Tweaking technique helps turtles stay off the hook

Simultaneously exceptionally charismatic and highly endangered, sea turtles are both popular targets for conservation and unfortunate victims of human industry. Incidental capture has been implicated as one of the greatest threats to global sea turtle populations – despite commercial fishing of these creatures being internationally prohibited, an estimated 250,000 are estimated to be killed as by-catch each year.

But researchers say a simple tweak to the design of fishing hooks could prove a simple way to turn this picture around.

Publishing their findings in Marine and Freshwater Research, researchers from Nha Trang University in Vietnam say that switching from a commonly-used hook type to an alternative design known as the C-hook could significantly reduce turtle by-catch in Vietnamese waters, without impacting the tuna hauls that underpin much of the region’s economic development.

The study collected data from 10 commercial tuna fishing vessels operating in shared fishing grounds off the coast of Vietnam throughout 2020, as well as from a single-vessel experimental fishing trial that operated from January to April of 2021.

The vessels were equipped with standard pole-and-line equipment, with half the fleet rigged with traditional hooks and the other half trialing the use of C-hooks.

To the untrained eye, the difference in hooks is barely noticeable. Contrasted against its more traditional counterpart, the JT-hook, The C-hook’s barb is curved in a longer sweep back towards the shank.

This tiny design tweak resulted in an enormous difference in the number of turtles hauled in as by-catch.

In total, 39 sea turtles were logged as by-catch in dockside analysis, comprising a mixture of loggerheads, green turtles and olive ridley turtles. A further two were caught during the experimental phase. Of these 41, only 10 were caught on C-hooks.

The researchers say these results indicate the potential to begin addressing the by-catch problem at the source, greatly reducing the numbers of non-target species that end up hauled on deck.

The research also indicates that for those sea turtles who do manage to get themselves snagged, the C-hook design offers vastly improved post-swallowing survival rates.

JT-hooks have a tendency to slip down the gullet of the creature that takes a bite, often becoming irretrievably lost in their bellies. Even if released from the line, animals that swim free with a JT-hook lodged in their innards are grievously injured, and face substantially higher mortality rates than un-snared animals.

C-hooks, on the other hand, tend to slide over soft tissues and rotate as the eye of the hook exits the mouth, catching most frequently in the jaw.

While it’s surely never pleasant to cop a barbed hook to any part of the body, encouraging piercing placement in accessible tissue such as the jaw allows for safer and simpler hook removal, significantly increasing the chance of survival of released animals.

Of the 31 sea turtles caught on JT-hooks in this study, 69% had hooks lodged in their throats. Of these, 17 couldn’t be safely removed – the turtles were instead snipped free from the attached line and released with the hooks still embedded.

In contrast, all the individuals caught with C-hooks were hooked in the mouth, and all but two hooks were able to be easily removed.

Importantly, the study found that the use of C-hooks over JT-hooks didn’t diminish the catch rate of target species at all – if anything, they were marginally increased. Conservation doesn’t happen in a vacuum, and socio-economic influences can make or break species protection efforts, particularly in less affluent communities.

The tuna fisheries of Vietnam are an important industry, contributing substantially to regional economic development. The industry has made vast improvements to its environmental impact since its inception in the early 1990s, transitioning away from pelagic longline operations, notorious for their associated by-catch rates, to the much more ecologically-sound pole and line method.

But further advances towards ecological sustainability need to come with minimal economic impact, and even a small reduction in target catches is significant.  Pole-and-line gear in the region has almost exclusively used JT-hooks, and the success of the set-up has left many tuna fishers reluctant to shift methods.

Demonstrating the ability to maintain catch-rates of target species while reducing the associated by-catch of sea turtles, this research paves the way for a painless yet effective conservation measure.