What if changing the shape of a fishing hook could leave swordfish catches unchanged, but send more blue sharks back in healthy condition? This study tests that idea in the South Adriatic Sea. Longline fishing gear has a higher by-catch rate than any other type of commercial fishing gear.

By-catch mitigation is therefore an urgent management need, and new hook types could be one possible strategy. The study examines a swordfish longline fishery in the South Adriatic Sea replacing the traditional J-type hook with a circle hook, or C-type hook, and assesses effects on target and by-catch species.

By-catch is the fraction of the catch unintentionally captured during a fishing operation in addition to the target species. Pelagic longline fishing mainly affects pelagic sharks and rays, and pelagic sharks are caught as by-catch by longline fisheries targeting swordfish and tunas.

Within the Mediterranean basin, the Alboran Sea had the highest pelagic-longline by-catch rate, followed by the Adriatic Sea, at thirty-four point three percent and fifteen point one percent of the total catch. The blue shark is the most caught species by pelagic longline, representing over seventy percent of the elasmobranch catch, and it is considered critically endangered in the Mediterranean by the International Union for Conservation of Nature.

Findings about hook shape and by-catch have been opposing. J-shaped hooks have a reputation for producing high by-catch of protected, endangered, or threatened species, while replacing them with circle hooks has been especially effective in reducing marine-turtle by-catch.

Overall, the effects of C-type hooks on by-catch species seem to be species- and area-specific. Because Mediterranean studies are limited and findings from other regions oppose one another, further research must consider multiple factors and potential trade-offs among species.

This preliminary study compares traditional J-type hooks with C-type hooks in a South Adriatic swordfish fishery, for target and by-catch species. To test C-type hook catch rates for target and by-catch species, a nine-day fishing trip in the Adriatic Sea was monitored.

The trip included seven fishing days and seven pelagic-longline sets, with one set per day. Look first at the setting panel, where the green J-type and red C-type segments alternate along the fishing tracks, with yellow dots marking starting points.

In the hauling panel, the corresponding tracks curve across the same Adriatic study area, and the date labels show the sequence of monitored trips from August sixth through August twelfth. A pelagic longline targeting swordfish was used, with a total mainline length between thirty and forty kilometers.

Each hook was attached to a dropline about thirteen meters long, and each dropline was attached to the main line every approximately fifty-eight meters. The longline configuration was the same as the configuration used in commercial fisheries.

The figure shows alternating sections of J-type and C-type hooks along the main line, with each labeled section containing the same sample size: one hundred and twenty hooks. This repeated sequence appears across three J-type and three C-type sections, making the comparison visually balanced along the gear.

For target and by-catch specimens, the study collected hook type and length in millimeters, using total length for blue sharks, carapace length for sea turtles, lower-jaw fork length for swordfish, and disc width for dark stingrays. The capture condition of each specimen was evaluated with a qualitative index: one meant healthy, two meant sluggish, and three meant moribund or dead.

The statistical analyses used R software version four point zero point four and a ninety-five percent level of significance. Catch rates were expressed as catch-per-unit-effort, calculated as the number of individuals caught per one thousand hooks. CPUE was estimated by fishing set for each species and hook type, then compared between J-type and C-type hooks with Wilcoxon’s test because the dataset contained seven fishing sets.

Lengths and capture-condition percentages were also compared by hook type using Wilcoxon’s test. Across the seven fishing sets, the catch included eighty-one swordfish, thirty-four blue sharks, sixteen pelagic stingrays, and eleven loggerhead turtles.

Swordfish made up fifty-seven percent of the total catch, while blue sharks, stingrays, and loggerhead turtles made up twenty-four, eleven, and eight percent, respectively. The average fishing-set CPUE by species showed no significant difference between C-type and J-type hooks.

Mean body size also did not differ between the two hook types, regardless of species. Across the four panels, the C-type and J-type bars are broadly similar, with overlapping error bars. The largest visible separation is in the swordfish panel, where the bars sit near fifteen and seventeen CPUE, while the loggerhead turtle bars are near two point four and two.

Only blue sharks showed significant differences in capture condition based on hook type. The percentage of blue sharks in healthy condition was significantly higher with a C-type hook, at seventy-one point five percent, than with a J-type hook, at twenty-two point six percent.

The opposite pattern appeared for sluggish blue sharks: fifty-seven percent were caught by J-type hooks, compared with nineteen percent by C-type hooks. The percentages of blue sharks that were dead or moribund did not differ significantly between the two hook types.

Across the other three species panels, the “NS” labels appear above every condition, indicating no statistically detectable separation between the blue and yellow bars. Loggerhead turtles are concentrated in conditions one and two, pelagic stingrays in condition one, and swordfish show their largest bars in condition three.

In the Mediterranean basin, where few studies of this type exist, the results show some positive effects of C-type hooks on the capture condition of blue sharks. Further research is needed to strengthen these results and better understand how fishing area or season, bait type, water temperature, and hook size influence by-catch in the Mediterranean Sea.

In this preliminary study, circle hooks did not significantly change catch rates or body size, but blue sharks were healthy far more often than with J-type hooks. The result is promising, but it needs confirmation with a larger sample.