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Juvenile lobsters Panulirus ornatus in the Torres Strait emigrate once a year from the Torres Strait to the Gulf of Papua, several hundred kilometres to the northeast. They mature and spawn during this emigration. After spawning the lobsters disappear. It has been suggested that the combined stress of reproduction and emigration results in mass mortality. To test this hypothesis, lobsters were collected at several stages of the emigration and their condition defined by the water content and composition of the digestive gland and abdominal muscle. A cage experiment, with 2 feeding regimes, was conducted to examine the likelihood of post-reproductive mortality. Compared to lobsters in the Torres Strait, before emigration takes place, lobsters that had con~pleted the breeding emigration were in very poor condition. There were changes in the size and composition of both the digestive gland and abdominal muscle. Three stages of deterioration were recognised; in the final stage the digestive gland tissue was severely atrophied. The physiological characteristics identified in this study provide a quantitative basis for future work on the mortality rates of lobsters that have completed the emigration. INTRODUCTION duction and emigration might cause the death of most of the lobsters (MacFarlane & Moore 1986). Mass migrations associated with reproduction are The lobsters at Yule Island are often lethargic, common in spiny lobsters, but that of Panulirus ornatus appear to be in poor condition and to have wasted in the Torres Strait, between Australia and Papua New abdominal muscle. These features have not been quanGuinea, is exceptional because the lobsters disappear tified, but in a pilot cage experiment, lobsters from Yule after they have finished spawning (Moore & MacFarIsland had a much higher mortality rate than lobsters lane 1984). Juvenile P. ornatus in the Torres Strait from the Torres Strait. These results support the emigrate northeastward each year, through the Gulf of hypothesis that large numbers at Yule Island die after Papua to the area around Yule Island, maturing as they spawning (MacFarlane & Moore 1986). travel. Fertilisation and spawning take place during the The emigrating lobsters would be subjected to 2 emigration and females may carry up to 3 broods in a principal sources of stress: (1) the energy requirements season. The routes they take and the sequences of associated with walking long distances, possibly with maturation and spawning have been studied in some limited opportunity for feeding, and (2) the physiologidetail (Moore & MacFarlane 1984, MacFarlane & cal and behavioural demands of maturation, fertilisaMoore 1986, Prescott et al. 1986). However, once tion and spawning. spawning is completed, the lobsters disappear from the In other Crustacea, physical and nutritional stress Yule Island area. There is no evidence that the lobsters can result in catabolism of both digestive gland and return to the Torres Strait or reappear at Yule Island in muscle (Heath & Barnes 1970, Dall 1974, 1975a, 1981, subsequent years. It has been suggested that the simulBarclay et al. 1983, Landau 1985, Whyte et al. 1986). In taneous physical and physiological demands of reproPanulirus cygnus, starvation was accompanied by an increase in the water content of the abdominal muscle Present address: Australian Independent Research, PO Box and digestive gland and a in muscle protein 7197, Cairns, Queensland 4870, Australia and digestive gland solids (Dall 1974). Starvation of O Inter-Research/Printed in F. R Germany 30 Mar Ecol. Prog. Ser. 58: 29-39, 1989

Severe physiological stress associated with the annual breeding emigration of Panulirus ornatus in the Torres Strait