Implementation of bycatch mitigation measures in Australia's pelagic longline fisheries

Circle hooks reduce marine turtle mortality:
Reports of unacceptable catch levels of marine turtles resulted in the closure of major United States (US) longline fisheries in the Pacific and Atlantic Oceans in 2001. Subsequent research demonstrated that large circle hooks and whole fish baits can significantly reduce longline catch rates and associated mortality of turtles. The US fisheries reopened in 2005 with stringent mitigation measures, including the use of large circle hooks and whole fish bait. More recently, the US is moving to ban imports of broadbill swordfish and other pelagic species from countries where longliners do not use acceptable mitigation measures, such as those required under US domestic legislation. These restrictions might eventually apply to Australia's Eastern Tuna and Billfish Fishery (ETBF), although interactions with turtles are quite rare here. Rather than investigating the efficacy of circle hooks in reducing turtle mortality, this study focused on the effects of circle hooks on catches of other non-target species and target species.

Large-scale experiment:
We conducted experiments during 2005-08 to test the effects of circle hooks on longline catches. The experiments involved ETBF longliners fishing primarily for yellowfin tuna, bigeye tuna and swordfish. Crew members alternated similar-sized circle hooks and control hooks along each longline. The control hooks were Japanese tuna hooks that ETBF longliners traditionally used. Observers monitored hook deployment and recorded the hook type, species, life status, hooking position and length of each animal caught. The experimental design, combined with the large sample size (> 95 000 hooks), provided a substantial dataset for investigating the relative performance of circle hooks.

Elevated catch rates:
For most species, catch rates on circle hooks exceeded those on tuna hooks. Overall catch rates on circle hooks were about 25 per cent higher than those on tuna hooks. The elevated catch rates were statistically significant for several commercially targeted species, including albacore tuna, yellowfin tuna, black oilfish, striped marlin and swordfish.

Other factors mask circle hook effects:
Variations in catch rates between longliners, trips and operations were often larger than the differences attributed to hook type. The effects of circle hooks on catch rates were masked by other factors, such as fluctuations in the availability and catchability of the different species, which were in turn driven by local environmental conditions, subtle differences in fishing gear and fishing practices and species' distribution and abundance. For commercial longliners, the development of techniques to cope with variations in those factors may have a greater impact on catches and financial returns than switching to circle hooks.

Similar body size:
For most species, there was no difference in the average size caught on the different hook types. Bigeye and yellowfin tuna caught on circle hooks were slightly smaller than those caught on tuna hooks, but these differences were not statistically significant. The difference was statistically significant for striped marlin. Striped marlin caught on tuna hooks were on average 10 kilogram larger than those caught on circle hooks. It is unclear how hook type affects the size of striped marlin caught on longlines.

Improved financial returns:
The differences in catch rates were statistically significant and large enough to affect catch levels of most species and financial returns. The superior catch rates of circle hooks mean that financial returns will be maximised with a complete switch to circle hooks rather than replacing existing hooks over a long period. The cost of converting to circle hooks is relatively small because additional fishing gear is not required and the cost of circle hooks has dropped to within about 5 per cent of that of tuna hooks. All else being equal, the adoption of circle