Severe mortality in a breeding population of ornate rock lobster Panulirus ornatus (Fabricius) at Yule Island, Papua New Guinea

Abstract

Ornate rock lobsters Panulirus ornatus, which emigrate from Torres Strait (north-east Australia) to breed each year, support a seasonal artisanal lobster fishery at Yule Island (eastern Gulf of Papua). The catches are highest in mid-January through mid-February, but then decline rapidly, so by late-March virtually no lobsters remain. It has been hypothesised that most of the decline in catch is due to natural mortality caused by the stress of migrating and breeding. However, there are alternative explanations: fishing mortality may be extremely high, or lobsters may move to deeper habitat, not accessible to divers. These hypotheses were examined through a combination of (i) deploying tangle nets in deep water near the main fishing area, (ii) daily monitoring of the catch and effort of the artisanal fishery, (iii) a tag-recapture study and (iv) daily assessment of physiological condition, using the water content of the hepatopancreas as an index. The tangle net catches showed that female lobsters temporarily moved into deeper water just off the reef to hatch their eggs at the January and February full moons, but then moved back onto the reef. Their emigration did not coincide with the decline in artisanal catch. The annual instantaneous total mortality rates ( Z ann), estimated from catch per unit effort (CPUE) curves, were extremely high: Z ann = 11.6 for females and Z ann = 12.0 for males. About one-third of the tags were returned. Analysis of the tag-return rate and total catch suggested the initial population was 19000–30000 lobsters. The total mortality rate, estimated from the tag-recapture data ( Z ann = 10.2), was consistent with the very high estimate from CPUE data. The annual instantaneous fishing mortality rate indicated that the fishery was responsible for about one-third of the mortality ( Z ann = 4.4), which corresponded well the proportion of tags returned. The remainder of the decline in catch was attributed to natural mortality. The hepatopancreas water content indicated that most lobsters were in poor condition. These results support the original hypothesis that there is extremely high mortality in this breeding population, a phenomenon not reported in any other species of lobster.