Preliminary estimates of the abundance and fidelity of dolphins associating with a demersal trawl fishery

Simon J Allen,Phil J Bouchet,Halina T Kobryn,Kenneth H Pollock,Neil R Loneragan,Krista E Nicholson,Joshua N Smith,Deirdre B Mcelligott

doi:10.1038/s41598-017-05189-0

Abstract

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.

Highlights

As both the human population and our demand for seafood grow, the incidental capture, or bycatch, of non-target species in fisheries continues to present a global conservation challenge[1,2,3]
As Mark-Recapture Distance Sampling (MRDS) analysis can capture some degree of perception bias, we focus on the results from the dual platform, port side only
This study presents the first abundance estimate for any pelagic delphinid interacting with an Australian fishery, and a fortiori, for any pelagic bottlenose dolphin community (Tursiops truncatus) in Australian waters

Summary

Introduction

As both the human population and our demand for seafood grow, the incidental capture, or bycatch, of non-target species in fisheries continues to present a global conservation challenge[1,2,3]. Policy controls implemented in order to curb marine megafauna bycatch in commercial fisheries include the Marine Mammal Protection Act (1972) in the United States and New Zealand’s Marine Mammal Protection Act (1978). Under such legislation, the maximum level of fishing-related mortality allowed for each impacted marine mammal population, stock or management unit is calculated using concepts such as Potential Biological Removal or the Maximum Allowable Level of Fishing-Related Mortality[9, 20]. Attempts have been made to estimate cetacean bycatch levels from observer data or implement trials for bycatch reduction around Australia[23, 24, 26] but without abundance estimates, trend data, or an understanding of what proportion of the impacted populations is affected, assessing the level of risk to the viability of cetacean populations subject to fisheries bycatch is not possible[20, 27]

Methods

Results

Conclusion