Characteristics of fish aggregation devices (FADs)and associated tuna schools behavior in Chinese tuna purse seine fishery.

Abstract

The use of fish aggregation devices (FADs) can increase catchability of tuna purse seine for associated tuna schools. In the past decades, large scale deployments of FADs have drawn the international attention due to their negative effects on the marine environment. Finding a FAD design ecologically and economically compatible has therefore become a challenge for tuna purse seine fishe-ry. Nowadays, knowledge and comprehension of interaction between FADs and catch species are quite lacking as a result of limited experimental investigation. We interviewed the captain of Chinese tuna purse seiner based on questionnaire to summarize the status of FADs deployment strategy, structural design, capture and ecological characteristics, and the behavior characteristics of associa-ted schools from the perspective of fishermen's knowledge and experience. We also quantified the cognitive differences of the investigated issues based on the index of consistency (IoC). Our results showed that: 1) the average number of year-round FADs deployment was (102±37) per vessel, and the number of that being lost was (72±1). 2) empirical knowledge from fishing masters indicated that man-made FADs could attract more tuna than natural floating objects (e.g. logs). Submerged structure was the predominant mechanism for aggregating tunas around the FADs. Hanging attractors (e.g. palm leaves) would facilitate the aggregation of tunas. 3) Fishing masters' observations in practical operation revealed that the duration needed for tunas to be associated with a FAD was around 32 days, and fish aggregation would tend to stabilize in the following month. Entanglement of sea turtles and sharks by netting was incidental events. 4) Most fishing masters had identical opinion on "bio-fouling was helpful for FADs to attract tunas" (IoC=0.73) and "modifying FADs design to reduce the detectability by other vessels" (IoC=0.73), while the answers for "the effect of raft type on tuna attraction capability" (IoC=0.34) were quite different. 5) The results of questionnaire suggested that the current design of FADs used by Chinese tropical tuna purse seiner had high risks of detected by other vessels and increased vulnerability of non-target species to fishery. Moreover, our results provided valuable references that promote sustainable exploitation of purse seine in a way of reconciling profitability with ecological preservation.