Assessing the life history, ecological role and spatio-temporal movements of a neritic predator, the bronze whaler (Carcharhinus brachyurus)

Author: Michael Drew

Drew, Michael, 2018 Assessing the life history, ecological role and spatio-temporal movements of a neritic predator, the bronze whaler (Carcharhinus brachyurus), Flinders University, College of Science and Engineering

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Many sharks are upper trophic level predators with wide-ranging distributions that play an integral role in the connectivity, maintenance, and stability of food webs. One such species is the bronze whaler (Carcharhinus brachyurus), which is a large-bodied species with a primarily temperate and coastal distribution in the Northern and Southern Hemispheres. Their coastal and nearshore distribution across southern Australia results in an overlap with anthropogenic stressors. However, information is limited for this wide-ranging predator and its vulnerability to the effects of fishing is poorly understood.

The overarching aim of this study was to assess the biology, ecological role, and movement of bronze whalers in temperate waters off southern Australia. I estimated the life history characteristics of bronze whalers by using vertebral growth counts to estimate age and biological data sampled from a seasonal commercial fishery. The ecological role and foraging ecology of bronze whalers in southern Australia was investigated by applying a combination of dietary assessment techniques, including stomach content and stable isotope analyses of muscle and liver tissue. Application of passive acoustic telemetry revealed insights into spatio-temporal patterns of occurrence, philopatry, and habitat use of juvenile bronze whalers in the inverse estuarine waters of northern Gulf St. Vincent. Broad-scale horizontal movements, and thermal and depth preferences of adult female bronze whalers were investigated using pop-up satellite archival tags.

Bronze whalers exhibited slow growth and a late ages-at-first maturity of 16 years for both males and females. Males matured at a smaller size than females. Fecundity estimated from four litters was low, ranging from 14 to 26 pups. Maximum age estimates were high for males and females at 25 and 31 years, respectively. Bronze whalers were identified as a generalist predator with a diverse prey field. Regional variations in prey diversity was evident with cephalopods (Sepia novaehollandiae and Sepioteuthis australis) and Australian sardine (Sardinops sagax) identified as the most important prey species. Fifty-six bronze whalers tagged with acoustic transmitters were monitored between 2009 and 2014 and showed a peak in seasonal presence in spring–early autumn (September–April). Philopatry to the study site was strong, with 77% of tagged sharks detected over multiple years. Time spent in the array was limited with a low estimate of mean residency index (mean Ri = 0.05 ± 0.01). Explanatory modelling indicated that water temperature and season had the most significant effect on presence, and juveniles exhibited the highest affinity to seagrass habitats (Posidonia spp.). Ten large bronze whalers (9 females and 1 unknown sex) ranging from 200–320 cm total length were tracked for 5–180 days (mean 106.5 ± 25.2 days) using pop-up satellite tags. Sharks tracked for >60 days moved from inshore waters (<50 m depth) to offshore shelf habitats (50–130 m depth) in late autumn, coinciding with the cooling of gulf and inshore coastal water temperatures. The maximum depth inhabited was 129 m, with four sharks inhabiting depths to >100m. Five sharks conducted broad-scale horizontal movements that covered distances ranging from ~200 km to coastal migrations of 1,600 km.

The findings of my study suggest that bronze whalers forage across several trophic levels and therefore play an important role in the ecosystem dynamics of temperate southern Australia gulf and shelf waters. The bronze whaler is a wide-ranging species that exhibit multi-jurisdictional movements from South Australia to Southeast Victorian and Western Australian waters. Sex- and stage-based segregation was evident within this population with shifts in habitat use occurring through life stages. Importantly, their predictable seasonal occurrence, coastal and neritic distribution, and slow life history traits combine to make the species potentially vulnerable to anthropogenic effects. The similarity in the life history characteristics of this species to the sympatric, highly migratory dusky shark (C. obscurus) which previous studies suggest is one of the most vulnerable shark species to extinction, highlights the need to ensure management and conservation improvements are considered for this important temperate marine predator. The findings of this study provided critical baseline biological and ecological information, that will be integrated into population assessments and ecosystem-based models recently developed for the southern Australian coastal, and shelf and oceanic waters.

Keywords: copper shark, ecology, telemetry, diet, life history

Subject: Biology thesis

Thesis type: Doctor of Philosophy
Completed: 2018
School: College of Science and Engineering
Supervisor: Charlie Huveneers