The ecology of shark-like batoids: implications for management in the Great Barrier Reef region

Shark-like batoids are a group of elasmobranchs with a body form similar to that of sharks (i.e. elongate body, well developed caudal and dorsal fins), but with head, gill and mouth morphology similar to that of skates and stingrays. Severe population declines of and reduction in geographic distribution throughout the South-East Asian portion of their range suggest Glaucostegus typus and Rhynchobatus spp. are vulnerable to depletion by unregulated take in fisheries. Shark-like batoids in fisheries have been poorly studied in comparison to sharks and data on these populations is severely lacking. Given the value of their fins, fishing is likely to continue in many locations. This research provides knowledge of their life history, ecology and how they interact with fisheries. This information is essential for informing decision making tools and the development of successful management strategies. Between 2007 and 2009, an on board vessel observer program was conducted to examine the composition of elasmobranch catch in Queensland's east coast inshore finfish fishery (ECIFF). Of the batoids, G. typus were most frequently caught in intertidal habitats, whereas Rhynchobatus spp. dominated the catch in inshore coastal habitats. Comparison of gill-net catches to research long-line sampling showed that not all size classes of shark-like batoids are captured by the gill-net fishery. Current mesh size restrictions in Queensland's gill-net fisheries limited interaction with G. typus to juveniles. Given that home-range size and habitat use by elasmobranchs can vary between ontogenetic stages and species, vulnerability to fisheries may vary depending on overlap of preferred habitats and fishing activity, and whether each size class is susceptible to the gear. Given the naturally low abundances of shark-like batoids, ecological sensitivity and structural complexity of some habitats within the Great Barrier Reef Marine Park (GBRMP), the use of conventional fisheries independent sampling across such a large geographic area would be impossible. The applicability of baited remote under-water video station (BRUVS©) to determine the distribution of elasmobranchs at broad spatial scales (12° latitude) and across a range of habitat types was assessed. This broad assessment allowed examination of distribution at the ecosystem scale in environments outside of those typically targeted by commercial gill-net fisheries. Shark-like batoids were observed across a wide depth range (10.4 - 87.5 m), showing highest affinity for 30 - 40 m. The depth range of highest affinity is outside the maximum depth at which the ECIFF operates. It is likely that preference for these deeper habitats may limit exposure of Rhynchobatus spp. to the commercial gill-net fishery. There was no difference in the proportion of BRUVS© in which shark-like batoids were observed between fished and unfished marine park zones. BRUVS© results suggest that marine protected areas (MPAs) may be of limited benefit to mobile habitat generalists like Rhynchobatus spp. Baited remote under-water video station surveys offer a standardized, non-extractive technique for quantifying the spatial distribution of mobile species that are difficult to sample using conventional techniques across broad spatial scales. The utility of BRUVS© was severely limited in high turbidity environments, such as inshore coastal embayment's within the GBRMP. Acoustic telemetry was used in these habitats to examine the space use and residency of shark-like batoids. Glaucostegus typus were monitored between 1 and 766 days (mean = 333 ± 69 days) and were present in the site from 1 to 198 days (mean 73 ± 25 days). Both adult male and female G. typus exhibited philopatric behaviour patterns, leaving the bay and returning after periods of about 9 -12 months to use the same areas where they were detected in previous years. Strong site fidelity observed in adult G. typus suggests that nearshore areas are a key component of the species' spatial ecology, and may form critical habitat. Strong philopatry evident in adult G. typus suggest this species may benefit from the use of discrete areas of protection in inshore coastal waters during austral summer months. Rhynchobatus spp. were monitored for 1 to 707 days (mean = 231 ± 50 days) and were present in the site from 1 to 350 days (mean 82 ± 24 days). Rhynchobatus spp. exhibited no synchronicity in use of the bay and size had no effect on residency. These findings support those from the BRUVS©, Rhynchobatus spp. is likely a habitat generalist with any patterns of spatial ecology possibly diluted by the presence of three species. The life histories of G. typus and Rhynchobatus spp. captured in the ECIFF were examined using vertebral ageing. The sigmoid growth functions, Gompertz and logistic, best described the growth of Rhynchobatus spp. and G. typus, providing the best statistical fit and most biologically appropriate parameters. The 2-parameter logistic was the preferred model for Rhynchobatus spp. with growth parameter estimates (both sexes combined); L(∞) = 2045 mm STL; k = 0.41 yr⁻¹. The 2-parameter logistic growth model was also the preferred model for G. typus with growth parameter estimates (both sexes combined); L(∞) = 2770 mm STL; k = 0.30 yr⁻¹. Annual growth-band deposition could not be excluded in Rhynchobatus spp. using mark-recaptured individuals. A single growth curve has been proposed for the Rhynchobatus spp. complex, given biological samples have been pooled between complex members. Further the complex is currently managed as a single species and a single growth model may prove useful in informing future management strategies. Although morphologically similar G. typus and Rhynchobatus spp. have differing life histories, with G. typus longer lived, slower growing and attaining a larger maximum size. Sensitivities to uncertainties in biological parameters and vulnerability to fisheries depletion of G. typus and Rhynchobatus spp. were examined using population matrix models. Unfished G. typus and Rhynchobatus spp. had robust populations with positive population growth (λ), of 1.38 yr⁻¹ and 1.27 yr⁻¹ and generation times between 7.05 and 7.54 years, respectively. Increasing longevity of G. typus had little effect on rates of population increase, while increasing natural mortality by 50 % reduced rates of population increase λ, from 1.55 yr⁻¹ to 1.22 yr⁻¹. Increased age at maturity produced the lowest rates of population increase λ, 1.13 yr⁻¹ and longest generation times (12.0 years). Rhynchobatus spp. were robust to increases in longevity; however increasing natural mortality by 50 % rapidly decreased rates of population growth λ, from 1.28 yr⁻¹ to 1.01 yr⁻¹. Models with earlier ages at maturity had higher rates of population growth λ = 1.27 yr⁻¹ in comparison to models with older ages at maturity (λ, = 1.04 yr⁻¹). Population models were also used to examine the effectiveness of alternative management strategies to the current initiatives in place in Queensland waters. Minimum size limits performed best for G. typus, while maximum size limits were the most beneficial management strategy for Rhynchobatus spp. The difference in strategies was the result of variation in life history between the species. Based on the results of demographic analysis, size limits may be a suitable management tool for shark-like batoids assuming they could be effectively implemented and may offer greater protection than current bag limit strategies that are in place for Rhynchobatus spp. The findings from this dissertation highlight the need for species specific investigations of biology, spatial ecology and fisheries interaction. Despite morphological similarities between focal species their habitat use and biology are different. This dissertation has provided substantial information necessary in both the assessment of shark-like batoid vulnerability within the GBRMP in addition to the development of targeted species-specific management strategies. Managing for individual species in multi-species fisheries is challenging. Understanding how and where fisheries interactions occur and the vulnerability of populations to these interactions is essential if bycatch species are to receive effective targeted management in mixed species fisheries