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Habitat use and trophic ecology of bronze whaler sharks (Carcharhinus brachyurus) in New Zealand

Sharks are among some of the most diverse and abundant top-level predators within marine ecosystems. Their predatory role in controlling the population size, distribution and behaviour of species within the lower levels of the food web make them an integral component in both coastal and oceanic communities. However, anthropogenic pressures are causing declines in many shark populations across their range, with the ecological consequences following the removal of top predators from marine communities yet to be fully understood. Insight into the dynamics of shark ecology is further complicated by the heterogeneity of variables regulating natural communities. Despite this, research on trophic cascades, and the direct and indirect effects of apex predators on successive levels of the food web has been achieved where top-level predators have been removed from systems. As a consequence of the difficulty in researching elusive species such as sharks in the marine environment, an understanding of the biology and ecology of many species remains limited. However, such information is critical if effective decisions on the management of shark species and the wider marine environment in which they interact, are to be made. Although considered common, the bronze whaler shark (Carcharhinus brachyurus) remains relatively unstudied. Found in warm temperate environments, this species is known to utilise coastal habitats during the summer months. However, little is known about their movements during the winter months when sightings and interactions with fishermen in nearshore waters cease. Using satellite-linked smart position or temperature transmitting (SPOT5) tags and stable isotope analysis, this research aims to investigate the habitat use and trophic ecology of bronze whaler sharks to establish their role as top-level predators within coastal New Zealand waters. This research further aims to build on current knowledge to ensure sound management decisions regarding this species, and to understand the possible ecological community effects if these predators are removed from local marine systems. Satellite telemetry has revolutionised the way in which scientists track highly mobile vertebrates, and continues to expand our understanding of the behaviour and distribution patterns of shark species across a range of environments and geographic scales. This study provides the first report of the use of satellite tags on bronze whaler sharks. Two mature female bronze whaler sharks were tagged with satellite-linked smart position or temperature transmitting (SPOT5) tags at the entrance to the southern Tauranga Harbour, New Zealand. Sharks were tagged during April, the beginning of the austral autumn, and tracked for up to 157 days until transmissions ceased towards the end of winter in September. While transmission periods were comparable to those found throughout the literature, transmission rates were low which could be a factor of the surfacing behaviour of this species. However, location accuracies exceeded those of other studies, providing viable tracks for analysis for both sharks. Movement and recapture data indicated a high level of residency and site fidelity to habitats within the north-eastern region of the North Island, New Zealand, suggesting bronze whaler sharks play an important top-down predatory role in structuring the food web dynamic in this region. This also indicated the potential for multiple segregated populations within New Zealand waters. Furthermore, a behavioural shift was evident with the onset of winter, with both sharks moving from inshore and shelf habitats during autumn, to primarily shelf-edge and oceanic habitats during winter. This movement coincided with a decline in coastal water temperatures below 15.1°C. The cessation of transmissions, likely due to biofouling, prevented the identification of a return migration to coastal environments the following summer. However, the data provided new evidence of the importance of shelf-edge and oceanic habitats to this species. Further understanding of their seasonal movements and feeding ecology would enhance our understanding of how their predatory influence may change across seasons and in turn, the implications for inshore and offshore prey assemblages. Stable isotope analysis presents an alternative method to track the movements and ecology of species, providing information of species interactions and trophic dynamics that cannot be determined through satellite tagging alone. This study examined stable isotope signatures δ13C and δ15N of muscle tissue of bronze whaler sharks in comparison with coastal and offshore teleost, invertebrate and algal species from the Bay of Plenty, New Zealand. A total of 40 bronze whaler sharks were caught from the Tauranga coastal region, ranging in length from 236–305 cm total length across the dorsal surface with the tail in a flexed position. Based on previous length at maturity studies, sharks in the data set were considered mature. Out of the 40 sharks captured, only two were male, supporting sex segregation in this species, and a strong association of females to inshore sites. Stable isotope analysis did not detect a change in δ13C and δ15N values with increasing length, informing that once bronze whaler sharks reached maturity, they fed off a similar food web and at a similar trophic level. A Bayesian mixing model was applied to estimate the contributions of potential prey species to the diet of bronze whaler sharks. The model identified kingfish as the most dominant prey source (mean = 50.7%), followed by piper (mean = 11.5%) which indicated the utilisation primarily of a coastal pelagic food web. Snapper also showed to be an important component, indicating trophic links also extended to reef and benthic habitats. The lack of a viable pelagic baseline signature prevented the ability to accurately calculate the trophic position of bronze whaler sharks. However, an examination of bivariate data from all species collected suggest a trophic position between 4 and 5, which was consistent with trophic position estimates previously calculated for bronze whaler sharks and other shark species of a similar size. This study adopted a multidisciplinary approach to investigate the movement patterns and trophic ecology of bronze whaler sharks captured from coastal sites within the Bay of Plenty. The use of stable isotope analysis can be a powerful tool to complement satellite tracking studies. However, in this case, each technique provided a somewhat contradictory result. It is evident that the ecology of this species is complex, with a larger, more diverse sample size over greater spatial and temporal scales required to further elucidate the many questions still pending over its habitat use and foraging behaviour. Such information is crucial if we are to fully understand the ecological role this species plays as a top-level predator within local marine communities, and its implications for the wider management of valuable ecosystems and fisheries within the region.
Type of thesis
Kellett, M. D. (2021). Habitat use and trophic ecology of bronze whaler sharks (Carcharhinus brachyurus) in New Zealand (Thesis, Master of Science (MSc)). The University of Waikato, Hamilton, New Zealand. Retrieved from https://hdl.handle.net/10289/14433
The University of Waikato
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