The mortality caused by demersal longliners operating in the Balearic Sea is high and may be jeopardizing the viability of the shearwater populations. Therefore, the identification and implementation of mitigation measures is urgently required.
This is the main conclusion reached in the work we recently published about seabird interactions with the artisanal demersal longliners of the northwestern Mediterranean and the resulting bycatch rates. This study constitutes part of my PhD work conducted in the University of Barcelona and directed by the researcher Jacob González-Solís. In addition, we also relied on the collaboration of Pep Arcos of SEO/Birdlife.
Cortés V., Arcos JM., González-Solís J. 2017. Seabirds and demersal longliners in the northwestern Mediterranean: factors driving their interactions and bycatch rates. MEPS 565: 1–16.
In this paper, we provide information about the characteristics of the demersal longline fishery operating in the area, the main factors influencing the seabird interactions with vessels (using the attacks on bait as a proxy of the bycatch risk) and an estimation of the bycatch mortality caused by the fishery. This information is derived from on-board observations carried out throughout the entire the Balearic Sea (2011-2015) and the seabird catches which were reported voluntarily by fishermen when observers were not in the boats.
We found that there is a higher risk of bycatch during spring, which coincides with the breeding season of seabirds, and also when fishermen set their longlines during sunrise. Other important factors were the bait type, wind conditions, gear configuration (distance between the weights that are attached to the lines) and proximity to the breeding colonies. The birds which are most vulnerable to bycatch in the Balearic Sea were the 3 Mediterranean species of shearwaters: Scopoli’s, Balearic and Yelkouan shearwater (Calonectris diomedea, Puffinus mauretanicus and P.yelkouan), often involving multi-catch events of several dozens of birds due to their flocking behaviour and deep-diving capabilities. We estimated an overall average bycatch rate of 0.58 birds per 1000 hooks, which would imply a conservative estimate ranging from 274 to 2,198 seabirds killed annually by demersal longliners in the area. This high mortality found in the Balearic Sea would be unsustainable and therefore of concern, especially because it is affecting threatened species that are suffering a severe decline of their already reduced populations, like the Critically Endangered Balearic shearwater.
We hope this work serves to draw attention to the high impact that the longline fisheries are having on local shearwater populations, and that it prompts urgent actions to reduce these incidental catches in the Balearic Sea. The Seabird Task Force is precisely using this previous experience to test and come out with the best solutions to minimize the problem. Indeed, currently we are testing the feasibility of using the vertical lines typically employed by Chilean fishermen (see last month’s post). This configuration increases the sink rate of the baited hooks and, hence, reduce the seabird access to the bait and the chance of the birds being hooked (Working together to make longline gear safe for seabirds in the Spanish Mediterranean).
My PhD work received the support of the Fundación Biodiversidad, EC LIFE+ Project INDEMARES, and a large number of fishermen, students and collaborators, as well as SEO/Birdlife.