Effectiveness of a two-chambered trap in reducing within-trap predation by octopus on southern spiny rock lobster

Abstract

Octopus are common bycatch species in many commercial fisheries for southern rock lobster ( Jasus edwardsii). In these fisheries, octopus predation on lobsters caught in traps is a significant cause of lobster mortality. In the South Australian commercial fishery for southern rock lobster ∼4% of the total annual catch is killed by the maori octopus ( Octopus maorum). A previous study showed that a new two-chambered trap design reduced the rate of octopus predation on lobsters in aquarium trials. The aim of the present study was to determine the effectiveness of the two-chambered design in conjunction with escape gaps in reducing octopus predation on lobsters in traps under commercial fishing conditions. The trial was conducted aboard several commercial fishing vessels and compared the performance of two-chambered traps (with and without escape gaps) with conventional traps in terms of lobster mortality rates and lobster catch rates. Octopus-related lobster mortality was reduced by 45–48% in two-chambered traps compared to conventional traps. The presence of escape gaps did not significantly affect the predation rates of lobsters above the minimum legal size, but significantly reduced the retention and subsequent mortality of undersized lobsters. Lobster catch rates were 28% lower in two-chambered traps compared to conventional traps, whilst the presence of an escape gap did not affect lobster catch rates. Further work is needed to improve the lobster catching efficiency of the two-chambered trap design which, if successful, could significantly reduce annual lobster mortalities in several commercial fisheries for southern rock lobsters.