Shark Bycatch Mitigation Measures
While there are several strategies currently being promoted, RFMOs have not mandated the use of one mitigation measure over another. Here we list some of the techniques that have been shown to be effective in reducing the catch of sharks.
Sharks appear to favor squid over fish as bait, as indicated by both scientific trials and reports from fishers. Using fish bait, such as mackerel, can reduce shark catch rates considerably, particularly for blue sharks. Remember that to reduce turtle catch, the use of fish bait is also recommended, so now you have two good reasons to consider using fish as your bait.
The data on the effect of hook type on shark catch rates are not very clear, but we do know that animals caught using circle hooks are not hooked as deeply, are less likely to suffer internal injury, and therefore have a higher likelihood of survival. Given the higher survival rates, the use of circle hooks—already a technology known to benefit sea turtles and seabirds—may also benefit sharks.
Shark catch rates are significantly higher on shallow-set longlines than deeper-set (deeper than 100 m) longlines. Some studies have found shark bycatch with shallow-depth hooks to be 3 to 10 times the rate of bycatch with deeper-set hooks.
It has long been known that the use of metal wire leaders maximizes the retention of hooked sharks. This is because sharks are unable to cut the wire and escape. For this reason, some countries have banned the use of wire leaders in pelagic longlining and require the use of nylon (monofilament and multifilament) leaders instead.
But another reason to use nylon over wire leaders is that catch rates of bigeye tuna are significantly higher using nylon leaders. Bigeye tuna have good eyesight, so they likely are able to see wire—but not nylon—leaders. Even when factoring in the extra cost of replacing lost hooks and nylon leaders, the financial benefit of the additional bigeye tuna catch makes the use of nylon leaders more profitable than the use of wire leaders (Ward et al., 2007).