Trap behaviour of the western rock lobster, Panuliruscygnus: in situ comparisons of the ‘white’ migratoryphase and the ‘red’ residential phase and the influence ofconspecifics in the laboratory

Nicholas Konzewitsch

The fishery for the western rock lobster (Panulirus cygnus) is one of the largest and most valuable single species fishery in Australia. However, few studies have examined the behaviour of this species and its interaction with traps (known as ‘pots’) in either the field or laboratory. The life cycle of the lobster incorporates a migratory phase (known as the ‘whites’), in which sub-adult (4 – 5 year old) P. cygnus moult into a creamy pale pink exoskeleton in November and migrate offshore to breed and return to the more sedentary ‘red’ phase. In this study I used direct in situ video observations of P. cygnus to investigate the effect of the migratory phase on trap behaviour. I also used video observations in the laboratory to investigate whether the presence of a large lobster in the trap affected the behaviour of smaller individuals outside the trap. Proportionally, the behaviour of the white migratory lobsters around a trap did not differ significantly from that of lobsters in the more sedentary red phase. However, there were a significantly higher number of observations during the whites phase (n = 224) than during the reds (n = 39). The difference is likely to be a reflection on the performance of the camera trap in the reds phase as there was a significant disparity between the catch rate of the camera trap and three standard control traps and as such, further replicates in a reds phase are required to increase the robustness of this study. On a few occasions, lobsters were observed to exit the trap through the neck entrance, however, a much higher proportion of lobsters in both phases escaped through the escape gap. Laboratory experiments were performed to observe the behaviour of small legal-sized P. cygnus (76 > 85 mm carapace length [CL]) when approaching a trap containing a large lobster (> 98 mm CL). The laboratory observations found that small lobsters of both sexes spent significantly more time on and around a trap containing a large female than one containing a large male. However, the number of entries and the catch rate did not differ significantly among treatments (trap with large male; trap with large female; and empty trap) and thus, a large trapped conspecific was not found to have any effect on the catchability of small approaching lobsters. Agonistic interactions resulting in the retreat of an individual were observed around the trap in both the field and laboratory studies. Species other than P. cygnus, most notably octopus (Octopus tetricus), were observed interacting with the trap in situ and may affect the catchability of lobsters. Population estimates of the western rock lobster fishery rely, in part, on trapping surveys to provide stock assessment data. The modelling used to predict abundance involves the catchability parameter (q) which can be affected by lobster behaviour. Direct observations of trap behaviour are useful for understanding how catchability is influenced by this factor and may lead to improvements in stock assessment and gear efficiency.

Trap behaviour of the western rock lobster, Panuliruscygnus: in situ comparisons of the ‘white’ migratoryphase and the ‘red’ residential phase and the influence ofconspecifics in the laboratory

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