BULLETIN OF MARINE SCIENCE, 54(1): 38-48, 1994 CHANGES IN THE SIZE STRUCTURE, SEX RATIO AND MOLTING ACTIVITY OF A POPULATION OF ORNATE ROCK LOBSTERS, PANULIRUS ORNATUS, CAUSED BY AN ANNUAL MATURATION MOLT AND MIGRATION Timothy D. Skewes, C. Roland Pitcher and Jasper T. Trendall ABSTRACT Each spring, most 2-year-old (2 +) Panu/irus ornatus undergo a maturation molt and then migrate from the fishing grounds in Torres Strait, which changes the size structure, sex ratio and molting activity ofthe lobster population. These changes are reflected to varying degrees by changes in the catch of Torres Strait lobster fishermen. The migration begins between early August and early September each year, and departure is spread over 4 to 8 weeks. More females migrate than males; thus the sex ratio of the remaining population becomes biased toward males. The maturation molt occurs during one to three peaks in molting activity among lobsters about to migrate. These peaks coincide with a general lunar rhythm in the molting activity of the lobster population, with peak rates of ecdysis about I week after each full moon. The ornate rock lobster, Panulirus ornatus (Fabricius), is widely distributed in the Indo-West Pacific (George, 1968). This species supports a diver fishery in Australian and Papua New Guinean waters of Torres Strait (Fig. 1). Lobsters, most of which are in their third year after hatching (2 +), migrate eastwards out of the Torres Strait fishing grounds during spring-some move as far as the eastern Gulf of Papua, 500 km east of Torres Strait, where they breed during the summer (MacFarlane and Moore, 1986; Bell et aL, 1987). There is no evidence of a return migration to the Torres Strait fishing grounds after the breeding season (Moore and MacFarlane, 1984; CSIRO, unpubL data). The fishery, therefore, is based mainly on two year classes; 1+ and 2+ lobsters (Moore and MacFarlane, 1984; Pitcher et aL, 1992). The migration of 2+ lobsters out of the Torres Strait fishing grounds causes changes to the size structure and abundance of the remaining population. Moore and MacFarlane (1984) found, in the Papua New Guinean (northern) sector of the fishery, "a loss oflarger lobsters during August so that, by September, smaller lobsters predominated," with an accompanying decline in the catch-per-unit effort of that sector of the fishery. Catch-per-unit effort in the Australian (central and southern) sector of the fishery generally shows a decline during September and October (Channells et al., 1987). More females than males migrate, with male to female ratios of from 1:1.5 to 1:2.9 recorded among migratory aggregations (MacFarlane and Moore, 1986). As the sub-adult population in Torres Strait has a sex ratio close to unity (Moore and MacFarlane, 1984), the migration would bias the sex ratio of the population remaining in Torres Strait toward males. Anecdotal evidence has suggested that P. ornatus in Torres Strait molt almost synchronously prior to migrating (Turnbull, 1989). Migrating lobsters mate and spawn during the migration (MacFarlane and Moore, 1986), but do not molt (Turnbull, 1989). This "synchronous molt" could represent a maturation molt as crustaceans generally do not mate prior to a pubertal or maturation molt (Sastry, 1983; Aiken and Waddy, 1980). This study was initiated to investigate changes in the size frequency, sex ratio and molting activity of P. ornatus in the catch of the Torres Strait fishery in 38