Observations of sharks swimming off the coast of Tahiti show the alarming degree to which fishing hooks remain attached to these marine predators.
A new paper published in Fisheries Research shows that 38 per cent of tiger sharks living near French Polynesia, also known as Tahiti, have at least one fishing hook stuck in them or show signs of having been previously been snagged by a hook. The health impacts of the hooks aren’t entirely clear, but these lingering nuisances have the potential to cause infections, damage organs, produce toxic effects, and inhibit normal feeding in sharks.
A residual hook “can have profound consequences for those animals,” said Carl Meyer, a marine biologist from the University of Hawaii and a co-author of the study, in a press release. “It can injure or even perhaps kill them because they’re unable to feed properly after these interactions.”
Hooks made from degradable materials could alleviate the problem, according to the new research.
Sharks get embedded by these hooks when they chomp onto bait at the end of longlines, which are intended for other prey, in this case tuna and swordfish.
“Longlines are lines that have multiple baited hooks—from tens up to several thousand—that are set either on the seabed or midwater supported by surface floats,” wrote Meyer in an email to Gizmodo. “They are typically left in place for several hours and then hauled in to recover the catch.”
The unfortunate “bycatch” sharks become free either by biting through the fishing line or by having a fishing crew member cut them loose. But the hooks often remain stuck to their mouths or even inside their bodies, with the associated fishing lines frequently trailing alongside them. These hooks can stay attached to the shark for years and possibly even for the rest of their lives, according to the research.
Meyer and his colleagues studied 55 tiger sharks (Galeocerdo cuvier) from 2011 to 2019 at a shark ecotourism site off Tahiti’s northwest coast. This allowed for long-term observations of the sharks, who frequently returned to the area. The purpose of these observations was to get a sense of how often these hooking events happen, the length of time these hooks stay attached to the sharks, and any associated impacts on the shark’s health or behaviour.
“Gear retention—when marine animals escape from fishing gear with parts of the gear stuck on their bodies—has been recognised as a potential problem for some time, but certain key questions have been very difficult to answer,” said Meyer. “We realised that we could answer… these questions using the tiger shark photo-identification data set where individual sharks are identified from unique characteristics and photographed on multiple occasions over multiple years.”
Results from the eight years of observations showed that 38 per cent of the tiger sharks had at least one hook attached to them or bore signs of a previous hooking, such as scars. It was very common to find sharks stuck to multiple hooks, including one shark with seven hooks and another with six.
Two primary types of hooks were identified in the paper, namely those made from stainless-steel and those that corrode quickly over time. No corrodible hooks managed to stay affixed to a shark for more than two and a half years, but the stainless-steel hooks lasted for upwards of seven years, with some “potentially retained for the lifetime of the shark,” according to the paper.
“After fishery interactions, sharks may swim away with hooks embedded in their stomachs, throats, mouths, or externally around the jaws, and may also be trailing line from those hooks,” said Meyer. “Internal hooks can cause internal bleeding, while external hooks can interfere with feeding. Trailing line can interfere with feeding, wrap around fins leading to necrosis and interfere with swimming.”
Thankfully, the hooks and trailing line didn’t appear to affect the health and growth of the 55 tiger sharks observed, which means they’re still able to eat properly despite the unwanted accessory. But tiger sharks are a particularly durable species, with Meyers referring to them as the “Sherman tank of the shark world.”
That said, only one shark with a hook attached from its insides, whether by throat or internal organs, was documented, which could mean these types of embeddings are not easily survivable. Indeed, the rate of sharks that are hooked internally could be quite high, as it’s difficult to visually identify such occurrences.
“Our study clearly shows that embedded non-stainless hooks will be shed significantly faster than stainless hooks,” said Meyers. “Fisheries should use non-stainless hooks to reduce impact on non-target species,” he said, adding that corrodible hooks are already mandatory in Australia and some U.S. fisheries.
It’s tough to extrapolate these findings to other regions, due to differences in the hooks used and differences in shark foraging behaviour, among other considerations. That said, these results, if generalised, are not encouraging, as “millions” of hooks are embedded in sharks around the world each year, according to the paper.