Thomas C. Barnes

Shifts in fish assemblages indicate failed recovery of threatened species following prolonged drought in terminating lakes of the Murray–Darling Basin, Australia

Freshwater fishes are vulnerable to changes in water quality, physical habitat and connectivity resulting from drought, particularly in regulated rivers. When adequate river flows return, the recovery of populations might depend on the duration and consequences of drought. Rivers of the highly regulated Murray–Darling Basin in south-eastern Australia terminate at two large, shallow lakes that are separated from the estuary by tidal barrages. Over-abstraction of water and widespread prolonged drought (1997–2010) placed the lakes under severe environmental stress, culminating in critical water level recession from 2007 to 2010. Concurrently, most freshwater fish populations collapsed. We investigate shifts in fish assemblages resulting from habitat inundation in the lakes following the drought. The inundation and re-connection of the lakes and fringing habitats led to a substantial reduction of salinity throughout the region, and aquatic vegetation shifted from salt-tolerant to freshwater species. Fish assemblages became increasingly characterized by common freshwater taxa (ecological generalists), including high proportions of alien species. There were no indications of population recovery for three threatened species. The findings emphasize that short-lived fishes with specialized habitat requirements are vulnerable to severe population declines during prolonged drought in regulated rivers, which might restrict their recovery when adequate flows return.

Fish as proxies of ecological and environmental change

Anthropogenic impacts have shifted aquatic ecosystems far from prehistoric baseline states; yet, understanding these impacts is impeded by a lack of available long-term data that realistically reflects the organisms and their habitats prior to human disturbance. Fish are excellent, and largely underused, proxies for elucidating the degree, direction and scale of shifts in aquatic ecosystems. This paper highlights potential sources of qualitative and quantitative data derived from contemporary, archived and ancient fish samples, and then, using key examples, discusses the types of long-term temporal information that can be obtained. This paper identifies future research needs with a focus on the Southern Hemisphere, as baseline shifts are poorly described relative to the Northern Hemisphere. Temporal data sourced from fish can improve our understanding of how aquatic ecosystems have changed, particularly when multiple sources of data are used, enhancing our ability to interpret the current state of aquatic ecosystems and establish effective measures to safeguard against further adverse shifts. The range of biological, ecological and environmental data obtained from fish can be integrated to better define ecosystem baseline states on which to establish policy goals for future conservation and exploitation practices.

Piscivory by alien redfin perch (Perca fluviatilis) begins earlier than anticipated in two contrasting habitats of Lake Alexandrina, South Australia

Abstract. Redfin perch (Perca fluviatilis) introduced to the Southern Hemisphere has contributed to the decline or localised extirpations of native fishes, principally due to predation. It has been widely recorded in the Murray–Darling Basin, south-eastern Australia, since the 1920s but the ecological consequences are largely undetermined. The purpose of this study was to examine the diet of juvenile redfin perch in Lake Alexandrina to assess its potential impacts on native biota in two distinct habitats (channel and lake). We proposed that the broad dietary composition of juvenile redfin perch matches that of its natural range (small decapods and insects). Most juvenile redfin perch with prey items in their guts, however, had consumed native fish. There was variability in the diet of redfin perch between the channel and lake where gudgeons and gobies, respectively, were targeted. Unexpectedly, otolith ageing revealed that the redfin perch were larger and shifted to piscivory at a much younger age compared with populations in its native range. Among other ecological issues, the findings are pertinent to threatened small-bodied fish populations in the Murray–Darling Basin. More broadly, they suggest that a generalist feeding behaviour can lead to the early onset of piscivory in alien fish populations.

Prey selection and diet overlap of native golden perch and alien redfin perch under contrasting hydrological conditions.

Abstract. Many freshwater fishes have been introduced outside their natural range. The consequences have included the decline or extinction of native fishes, principally due to competition and predation. Redfin perch (Perca fluviatilis) is a highly efficient predatory fish species that was introduced to Australia in the 1800s. It now has a broad distribution in the Murray–Darling Basin, but its impacts on native fishes are largely unstudied. It often cohabits with native golden perch (Macquaria ambigua ambigua), which is similar from a trophic ecomorphology perspective. We examine prey selection and diet overlap of adult redfin perch and golden perch under contrasting hydrological conditions in terminating lakes of the Murray–Darling Basin. Prey selection by both species varied substantially between drought and flood conditions. Diet overlap of redfin perch and golden perch was significant only during flood, and was apparently related to pelagic prey availability. There were dietary differences during drought that imply that resource partitioning occurred between the perches, possibly because competitive interactions were intensified. Conversely, the promotion of pelagic prey fishes during flooding apparently facilitated resource sharing. The findings suggest that redfin perch can directly compete with native piscivores for prey. The potential impacts on native piscivores and small-bodied fish populations warrant further experimental and field investigations.

Population structure in a wide-ranging coastal teleost (Argyrosomus japonicus, Sciaenidae) reflects marine biogeography across southern Australia

Population structure in marine teleosts is often investigated to aid conservation and fisheries management (e.g. to assess population structure to inform restocking programs). We assessed genetic population structure of the important estuary-associated marine fish, mulloway (Argyrosomus japonicus), within Australian waters and between Australia and South Africa. Genetic variation was investigated at 13 polymorphic microsatellite markers. FST values and Bayesian estimates in STRUCTURE suggested population differentiation of mulloway within Australia and confirm strong differentiation between South Africa and Australia. The 12 Australian sample sets fell into one of four spatially separated genetic clusters. Initially, a significant signal of isolation-by-distance (IBD) was evident among Australian populations. However, further investigation by decomposed-pairwise-regression (DPR) suggested five sample sets were influenced more by genetic-drift, rather than gene-flow and drift equilibrium, as expected in strong IBD cases. Cryptic oceanographic and topographical influences may isolate mulloway populations from south-western Australia. The results demonstrate that DPR is suitable to assess population structure of coastal marine species where barriers to gene flow may be less obvious than in freshwater systems. Information on the relative strengths of gene flow and genetic drift facilitates a more comprehensive understanding of the evolutionary forces that lead to population structure, which in turn informs fisheries and assists conservation management. Large-bodied predatory scale-fish may be under increasing pressure on a global scale, owing to a variety of anthropogenic reasons. In southern Australia, the iconic sciaenid A. japonicus (mulloway, jewfish or kob) is no exception. Despite the species supporting important fisheries, much of its ecology is poorly understood. It is possible that a greater understanding of their genetic population structure can help ensure a sustainable future for the only southern Australian sciaenid.