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Superior Tuna Catches With Circle Hooks, Saves Turtle But Not Sharks

An extensive study conducted by the Bureau of Rural Sciences, Canberra, Australia and presented at the last WCPFC Scientific Meeting (October, 2008) states that financial returns for tuna longliners can be maximized with a complete switch from tuna hooks to circle hooks.

The study entitled Implementation Of Bycatch Mitigation Measures In Australia's Pelagic Longline Fisheries: The Effects Of Circle Hooks On Target And Non-Target Catches evaluated for three years, 2005-08, the effects of circle hooks on longline catches at the Australia’s Eastern Tuna and Billfish Fishery (ETBF).

During that period, data from 95.150 hooks were analyzed, consisting of 47.575 circle hooks and 47.575 tuna hooks (see picture). Longliners involved in the project were typical ETBF longliners, which deploy around 1.200 hooks per day and fish for around 107 days per year on average.

“Circle hooks are considered to be effective in reducing bycatch mortality because they are more likely to lodge in the jaw. By contrast, tuna and “J” hooks are believed to lodge in other locations, including the throat and stomach, as well as the jaw” states the report.

Circle hooks are often used to prevent sea turtles bycatch, however, since there are a small number of turtle interactions with tuna fishing in that area, the study focused on the effects of circle hooks on other non-target species and target species.

The results showed that for most species, catch rates on circle hooks exceeded those on tuna hooks, about 25% higher: “The elevated catch rates were statistically significant for several commercially targeted species, including albacore tuna, yellowfin tuna, black oilfish, striped marlin and swordfish”, stated the report.

In addition, the study confirms that no relevant size difference was reported among tuna species caught on circle hooks and predicts that the adoption of circle hooks should result in increased catches and financial returns across the tuna fleet over time.

The small number of turtle bycatch by circle hooks is an indication of their efficiency for that purpose in the ETBF. The study has similar conclusions based on experiments made in other areas of the world. However, the results show that catch rates of most shark species were higher on circle hooks.

The study recommends fishery managers stakeholders to consider the “wider implications of bycatch mitigation measures” such as the effect on other species other than turtles, but concluded saying that the adoption of circle hooks won’t be financially detrimental for longliners targeting yellowfin and bigeye tuna in the ETBF.