Movements of Diadromous Fish in Large Unregulated Tropical Rivers Inferred from Geochemical Tracers

Patterns of migration and habitat use in diadromous fishes can be highly variable among individuals. Most investigations into diadromous movement patterns have been restricted to populations in regulated rivers, and little information exists for those in unregulated catchments. We quantified movements of migratory barramundi Lates calcarifer (Bloch) in two large unregulated rivers in northern Australia using both elemental (Sr/Ba) and isotope (87Sr/86Sr) ratios in aragonitic ear stones, or otoliths. Chemical life history profiles indicated significant individual variation in habitat use, particularly among chemically distinct freshwater habitats within a catchment. A global zoning algorithm was used to quantify distinct changes in chemical signatures across profiles. This algorithm identified between 2 and 6 distinct chemical habitats in individual profiles, indicating variable movement among habitats. Profiles of 87Sr/86Sr ratios were notably distinct among individuals, with highly radiogenic values recorded in some otoliths. This variation suggested that fish made full use of habitats across the entire catchment basin. Our results show that unrestricted movement among freshwater habitats is an important component of diadromous life histories for populations in unregulated systems

Habitat partitioning and vulnerability of sharks in the Great Barrier Reef Marine Park

Sharks present a critical conservation challenge, but little is known about their spatial distribution and vulnerability, particularly in complex seascapes such as Australia's Great Barrier Reef Marine Park (GBRMP). We review (1) the distribution of shark species among the primary habitats of the GBRMP (coral reefs, inshore/shelf, pelagic and deep-water habitats) (2) the relative exploitation of each species by fisheries, and (3) how current catch rates interact with their vulnerability and trophic index. Excluding rays and chimaeras, we identify a total of 82 shark species in the GBRMP. We find that shark research in the GBRMP has yielded little quantitative information on most species. Reef sharks are largely site-fidelic, but can move large distances and some regularly use non-reef habitats. Inshore and shelf sharks use coastal habitats either exclusively or during specific times in their life cycle (e.g. as nurseries). Virtually nothing is known about the distribution and habitat use of the GBRMP's pelagic and deep-water sharks. At least 46 species (53.5 %) are caught in one or more fisheries, but stock assessments are lacking for most. At least 17 of the sharks caught are considered highly vulnerable to exploitation. We argue that users of shark resources should be responsible for demonstrating that a fishery is sustainable before exploitation is allowed to commence or continue. This fundamental change in management principle will safeguard against stock collapses that have characterised many shark fisheries

Response of diurnal and nocturnal coral reef fish to protection from fishing: An assessment using baited remote underwater video

Diel variation is known to alter the compositionand structure of reef fish assemblages. What is unknown ishow nocturnal fish assemblages respond to closed fisheryarea management. Diurnal and nocturnal reef fishassemblages at the Houtman Abrolhos Islands, WesternAustralia, were studied using baited remote underwaterstereo-video systems (stereo-BRUVs). Surveys were conductedduring the day and at night (using blue illumination)from three sites inside and three sites outside a closedfishery area (CFA). The relative abundance of 116 fishspecies from 41 families was recorded. Significant changeswere observed in fish assemblage structure from day tonight (driven by high dispersion at night) and in areas openversus closed to fishing (driven by increased abundancewithin the CFA). The effect of protection from fishing wasconsistent for both diurnal and nocturnal fish assemblages.Closer examination of six targeted fish species showed that their response to diel changes and the absence of fishingpressure varied from species to species. The targeted fishesPagrus auratus and Glaucosoma hebraicum were sampledduring the day and at night with both species respondingpositively to protection from fishing. Results suggest thatthe inclusion of diurnal and nocturnal fish assemblage datawill provide an improved ability to assess the effectivenessof closed fishery area management

Targeted demersal fish species exhibit variable responses to long-term protection from fishing at the Houtman Abrolhos Islands

Natural fluctuations in the abundance and length of targeted fish are often disrupted by acute environmental changes and anthropogenic impacts, particularly fishing pressure. Long-term assessments of targeted fish populations inside and outside areas closed to fishing are often necessary to elucidate these effects, yet few of these studies extend over long time periods. We assessed trends in the abundance and length of six targeted fish species in areas open and closed to fishing on seven occasions spanning a 9-year period (2005–2010 and 2013) at the Houtman Abrolhos Islands, Western Australia. Shallow (8–12 m) and deep (22–26 m) coral-dominated reef sites were sampled across four geographically separated island groups using baited remote underwater stereo-video (stereo-BRUV). Between 2005 and 2010, populations of Lethrinus miniatus, Lethrinus nebulosus, Plectropomus leopardus, and Chrysophrys auratus became increasingly dominated by larger individuals, potentially indicative of an ageing population. Between 2010 and 2013, however, there was a significant increase in the proportion of smaller L. miniatus, L. nebulosus, and P. leopardus in both open and closed areas, reflecting increased recruitment perhaps due to changing environmental conditions associated with a marine heat wave anomaly.This recruitment pulse was not observed for the other species in this study (Chr. auratus, Choerodon rubescens, and Glaucosoma hebraicum). Lethrinus miniatus, L. nebulosus, Chr. auratus, and P. leopardus were larger in closed areas relative to open areas; however, they were not more abundant. These complex responses to protection also varied across sampling years for certain species (e.g., P. leopardus). Monitoring changes over the long-term in areas open and closed to fishing provides a sound basis for separating environmental variability from that associated with fishing mortality, which is crucial for optimising fisheries management

Contrasting patterns of reef utilization and recruitment\ud of coral trout (Plectropomus leopardus) and snapper (Lutjanus carponotatus) at One Tree Island, southern Great Barrier Reef

Patterns of abundance, age structure and\ud recruitment of coral trout (Plectropomus leopardus) and\ud snapper (Lutjanus carponotatus) were described in different\ud environments, which varied in benthic cover, in a 12-yr study\ud at One Tree Island. It was hypothesized that both taxa would\ud show strong preferences to different environments and\ud benthic cover and that patterns would be consistent through\ud time. Plectropomus leopardus were abundant on the reef\ud slope and seaward edge of the lagoon, where live coral cover\ud was high, and recruitment was generally low, in all environments.\ud The population was sustained by a trickle of\ud recruits, and total abundance varied little after 10 to 25 yr of\ud protection in a no-take area, suggesting P. leopardus had\ud reached an environment-related carrying capacity. Protogynous\ud P. leopardus recruited to shallow environments at\ud sites with 20% or more hard live coral and age data indicated\ud the abundance of fish on the reef slope was from redistribution.\ud Most recruits of gonochoristic L. carponotatus\ud (\150 mm Standard length, SL) were found in the lagoonal\ud environments, and adults were rare on the reef slope.\ud Abundance of recruit L. carponotatus and P. leopardus did\ud not correlate with percent cover of live and soft coral within\ud environments. Recruits of L. carponotatus were usually rare\ud in all lagoonal environments, but in 2003, many recruits (80\ud to 120 mm SL) were found in lagoonal environments with\ud low and high hard live coral cover. A substantial proportion\ud of the population (age max 18 yr) was from strong\ud recruitment events. In 2003 and 2004, total abundance of\ud L. carponotatus was supported by 1 year class 51.7 and 41%\ud respectively. The utilization of environments and types of\ud substrata varied among taxa and in some cases among lifehistory\ud stages. There was also temporal variation in the\ud importance of some environments (e.g. Lagoon Centre)