Tori line experiments on Taiwanese tuna longline fishing vessels in the North Pacific Ocean

Many seabird species are facing a significant threat from the bycatch associated with longline fisheries. To mitigate this issue, bird-scaring lines, also known as tori lines, are commonly employed on tuna longline vessels to minimize seabird bycatch. While numerous tuna regional fisheries management organizations (RFMOs) have imposed regulations requiring the use of tori lines that adhere to specific specifications, no research has been conducted to assess the suitability of such regulations for Taiwanese tuna longline vessels. In order to address this knowledge gap, our study aimed to conduct experiments on three large vessels in the North Pacific Ocean to evaluate the effectiveness of internationally standardized tori lines in comparison to the lines made by the captains on these vessels. The results indicated that the seabird bycatch per unit effort ranged from 0.07 to 0.63 birds per 1000 hooks per vessel. Using zero-inflated generalized linear mixed models, we identified that the probability of seabird bycatch increased in higher latitudes, while the bycatch rate was higher when using tori lines with the international standard than in the tori line made by the captains. This discrepancy may be attributed to the standard tori lines broke more frequently during the experiment. Based on the findings, we recommend the adoption of "O-Kuan" as the material for the main rope of the tori line, which is the same material used for the main fishing rope on Taiwanese vessels. Additionally, we suggest retaining the dragging part of the line while reducing the total length of the tori line. Furthermore, we conducted measurements on the sinking rates of the hooks for the three experimental vessels, which ranged from 0.19 to 0.43 m/s. Conducting further research to explore the sinking rate, while utilizing toriline to prevent seabird bycatch, would be valuable in establishing effective seabird mitigation practices for Taiwanese longline vessels.