Incidental bycatch of short-beaked common dolphins (Delphinus delphis) by pairtrawlers off northwestern Spain

Abstract M. M. Fernández-Contreras, L. Cardona, C. H. Lockyer, and A. Aguilar. 2010. Incidental bycatch of short-beaked common dolphins (Delphinus delphis) by pairtrawlers off northwestern Spain. – ICES Journal of Marine Science, 67: 1732–1738. The numbers of short-beaked common dolphins captured annually by pairtrawlers operating off Galicia (northwestern Spain) and the operational factors influencing the bycatch were evaluated using on-board observations. Hauling time, fishing depth, and season of the year were identified as the key factors involved in the incidental capture. The dolphins were most vulnerable to trawls at night from May to September, around the continental shelf break. Most of the dolphins in the bycatch were males, and the average age was 13.4 ± 4.4 years for males and 11.5 ± 4.8 years for females. The sex ratio was male-biased owing to a few capture events involving several males each, supporting the notion that bachelor groups exist in the area. The annual bycatch in 2001 and 2002 was an estimated 394 dolphins [95% confidence interval (CI) 230–632], most taken from May to September (mean 348 dolphins, 95% CI 200–590) and just a few from October to April (mean 46 dolphins, 95% CI 0–132). This level of bycatch could be reduced significantly if trawlers were restricted to operating in water deeper than 250 m and likely avoided entirely if they were restricted to water deeper than 300 m.