Spatial variation in mortality by in-pot predation in the Tasmanian rock lobster fishery

The capture of target species using pots or traps can be affected by interactions that occur inside fishing gear. In the Tasmanian southern rock lobster (Jasus edwardsii) fishery (SRLF), predation by octopus is known to vary spatially, assumed to be in response to different environmental and ecological conditions. However, these effects are not well understood and there is concern that changes in the system, as a result of climate change, may alter the spatial distribution and extent of octopus predation. Using fishery-dependent commercial time series data (2000-2011), we first examined how in-pot lobster mortality by octopus predation varied across fishing areas at inter- and intra-annual scales. These spatial and temporal patterns were examined using dynamic regression models (DRM) using lagged variables of lobster abundance (lobster catch per unit effort, CPUE), fishing effort (potlifts) and environmental variables (sea surface temperature, SST). This demonstrated that in-pot mortality in most areas was strongly affected by the current lobster CPUE (indicating an instantaneous direct impact), whereas fishing effort and SST differed spatially in terms of lag selection and correlation sign. Most eastern Tasmanian areas showed a stronger effect of fishing effort rather than SST, with the opposite pattern along the west coast. We provide the first classification of SRLF stock assessment areas based on predation risk level and identify the explanatory fishing and environmental factors.