. ........................................................... 3 2 Methods: ............................................................................................................... 4 2.1 Study Area and Survey Localities ................................................................... 4 2.2 Spotlight Surveys............................................................................................ 6 2.3 Habitat Measurements .................................................................................... 6 2.4 Field Swabbing for Pathogen Testing.............................................................. 6 2.5 Statistical Analysis.......................................................................................... 6 3 Results................................................................................................................... 7 3.1 Presence absence surveys and extent of potential breeding habitat. ................. 7 3.2 Results of swabbing for the amphibian chytrid fungus. ................................... 8 4 Discussion............................................................................................................. 8 4.1 Direct Population Level Impacts of Trout Stocking......................................... 8 4.2 Potential for Trout Stocking to Exacerbate the Impact of the Amphibian Chytrid Fungus...................................................................................................... 9 4.3 Implications of these findings to the conservation management of the Booroolong frog. ..................................................................................................10 4.4 Summary of Specific Recommendations........................................................10 5 References.............................................................................................................11 Acknowledgements: This study was funded by the Recreational Fishing Trust, NSW Department of Primary Industries. This study was covered under a NSW NPWS scientific license (No. S11409), and a NSW DECC Animal Ethics license (No. 041025/02). Craig Watson and Cameron Westaway provided valuable advice and comments on a draft of this report. We are particularly thankful to the property owners who granted us access to undertake the surveys in this study. NSW Dept. of Environment and Climate Change Trout Stocking and the Endangered Booroolong Frog 1 Background: 1.1 Potential Impact of Non-Native Salmonid Stocking on Amphibians. Predatory fish species can be an important factor regulating amphibian populations through predation on the eggs and tadpoles (Alford 1999). The capacity for some amphibian species to breed in aquatic environments occupied by predatory fish is typically the result of coevolution between predator (fish) and prey (tadpole) resulting in tadpole defence mechanisms to avoid being eaten by the fish (Kats et al. 1988, Sih et al. 1988, Werner and McPeek 1994). A number of experimental studies have shown that tadpole defence mechanisms are often predator or predator group specific, and may have limited effectiveness against fish species for which there has been minimal co-evolutionary history (Kats et al. 1988, Gillespie 2001). Hence, it is not surprising that the introduction of a non-native predatory fish species may result in the decline of native frog populations (Denoel et al. 2005). The release of non-native fish species may also have indirect impacts on amphibian populations. Fish predation may exclude species of amphibian from some habitats or sections of stream, resulting in greatly reduced and fragmented populations which are more prone to loss of genetic variation or local extinction (Bradford et al. 1993, Shaffer et al. 2000). Moreover, some diseases have been shown to infect both fish and amphibians and so the movement of fish to different water bodies may inadvertently result in the spread of pathogens that are harmful to amphibians (Kiesecker et al. 2001). Similarly, water used to transport fish may also contain pathogens that are harmful to amphibians, even if the fish themselves are not infected (Johnson and Speare 2003). There is also the potential for fish to compete with tadpoles for resources and so indirectly reduce their fitness (Resertarits 1995). Because of their popularity as a recreational angling species, several fish species within the family salmonidae have been extensively liberated outside their natural range. Salmonids are highly efficient predatory species, and so not surprisingly, their recent introduction into waterways in various parts of the world has been associated with the decline of native frog species (Mathews et al. 2001, Lowe and Bolger 2002, Bosch et al. 2006, Orizaola and Brana 2006). The capacity for non-native salmonids to regulate the population density of a native amphibian species was particularly well demonstrated by Vrendenburg (2004), who found that the removal of two salmonids from high altitude lakes in California resulted in the rapid recovery of Rana mucosa populations in those water bodies. While this study demonstrated a clear negative impact from the presence of introduced salmonids, it is also clear from correlative distributional studies and controlled experiments in artificial environments that susceptibility to salmonid predation may vary greatly among amphibian species (Gillespie 2001, Orizaola and Brana 2006). For over a centaury two salmonid species, brown trout, Salmo trutta, and rainbow trout, Oncorhynchus mykiss, have been widely released into rivers and dams in south- eastern Australia, and have established self-sustaining populations (McDowall 1996). Predation by introduced trout has been identified as a potential causal factor in the decline of one frog species in south-eastern Australia, the spotted tree frog, Litoria spenceri, through a combination of field observations, palatability experiments, and an in-stream experiment (Hunter and Gillespie 1999, Gillespie 2001). Since several other threatened frog species occur in streams where O. mykiss and S. trutta have become established, it has been strongly recommended that further studies examining NSW Dept. of Environment and Climate Change 1 Trout Stocking and the Endangered Booroolong Frog the impact of introduced