Use of fishery-dependent data for the evaluation of depensation: case study involving the predation of rock lobster (Jasus edwardsii) by octopus (Octopus maorum)

The relationship between the level of octopus (Octopus maorum) predation and the daily-average number of lobsters (Jasus edwardsii) per pot was investigated using commercial catch statistics. Octopus predation was found to be inversely correlated with lobster catches, such that, lobsters experienced reduced survival when their daily-average density in a pot was lower. The reduction in survival at lower densities provides evidence for a depensatory mechanism underlying the predator-prey interaction between pot-caught octopus and lobster. The effect of lobster density on the feeding response of octopus is unknown so this study could not determine whether depensation was brought about by either predator saturation or predator avoidance tactics used by lobsters. The use of commercial catch statistics to investigate this depensatory effect has disadvantages because the lobster mortality estimates can be biased by the non-reporting of undersized lobsters caught inside pots, under-reporting of lobsters killed by octopus, the inability to identify unsuccessful predation attempts made by octopus, and the ability of some lobsters to escape from pots. All the sources of bias would tend to make it difficult to conclusively determine both the absolute proportion of lobsters killed by octopus and the true scale of the depensatory mortality, as the scale of the mortality may be greater than the commercial data suggest. However, the bias will not necessarily reduce our ability to detect the depensatory effect if, as seems likely, the sources of bias are relatively constant across the range of daily-average catches. Depensation in crustacean stocks affected by pot-related octopus predation has been rarely studied, and, because depleted stocks lead to fewer lobsters being caught per pot, such depensatory mortality has implications for the population dynamics and management of crustacean stocks. © The Royal Society of New Zealand 2005.