What if a small change to a fishing hook could help sharks survive—without reducing the fishers’ catch? In the South Adriatic, researchers found a striking difference in the condition of blue sharks brought aboard. Longline fishing gear has a higher by-catch rate than any other type of commercial fishing gear.

By-catch means animals caught unintentionally while fishing for another species. The study investigates replacing the traditional J-shaped hook with a circle-shaped hook in a swordfish fishery in the South Adriatic Sea, and examines the effects on both the intended catch and animals caught by accident.

These two hook designs differ in both shape and size: one is shorter and more rounded, while the other is longer and more open. That matters because hook design can affect which animals are caught and how much harm they suffer. Measures to reduce accidental catches are needed, but simple solutions are not available because fisheries interact with many different animals in different ways.

Animals caught accidentally can differ in how much harm they can tolerate, both from being caught and from being vulnerable to capture in the first place. Reducing accidental catches can also be harder for sharks because some may be landed and sold, although for less money than the intended catch.

Earlier studies have found opposing results about how hook shape affects accidental catches. Circle-shaped hooks have been especially effective at reducing accidental catches of marine turtles in some settings. The effects seem to depend on both the species and the area, while the small number of Mediterranean studies and conflicting findings elsewhere show that further research is needed.

That is why this preliminary study compares traditional J-type hooks with C-type hooks in a South Adriatic swordfish fishery for target and by-catch species. For each intended or accidental catch, the study recorded which hook was used and measured the animal’s length.

Each animal’s condition when brought aboard was also placed into one of three groups: healthy, sluggish, or dead or close to death. Across the animals studied, the two hook shapes produced no significant difference in catch rates. The average body size was also not different between the two hook types, regardless of species.

Here is the surprise: only blue sharks showed a significant difference in condition based on hook shape. The percentage of blue sharks in healthy condition was significantly higher with the circle-shaped hook—71.5 percent, compared with 22.6 percent with the J-shaped hook.

The reverse pattern appeared for sluggish sharks: a higher percentage were caught with J-shaped hooks than with circle-shaped hooks. The percentages that were dead or close to death did not differ significantly. Overall, circle-shaped hooks did not significantly affect catch rates or size ranges for the intended catch or the animals caught accidentally.

But blue sharks were in significantly better condition when caught with circle-shaped hooks. Because blue sharks were in significantly better condition when caught on C-type hooks, they may survive more often after release. Fishers may be more willing to adopt them when economic losses are little or none and the change takes little effort.

Circle-shaped hooks are a simple technology that could support marine conservation, but the study’s preliminary results and variation among studies mean that more Mediterranean research is needed. Changing to a circle-shaped hook did not significantly alter catch rates or the sizes of caught animals, but blue sharks arrived in better condition.

That could make releasing them alive more achievable, though larger studies are still needed.