Author: Vero Cortés


New publication about seabird bycatch in the artisanal demersal longliners of the Balearic Sea (northwestern Mediterranean)

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cortes_yel2The 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.becares_cona

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.

Photo: Vero Cortés

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.


Working together to make longline gear safe for seabirds in the Spanish Mediterranean

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Working together with fishermen is essential for finding suitable solutions to reduce seabird bycatch. Their specialist knowledge about how their gear works and how to effectively catch their target species is helping us to design the most appropriate bycatch reduction methods without affecting their valuable catches. Hence, the main purpose of our demersal longline work for the next months is to encourage fishermen to cooperate and to enlist their direct involvement in finding solutions.

Our first challenge is testing the feasibility of a fishing gear configuration that is commonly used by artisanal fishers in Chile to target Austral hake (Merluccius australis). It has been demonstrated that this fishing gear has less impact on seabirds, as its rapid sink rate limits the seabird’s access to baited hooks. This is a key feature to reduce the catches of the diving species which can reach a depth of several meters, such as the shearwaters. Through the support of the ACAP Secondment Programme, I was in Chile for two months with the aim of understanding “in situ” how the Chilean gear works, its main components and characteristics. The experience I gained is now being used to tackle bycatch mitigation in the local demersal longline fishery in Catalonia- essentially adapting the Chilean gear to Spanish boats.

The gear is composed of a number of vertical lines o “espineles” distributed along a horizontal line (Figure 1). Each vertical line contains several hooks located at different depths and, at the end, there is a large weight that pulls the hooks down very quickly, reducing the time that bait is available to seabirds and therefore their risk of being bycaught.

Figure 1. Longline types used by Austral hake artisanal fleet. Moreno et al 2006


Antonio, a longline fisherman from Llançà (Northern Catalonia), is  working with our team on the process of adapting the Chilean gear to the local boats. His long experience and contribution are crucial to building the gear correctly, to finding the most effective variations and to assessing its feasibility.

Antonio is working with our team to test the feasibility of the vertical lines. Photo: Vero Cortés

In these last weeks, we have gone to sea with Antonio to make the first tests with the new gear. These initial trips served to learn how the gear works and to reveal possible problems or what we should improve. We also collected data on how fast the baited hooks are sinking, and our preliminary outcomes showed that they sink at around 0.8 m/s in first 10m depth, this being far greater than the sink rate recommended to avoid seabird catches (0.3 m/s). Regarding the commercial fish catches, unfortunately we haven’t caught any individuals of the target species yet, although at this moment the number of settings achieved is still insufficient to make a proper assessment of these results. We will post again soon to let you know how our trials get on!


Moreno, C. A., Arata, J. A., Rubilar, P., Hucke-Gaete, R., & Robertson, G. (2006). Artisanal longline fisheries in Southern Chile: Lessons to be learned to avoid incidental seabird mortality. Biological Conservation, 127(1), 27–36.