Estimating common dolphin bycatch in the pole-and-line tuna fishery in the Azores

© The Author(s), 2018. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in PeerJ 6 (2018): e4285, doi:10.7717/peerj.4285.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.This work was supported by the Fundac¸a˜o para a Cieˆncia e a Tecnologia (FCT) and the Fundo Regional da Cieˆncia e Tecnologia (FRCT), through research projects TRACE (PTDC/MAR/74071/2006), MAPCET (M2.1.2/F/012/2011), and the FCT Exploratory project (IF/00943/2013/CP1199/CT0001), through funds from FEDER, the Competitiveness Factors Operational (COMPETE), QREN, POPH, the European Social Fund, the Portuguese Ministry for Science and Education, and the Proconvergencia Ac¸ores/EU Program. Funds were also provided by FCT to MARE, through the strategic project UID/MAR/04292/2013. Maria Joa˜o Cruz is supported by a DRCT doctoral grant (M3.1.2/F/008/2009-RTF/1) and Mo´ nica Almeida e Silva is supported by an FCT-Investigator contract (IF/00943/2013)