Effects of 17/0 circle hooks and bait on fish catches in a Southern Atlantic swordfish longline fishery

* This paper reports the results of using different hook style and bait type combinations on the catches of targeted, bycatch and discarded fishes in the Portuguese commercial longline fishery targeting swordfish (Xiphias gladius) in the South Atlantic Ocean.
* In total, 310 longline experimental sets (446 400 hooks) were deployed between October 2008 and February 2012. Three different hook styles were tested; the traditional J-hook (9/0) 10o offset was compared with two 17/0 circle hooks (a non-offset and a 10o offset), and squid (Illex spp.) bait was compared with mackerel (Scomber spp.).
* Catch per unit effort (CPUE) was calculated for each fish species per fishing set and compared between the different hook style and bait type combinations. Results indicated that the effects of hook style and bait on the CPUEs were species-specific. For example, swordfish (target species, Xiphias gladius) CPUEs were higher with J-hooks baited with squid, while for the blue shark (most important bycatch species, Prionace glauca) the highest CPUEs were obtained with circle hooks baited with mackerel. For tuna (Thunnus spp.) and marlin (blue, Makaira nigricans and white, Kajikia albida) only the bait effect was significant, with higher catches with squid.
* For the discarded species, the proportions of alive vs dead specimens at the time of fishing gear retrieval were also species-specific.
* The total retained catch value per unit of effort (VPUE) did not change between the different hook and bait combinations, but these VPUEs are highly dependent on market fluctuations.