The Effect of Bait Type on Hooking Responses of Target and Non-target Species on Pelagic Longlines: Preliminary Results From Fishing Experiments in the Seychelles
Bait is a major factor affecting pelagic longline catches in terms of efficiency and species targeting, yet few studies are carried out to quantify its role within this context. Three principal reasons justify bait based research in longlining : (i) in terms of responsible fishing, the pelagic longlines suffer from non-selectivity of both fish size and species, (ii) the economic efficiency of a longline fishery is partially driven by bait, especially when fisheries depend on imported bait and (iii) bait type strongly influences catch rates, which are often used to derive abundance indices for stock assessments. Eight fishing experiments using an instrumented longline (hook timer, temperature depth recorder) were carried out in May and October 2005 on board the SFA’s research vessel “L’Amitié”. These experiments compared hooking responses of large pelagic fishes according to three types of bait: imported bait (squid and saury) and bonitos frozen in brine obtained locally from purse seiners. Fishing experiments were done at night using a strategy to target swordfish. Longlines were set after sunset and hauling started just before sunrise. For each fishing set, one half of the baskets were baited with squid or saury and the other half with bonitos. Sizes of bonitos used during this trip ranged between 37 cm (~ 0.9 kg) and 45 cm (~ 2 kg). The weight of the other baits was much lower and ranged between 0.1 kg and 0.35 kg. A total of 2,684 hooks were deployed during these experiments (i.e. an average of about 310 hooks per set). Capture analyses show differences between hooking responses (hooking contact, hooking success) recorded on bonito versus the other baits. The highest value for hooking contact was obtained with bonito, however, the lowest hooking efficiency was also calculated for this bait. Both targeting and commercial efficiencies are highest for bonitos while squid, which is the common bait used to target swordfish in longline fisheries, was less efficient. Shark, however, is the only bycatch group caught with bonitos and this aspect must be considered in terms of conservation. Bycatch mitigation measures appear as a keystone issue of longlining management.