Preliminary estimates of the abundance and fidelity of dolphins associating with a demersal trawl fishery
Simon Allen, Phil Bouchet | Jul 10, 2017
Aggregations of dolphins follow the trawlers day-in day-out, leading to the impression that the population as a whole is large, but these preliminary results indicate that the population is smaller than expected.
Photo: Simon Allen
Allen SJ, Pollock KH, Bouchet PJ, Kobryn HT, McElligott DB, Nicholson KE, Smith JN, Loneragan NR. 2017. Preliminary estimates of the abundance and fidelity of dolphins associating with a demersal trawl fishery. Scientific Reports, 7: art4995.
- An aerial survey to estimate dolphin abundance in the PTF suggests population size is smaller than expected at somewhere between 2,000 and 5,000 individuals.
- As expected, boat-based photo-ID and genetic sampling suggest a community of dolphins show fidelity to trawler-associated foraging over days, weeks and years.
- Fisheries-related mortality events are likely to have an impact on the viability of a small dolphin population, but this cannot be demonstrated without more precise estimates of mortality rates (through an observer program) and/or further estimates of abundance (to detect trends).
- The tendency for the dolphins to exploit the fishery as a high-risk but efficient means of procuring resources is problematic for bycatch mitigation, as foraging “traditions” tend to be handed down from one generation to the next.
- The data presented here do, however, provide the basis for some further assessment of the level of impact that dolphin bycatch has on the population, through either the application of Potential Biological Removal estimation (as per the US system) or completing a Population Viability Analysis.
The incidental capture of wildlife in fishing gear presents a global conservation challenge. As a baseline to inform assessments of the impact of bycatch on bottlenose dolphins (Tursiops truncatus) interacting with an Australian trawl fishery, we conducted an aerial survey to estimate dolphin abundance across the fishery. Concurrently, we carried out boat-based dolphin photo-identification to assess short-term fidelity to foraging around trawlers, and used photographic and genetic data to infer longer-term fidelity to the fishery. We estimated abundance at ≈ 2,300 dolphins (95% CI = 1,247–4,214) over the ≈ 25,880-km2 fishery. Mark-recapture estimates yielded 226 (SE = 38.5) dolphins associating with one trawler and some individuals photographed up to seven times over 12 capture periods. Moreover, photographic and genetic re-sampling over three years confirmed that some individuals show long-term fidelity to trawler-associated foraging. Our study presents the first abundance estimate for any Australian pelagic dolphin community and documents individuals associating with trawlers over days, months and years. Without trend data or correction factors for dolphin availability, the impact of bycatch on this dolphin population’s conservation status remains unknown. These results should be taken into account by management agencies assessing the impact of fisheries-related mortality on this protected species.
Bottlenose dolphins following a trawl net in the Pilbara as it is being winched up. Photo: Simon Allen.
FROM DAYS TO YEARS
Locations and times between sampling events of dolphins around trawlers. Genetic re-sampling and opportunistic photo-ID confirmed that some dolphins also follow the trawlers in the Pilbara over months and years. Figure: Allen et al. 2017.
This newborn is likely to learn about foraging around trawlers from its mother. Unfortunately the level of risk associated with foraging in and around fishing nets is not so easily taught. Photo: Simon Allen.
FUNDING & ACKNOWLEDGEMENTS
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