Aggregation characteristics of fish aggregating devices (FADs) in tuna purse seine fisheries in the Western and Central Pacific Ocean

Fish aggregating devices(FADs) have been used for large-scale aggregation of tuna species for more than 30 years. In order to understand the aggregation characteristics of the fish around FADs, this study explored the day of first arrival day and the dynamic aggregation process of fish after the deployment of FADs based on the echo-sounder buoy data of Chinese tuna purse seine vessels in the Western and Central Pacific Ocean in 2021. The U and H tests were used to analyze the difference of fish first arriving day around FADs, and the generalized additive mixed models (GAMMs) was used to analyze the variation of tuna biomass with the change of days at sea and drifting speed of FADs in the study. The results showed as follows: ① The first arriving day of fish was (8.9±9.0) days, including (3.8±4.2) days for tuna species and (16.0±8.9) days for non-tuna species; ② There was a significant difference on the first arriving day of tuna species around different submerged depth of FADs (P<0.05), but no significant difference was found in non-tuna species (P>0.05); ③ The aggregation biomass of tuna species around FADs varied dynamically with the drifting days of FADs, and reached the peak at around the 25th day after deployment, then gradually decreased; ④ The aggregation biomass of tuna decreased with the increase of drifting speed of FADs; ⑤ The random effect showed that the aggregation biomass of tuna around FADs of 80 m was generally the highest, followed by FADs of 60 m, while the FADs of 90 m was the lowest. This study showed that tuna species usually arrived earlier than non-tuna species after FADs were deployed, and the day of first arrival day of tuna species was related to the underwater length of FADs. FADs with shallower submerged length and slower drifting speed were more likely to attract tuna to aggregate around them. Meanwhile, tuna species gradually aggregated after FADs were deployed, and the biomass reached peak at around one month, which might reflect the optimal fishing time after the deployment of FADs. This study was conducive to further understanding the behavior pattern of floating objects associated schools, and provides scientific basis for the design of efficient and eco-friendly FADs, optimization of setting strategies and formulating of conservation and management measures for floating object associated schools.