Influence of electric fishing lights on sink rates of baited hooks in Brazilian pelagic longline fisheries: implications for seabird bycatch

Abstract

The incidental mortality of seabirds in longline fisheries is the major cause of the population declines threatening most albatross and large petrel species (LEWISON; CROWDER, 2003; ANDERSON et al., 2011). Currently there is a large and growing number of solutions for reducing seabird mortality on longlines. The combination of certain mitigation measures can greatly reduce seabird bycatch, although no single mitigation measure can reliably prevent seabird mortality. The combination of night setting, bird scaring lines and well-weighted branch lines are the best practice for the mitigation of seabird bycatch in pelagic longline fisheries (ACAP, 2014).The sink rate of baited hooks is arguably the major issue to be addressed to reduce seabird-longline interactions. Even when scaring lines are used, to make them effective the baited hooks must attain safe depths (i.e. deeper than the diving range of most petrel species) within the area protected by a scaring line. Many factors are known to affect the sink rate of baited hooks on pelagic longlines, such as the bait type (e.g. squid, mackerel, sardine, skipjack flash) and its state (e.g. dead or alive, frozen or thawed); the mainline tension during deployment; and line weight regimes (the use or not of leaded swivels, as well as their mass and distance from the hooks) (ANDERSON; MCARDLE, 2002; PETERSON et al., 2008, ROBERTSON et al., 2010; ROBERTSON et al., 2013).The best weighting regimes recommended are those that reach a depth of 10 m while under the protection of a scaring line with ~100 m aerial coverage (PETERSEN et al., 2008, MELVIN et al., 2009a). Experiments have indicated that ≥ 60 g placed no more than 3 m from the hooks is likely to achieve satisfactory sink rates under most operational conditions (MELVIN et al., 2009b; ROBERTSON et al., 2010; GIANUCA et al., 2011). Accordingly, among the best practices to reduce seabird mortality in pelagic longline fisheries recommended by the Agreement on the Conservation of Albatrosses and Petrels (ACAP, 2014) and the International Commission for the Conservation of Atlantic Tunas (ICCAT, 2011), is to use specific line-weighting regimes that ensure appropriate sink rates. The three recommended configurations are: weight greater than 45 g attached within 1 m of the hook or greater than 60 g attached within 3.5 m of the hook or greater than 98 g attached within 4 m of the hook. Positioning the weight farther than 4 m from the hook is not recommended.The use of chemical light sticks and more recently battery powered electric fishing lights (EFL), increases catch rates by attracting target species directly or by attracting their prey (BERKELEY et al., 1981; FREEMAN, 1989; ORTIZ; SCOTT, 2001; HAZIN et al., 2005). Despite the addition of this kind of device (usually ~2 m from the hooks) alters the surface/volume ratio of the terminal portion of the branch lines and potentially the sink rate of baited hooks, no investigation has yet been undertaken on this issue.EFL have recently been adopted by the southern Brazilian pelagic longline fleet, and their use and popularity are growing among fishermen and ship owners. Each EFL carries two AA batteries and, given its weight out of the water (~160 g), some fishermen argue that the use of this kind of device increases the sink rate of baited hooks, and using this untested premise as justification for not adopting the required line-weighting regimes.In the light of this scenario, the aim of the present