Abstract
Small-scale artisanal fisheries can have a significant negative impact in cetacean populations. Cetacean bycatch has been documented in the pole-and-line tuna fishery in the Azores with common dolphins being the species more frequently taken. Based on data collected by observers on ∼50% of vessels operating from 1998 to 2012, we investigate the influence of various environmental and fisheries-related factors in common dolphin bycatch and calculate fleet-wide estimates of total bycatch using design-based and model-based methods. Over the 15-year study dolphin bycatch occurred in less than 0.4% of the observed fishing events. Generalized additive modelling results suggest a significant relationship between common dolphin bycatch and duration of fishing events, sea surface temperature and location. Total bycatch calculated from the traditional stratified ratio estimation approach was 196 (95% CI: 186–205), while the negative binomial GAM estimated 262 (95% CI: 249–274) dolphins. Bycatch estimates of common dolphin were similar using statistical approaches suggesting that either of these methods may be used in future bycatch assessments for this fishery. Our work shows that rates of common dolphin bycatch in the pole-and-line tuna fishery in the Azores are low, despite considerable variations between years. Dolphins caught were released alive although the fate of these individuals is unknown. Continued monitoring will provide a better understanding of dolphin bycatch and more accurate estimates essential in the development of potential mitigation measures.
Highlights
The incidental catch of non-target marine mammals in fisheries, termed bycatch, is a worldwide problem, with several hundred thousand animals killed per year across the globe (Lewison et al, 2004; Read, Drinker & Northridge, 2006; Reeves, McClellan & Werner, 2013; How to cite this article Cruz et al (2018), Estimating common dolphin bycatch in the pole-and-line tuna fishery in the Azores
A single dolphin was captured per fishing event, with a maximum of three dolphins captured at the same time
The present study revealed that duration of fishing events, sea surface temperature and location were significant predictors of common dolphin bycatch
Summary
The incidental catch of non-target marine mammals in fisheries, termed bycatch, is a worldwide problem, with several hundred thousand animals killed per year across the globe (Lewison et al, 2004; Read, Drinker & Northridge, 2006; Reeves, McClellan & Werner, 2013; How to cite this article Cruz et al (2018), Estimating common dolphin bycatch in the pole-and-line tuna fishery in the Azores. Bycatch of marine mammals occurs in most gears, with gillnets being responsible for the highest mortality, followed by trawls and longlines, in the eastern Pacific Ocean and Mediterranean Sea (Read, Drinker & Northridge, 2006; Soykan et al, 2008; Lewison et al, 2014). Incidental catches of dolphins in small-scale fisheries in East Malaysia exceed the maximum 2% annual anthropogenic removal tolerated by the population according to the potential biological removal (PBR), and may be unsustainable (Jaaman, Lah-Anyi & Pierce, 2009)
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