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Incidental bycatch in fishing nets is a global cause of incidental mortality of marine mammals. Two classes of approaches attempt to mitigate this impact: (1) approaches that change the behaviour of the fisher (e.g. closures and gear modifications), (2) approaches that attempt to change the behaviour of the bycatch species (e.g. acoustic alarms or pingers). Even though the effectiveness of pingers has been established for very few bycatch species, pingers are now mandatory in many fisheries throughout the world. Pingers are being trialled in commercial gill net fisheries in tropical Australia to reduce the bycatch of the dugong and three species of coastal dolphins, despite an absence of robust assessments of: (1) their effectiveness in reducing bycatch, (2) the likelihood of alienating bycatch species from critical habitats. We conducted replicate experiments to test the behavioural responses of dugongs to 4 and 10 kHz pingers in an array simulating a net. Each experiment comprised three sequential 10-min treatments in which two pingers were: (1) inactive, (2) active, (3) inactive. The rate of decline of the number of dugongs within the focal arena did not change significantly while pingers were activated. Dugongs passed between the pingers (where a net would be located) irrespective of whether the alarms were active or inactive, fed throughout the experiments and did not change their orientation to investigate pinger noise, or their likelihood of vocalizing. We conclude that: (1) pingers are unlikely to alienate dugongs from critical habitats or reduce dugong mortalities in fishing nets, (2) bycatch mitigation strategies such as pingers that rely on changing animal behaviour should only be used after rigorous testing on all likely bycatch species.

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Stable isotope analysis of dermis was used to examine foraging behavior of whale sharks at Ningaloo Reef in Western Australia. Values of δ 13 C and δ 15 N in dermis were compared to those obtained from likely species of local prey. The δ 13 C values of zooplankton and nektonic taxa at Ningaloo ranged from −18.9 to −16.5 reflecting the different carbon sources (from pelagic to more inshore and benthic) entering the food web. Isotopic values also varied depending on the diet-to-tissue discrimination factor applied in the analysis. When data was corrected using factors derived from slow turnover, structural cartilage in fins, whale sharks showed a greater reliance on pelagic food webs, whereas analyses using raw data suggested a greater dietary component from benthic and inshore habitats. Variability in δ 15 N values (6.9 to 10.8) implied different patterns of foraging among whale sharks, likely indicating movement among foraging localities that occur at Ningaloo Reef and along the Western Australian coast. There was evidence of enrichment in 15 N occurring with increasing size in males and females, a pattern that could have been due to changes in growth rate and trophic level with age and/or an ontogenetic shift in feeding grounds. Given the variability potentially induced in stable isotope values by differences in rates of turnover of tissues and the use of diet-to-tissue discrimination factors, future studies would benefit from a multi-technique approach using different tissues to identify the diet of whale sharks.

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