Octopus magnificus predation and bycatch in the trap fishery for spinylobsters Palinurus gilchristi off South Africa

摘要

Octopus magnificus inhabits the deep-shelf waters of southern South Africa, where it is a regular bycatch of the trap-fishery for spiny lobster Palinurus gilchristi. We used observer data collected on commercial lobster vessels to assess octopus size distribution, effects of trap location, soak-time and season on octopus catch rates, prey preference upon entering traps (bait or lobster), and octopus-induced lobster mortality rates. Octopuses were absent from traps set at the Agulhas Bank and east of Algoa Bay, and in the remaining area they were present in 54% of longline sets. Soak-times of -24 h recorded the highest mean catch rates, suggesting that many octopuses enter, feed and escape before traps are retrieved. Octopus catch rates peaked in autumn and ebbed in winter, and gut contents showed that 80% of octopus that had a choice between bait and lobsters in traps, ate the bait first. Lobster catch rates were unaffected by octopus bycatch catch rates, apart from -1% of sets which captured >=8 octopuses per 100 traps. Octopus-induced lobster mortality rates in traps are estimated at 2.1%, and octopus is considered to be an additional product of the fishery, rather than a hindrance.