Mitigating Turtle Bycatch
While there are many fishing methods and gear modifications that can reduce sea turtle interactions in longline fisheries, the following practices are known to be highly effective without compromising catch rates of target species:
- Use wide circle hooks (18/0 or larger)
- Use fish (e.g., mackerel/opelu/saba/sanma), rather than squid, for bait
- Set hooks deeper than turtle-abundant depths (40–100 m)
Circle hooks appear to reduce the capture of turtles because they are wider at their narrowest point than J hooks and tuna hooks, making it difficult for the circle hook to fit inside a turtle’s mouth. And if a turtle does bite a circle hook, they are less likely to be deeply hooked (where the hook is swallowed down the throat or pierces the roof of the mouth), making it easier to dehook the turtle. Lightly hooked turtles also have a greater chance of surviving than deeply hooked turtles.
Turtles eat squid differently than they eat fish. With squid, they tend to swallow the whole animal in one gulp, whereas with fish they take several, smaller bites. For this reason, fishing with squid-baited hooks captures turtles at a higher rate than fishing using mackerel or other baitfish, where turtles are more likely to eat around the hook instead of ingesting it.
If economically feasible, setting gear deeper than 100 m is a good way to avoid turtle interactions (as turtles tend to prefer shallower water). There are several ways to set gear more deeply:
- Make the branch lines next to buoys longer, as those lines are effectively the shallowest set hooks
- Leave a longer gap on each side of the buoy line before adding branch lines
- Increase the length of buoy lines rather than having short buoy lines and longer branch lines