Behavior of target and non-target species on drifting FADs and when encircled by purse seine gear

Empirical and observational data were collected from 31 sets made from a tuna purse seine vessel during the International Seafood Sustainability Foundation’s (ISSF) WCPO bycatch reduction research cruise. Simultaneously, skipjack (Katsuwonis pelamis), yellowfin (Thunnus albacares), and bigeye (T. obesus) tuna and silky sharks (Carcharhinus falciformis) were implanted with pressure sensing acoustic transmitters and monitored on drifting FADs in the same areas. Untested assumptions relating to natural behavior of encircled fish in purse seine gear were clarified; divers observed separation of target species by species as well as size. Additionally, nontarget species consistently separated themselves from target species and remained shallow. This species segregation was consistently observed during all sets and lasted throughout the duration of the haul until the beginning of sacking was reached, at which point individuals and schools were forced to intermingle. Acoustic tagging data confirms these behavioral characteristics displayed naturally while associated with a floating object, which is well documented by other tagging studies on FADs. These observations provide much needed insight to behavioral patterns of non-target and unwanted species, which may inform further research and development of options to selectively release or sort these from the net before these animals sustain mortal injuries.