Distribution and Recruitment Patterns of Fish in the Lower Lachlan River

"December 2009".

Project Number: Fish Spawning & Recruitment - M/BUS/295.

MDFRC item.

The Lachlan catchment is a unique terminal system in the Murray-Darling Basin. Historically, a relatively diverse fish fauna has been recorded from the Lachlan River; however, native fish populations have declined and introduced fish species now dominate in many areas. The Lachlan Catchment Management Authority has identified a number of threats to the health of the system and the Lachlan Catchment Action Plan seeks to address many of these through a variety of management targets. Whilst the activities planned to achieve these management targets will certainly improve riverine ecosystems within the catchment, it will be critical to establish whether a viable, self-sustaining fish community is present, and to determine what factors may be responsible for fish declines, before undertaking significant works aimed at improving water quality, habitat and connectivity for native fish. This project represents one component of a larger project proposal aimed at investigating the distribution and long-term viability and sustainability of native fish populations in the Lachlan River through the examination of spawning and recruitment patterns. The current project aimed to investigate the distribution and recruitment patterns of fish and to assess fish condition in the Lachlan River downstream from Wyangala Dam. Twelve sites downstream from Wyangala Dam were sampled in February-March 2009. At each site, the fish community was surveyed using electrofishing and fyke netting and a range of water quality, nutrient, primary production, food availability and habitat variables were measured. A total of 3817 fish from nine species (six native and three introduced) were collected in the lower Lachlan River. Overall, samples were dominated by introduced species and the proportion of large-bodied native species was very low at all sites. Only four species, the introduced species common carp, eastern gambusia and goldfish, and the small-bodied native carp gudgeon were found to be widely distributed. The distribution of the remaining species was patchy and abundances were low. There was evidence of recent recruitment for all species except golden perch and flathead gudgeon. However, with the exception of eastern gambusia and carp gudgeons, recruitment was spatially patchy and numbers of recruits was low. The highest number of recruits overall and for carp gudgeons, carp, goldfish and bony herring was found at Braebuck Woolshed and the largest number of gambusia recruits were found at Corrong, suggesting that the lower end of the river system may be a recruitment 'hotspot' for a number of species. Fish condition was only able to be compared among sites for four of the species. Results indicated that fish condition varied among sites; however, no interpretable spatial patterns were evident. Examination of relationships between various fishrelated variables (species richness, total abundance of all fish, total abundance and number of recruits of the dominant species and for species for which enough data was available, fish condition) and the environmental and food availability variables revealed some significant relationships. Some relationships were found between gambusia and nutrient concentrations and food availability variables; however, the majority of the relationships found were between fish-related variables and hydrological variables. The nature of these relationships varied among species. On the basis of the results from this work and from recent and historical fish surveys undertaken in the lower Lachlan River, we propose that the current fish fauna of the lower Lachlan River can be classified into four groups based on their relative distribution and abundance patterns. The first group is characterised by species which are currently maintaining viable populations in the lower Lachlan River. This group is primarily comprised of introduced species. The second group is made up of six native species with patchy distributions and low to moderate abundances. It is unclear whether species in this group are maintaining viable populations at present. The third group is comprised of native species which, although present in low numbers, do not appear to be sustaining viable populations. The fourth group is made of native species which historically were recorded within the lower Lachlan River but now appear to be locally extinct. In the short-term, managing the current water crisis to minimise impacts on native fish will be critical. It is recommended that in the medium term, management activities are focussed on native species from the second group; that is species which may still have the capacity to maintain viable populations in the lower Lachlan River. Management options for these species include the provision of suitable flow regimes and habitat and enhanced connectivity for all life-stages. For species in groups three and four, restocking activities may be needed to achieve population sustainability. In order to maximise the long-term success of re-stocking activities, it would be desirable to ensure that the factors responsible for the original decline of these species have first been identified and addressed. However, there are inherent difficulties in determining the causal factors responsible for population decline for species whose abundances are already very low. Thus, it is recommended that any re-stocking programs that are undertaken be done in conjunction with carefully planned management activities and that these programs be designed and monitored in such a way that the causal factors responsible for possible future declines are able to be determined thus allowing for an adaptive management process to be implemented.