Ecology of the Australian bass in tributaries of the Hawkesbury-Nepean River

The Australian bass Macquaria novemaculeata, belonging to the family Percichthyidae, is a carnivorous fish endemic to Australia. Its distribution extends throughout Eastern-Australian streams from the Mary River (Queensland) to Lakes Entrance (Victoria). The focus of this study was to determine the ecological attributes of M. novemaculeata in open and obstacle strewn tributaries within the Hawkesbury-Nepean River Catchment; two unique habitats that M. novemaculeata commonly inhabit. Specifically, this study focused on the demography, diet, and migratory patterns of M. novemaculeata between open and obstacle-strewn tributary reaches. In addition, the attitudes and behaviours of recreational fisherman who target M. novemaculeata in their natural environment on the east coast of Australia were also investigated. Recreational fishing pressure on M. novemaculeata in their natural environment was obtained via a survey of recreational anglers by contacting fishing clubs and affiliates. The survey included angler gender, age class, State of residence, fishing effort specifically for M. novemaculeata, including years spent targeting the species, size of angling group, areas fished and watercraft used, how fishing sites were selected, the average size of M. novemaculeata caught, and the level of catch-release employed by anglers. In order to investigate demography data, diet and migratory patterns, overall 120 M. novemaculeata were caught by hand-held rod with lure in obstacle-strewn and open tributaries and euthanised. This provided a sample to collect data on length, weight, condition, age, growth, sex ratio, density, and in-stream habitat selection. Stomach contents of euthanised fish were analysed for diet. Otoliths of these fish were removed to determine age and migratory movement using a micro-chemical procedure, involving a laser ablation-inductively coupled plasma-mass spectrometer (LA-ICPMS). The average angler who targeted M. novemaculeata was male, between the ages of 25 and 34, had fished for this species for less than five years and typically fished with one companion. Those that target this species generally devote considerable fishing effort to their sport, identifying fishing spots commonly through online methods, releasing at least a portion of their catch and utilising watercraft to gain greater access to fishing grounds. Smaller fish were consistently captured in the Hawkesbury-Nepean River than other water bodies examined, likely as a result of the removal of larger fish. The few anglers that did take fish for consumption were more likely to fish in readily accessible areas. Despite the habitat differences evident between tributary types, parameters measured - fish length, weight, condition, age, growth, sex ratio, sex and growth, density and habitat selection - were not significantly different between open and obstacle-strewn tributaries. In both tributary types females were larger and more likely to be caught than males, and smaller fish were more likely to frequent habitat away from structure (e.g., coarse woody debris) than adult fish although, overall, fish of all sizes preferred habitat near structure. Such structure is, therefore, preferred habitat for this species. A comparison of fish length between tributaries and the riverine channel determined that longer fish typically occupied tributaries. This correlated to the female sex bias in the tributaries as this sex was observed to grow to a larger length than males. The study re-confirmed that M. novemaculeata is a generalist carnivore, opportunistically feeding on prey from a variety of taxonomic groups from both aquatic and terrestrial habitats. The tributary types studied offered dissimilar habitats for prey, which influenced the dietary consumption of M. novemaculeata. Diversity and abundance of prey was higher in the stomachs of fish from obstacle-strewn tributaries than open tributaries. Despite the variation in diet between habitats, however, this did not appear to affect this species ability to live and thrive in either habitat. This was evidenced by the lack of difference in the length, weight, condition and stomach fullness of fish. Although previous studies have shown that natural barriers to movement may result in fish assemblage changes between stream reaches, this does not appear to be the situation for M. novemaculeata in tributary habitat. There was no indication that individuals were impeded in their movement to the estuary to spawn from either obstacle-strewn or open tributaries. The reason for not moving to the estuary annually may, therefore, be related to other factors associated with the ecology of this species. This study found that the impacts of recreational fishing on M. novemaculeata are more likely to occur in easy to access areas, and this may have catchment-wide implications. Since entering the fishery almost half of respondents had switched to a more sporting focus when targeting this species. However, regardless of motivation, catch-and-release angling, coupled with the high survivorship of this species after angling release is likely to play a key role in management of this species. Maintaining tributary populations of M. novemaculeata is desirable for the species sustainability as it is apparently preferred habitat for females based on the observation that they are in greater abundance in tributaries. Tributary habitat thus would serve to maximise M. novemaculeata recruitment. The apparent importance of the riparian zone to primary and secondary production and subsequently fish diet, highlights the complexities in managing M. novemaculeata habitat for the species long-term sustainability. Degradation of the riparian zone at the aquatic/terrestrial interface is likely to result in reduction in the availability of terrestrial prey, and also indirectly aquatic prey, through a loss of allochthonous input. The observation that Macquaria novemaculeata from the different tributary types made equivalent numbers of estuarine visits, indicated that any fish taken by anglers is unlikely to differentially impact on populations in the two tributary habitat types studied