The effects of hook and bait sizes on size selectivity and capture efficiency in Icelandic longline fisheries

One of the major problems in marine fisheries is that species and sizes outside the range of those targeted may be captured, leading to discards. Although longline fishing is characterised by low energy consumption, low bycatch of non-target species and high catch quality, bycatch of undersized fish has been found to be a problem in some fisheries. We studied the effects of bait size and hook size on catching efficiency and size selectivity in the Icelandic longline fisheries targeting cod (Gadus morhua), haddock (Melanogrammus aeglefinus), tusk (Brosme brosme), ling (Molva molva) and wolffish (Anarhichas lupus). Five sizes of EZ-Baiter hooks (sizes 10 − 14) and two sizes (10 and 30g) of Pacific saury (Cololabis saira) bait were tested in six fishing trials. There was a significant size selectivity effect for bait size in that larger bait caught more large fish and fewer small fish of all species, apart from ling. The large bait caught 45, 56, 35 and 76% more (by weight) cod, wolffish, tusk and ling, respectively. Haddock catches were reduced by 57% when the large bait was used. Increasing hook size lowered capture efficiency for all species, but had only a minor effect on size selectivity. Our results thus demonstrate that hook size and bait size affect the profitability of longline fisheries, in that smaller hooks improve capture efficiency, while larger baits increase catches of large fish and reduce those of undersized fish.