Seabird bycatch mitigation trials in artisanal demersal longliners of the Western Mediterranean

Abstract

High numbers of seabirds are killed annually worldwide in longline fisheries. In the Mediterranean, this mortality is seriously affecting the viability of seabird populations, in particular of the three endemic shearwaters. Even so, there is currently no specific seabird mitigation requirements for the longline fleet operating in this area. From 2013 to 2014, we assessed the efficiency and practical applicability of four mitigation measures on artisanal demersal longliners targeting European hake (Merluccius merluccius) in the western Mediterranean: night setting, tori line, weighted lines and artificial baits. We performed fifty-two pairs of experimental (with the tested mitigation measure) and control settings (without any measure), and compared their effects on seabird interactions and fish catches. In addition, we estimated the longline sink rates and the seabird access area to baited hooks in different longline configurations. Night setting reduced bycatch risk without affecting target and non-commercial fish catches. The tori line may have reduced the bycatch risk by displacing bait attacks beyond the end of the line, but at this distance shearwaters could still access to the baits and the streamers did not deter birds under calm wind conditions. Weighted lines increased sink rate, but it resulted in only a minor reduction of the seabird access window to baited hooks and led to some operational problems during the setting. Artificial baits substantially reduced commercial catches. Moreover, the seabird access to the baited hooks was influenced by the longline configuration, the setting speed and the relative position to the floats and weights. So far, night setting stands out as the best mitigation measure for reducing bycatch levels without compromising target catches in demersal longliners. Ideally, these results should be confirmed in longliners targeting species other than European hake.

A contribution to reducing bycatch in a high priority area for seabird conservation in Portugal

“…Several approaches have been tested around the world to reduce seabird bycatch in longlines. Night setting showed a greater potential to reduce seabird bycatch in small scale fisheries operating in the Western Mediterranean when compared to tori lines, weighted lines or artificial bait (Cortés and González-Solís 2018). In our case, seabird bycatch occurred while longlines were soaked in the water, preventing the successful use of tori lines or other devices, like underwater setting and line shooters installed in the fishing boats and tested by Løkkeborg (2003).…”

“…In fact, at a global level, gaps in knowledge of seabird bycatch are more noted in areas where small-scale fisheries are more prevalent. Nevertheless, seabird bycatch has already been reported for some regions, namely the Pacific (Melly et al 2006, Moreno et al 2006), Atlantic (Bugoni et al 2008, Shester and Micheli 2011) and Mediterranean (Cooper et al 2003, Cortés et al 2017, Cortés and González-Solís 2018). The main constraints on collecting bycatch data from artisanal fisheries are related to the limitations on accommodating observers on such small-sized vessels.…”

Slow sink rate in floated-demersal longline and implications for seabird bycatch risk

“…For instance, a switch from current 3kg to 5kg weights for a set of ten thousand hooks would represent roughly an additional half a tonne to be carried onboard and handled by fishers. Cortés & González-Solís [ 51 ] also found that in artisanal demersal longlines, more weight is susceptible to increase entanglement risks between the branchlines and hooks during the setting operations. However, change in the longline weighting regime is the single measure most likely to deliver a significant reduction in seabird bycatch, and a series of measures that could help reduce operational issues should be investigated such as alternative spacing between branchlines and between weights [ 52 ], use of steel weights instead of concrete [ 29 ], etc.…”

“…Several simple and effective seabird bycatch mitigation measures for longline fisheries already exist [ 6 ] but most have been developed and tested in the southern hemisphere longline fisheries [ 7 – 10 ]. To date, limited work has been conducted to test, adapt or implement these measures for the unique set of challenges posed by ‘floated’ demersal longlines (also referred to as ‘ Spanish longline’ [ 11 ], ‘ piedra bola’ [ 12 ] or ‘semi-pelagic longline’ [ 13 ]), which are predominantly used in the North-East Atlantic to target Hake and other whitefish fisheries. This fishery operates mainly in the ecoregion known as the Celtic Seas (ranging from the north of the Shetland Islands to Brittany in the south [ 14 ]) and towards the Bay of Biscay, including the area referred to as the ‘ Gran Sol ’ [ 3 , 15 ].…”

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“…Several simple and effective seabird bycatch mitigation measures for longline fisheries already exist [6] but most have been developed and tested in the southern hemisphere longline fisheries [7][8][9][10]. To date, limited work has been conducted to test, adapt or implement these measures for the unique set of challenges posed by 'floated' demersal longlines (also referred to as 'Spanish longline' [11], 'piedra bola' [12] or 'semi-pelagic longline' [13]), which are predominantly used in the North-East Atlantic to target Hake and other whitefish fisheries. This fishery operates mainly in the ecoregion known as the Celtic Seas (ranging from the north of the Shetland Islands to Brittany in the south [14]) and towards the Bay of Biscay, including the area referred to as the 'Gran Sol' [3,15].…”