Trawl surveys in the north-western Gulf of Carpentaria were carried out each lunar month from August 1983 to March 1985 to assess the temporal and spatial distribution and abundance of P. esculentus and P. semisulcatus. The information obtained was then compared with that from fishermen's logbooks. Water temperature and salinity were monitored during the study and their possible influence on the distributions has been inferred. The distributions of juveniles of less than 20 mm carapace length indicated that, for both tiger prawn species, the main nursery areas in the region were in Blue Mud Bay and in the bays along the northern coast of Groote Eylandt. Although the two species shared the same nursery areas, the juveniles were concentrated in different parts of Blue Mud Bay and were most abundant at different times. Catches of juvenile P. esculentus increased substantially in October and peaked in November, whereas catches of juvenile P. semisulcatus increased in November and peaked in January. Catches of both species showed a secondary peak in March 1984, coincident with the heaviest monthly rainfall of the summer monsoon season. The distribution of larger prawns showed spatial separation of the two species in the offshore fishery. The monthly pattern of the catch-per-unit-effort (CPUE) of the two tiger prawn species combined was similar to that obtained from fishermen's logbooks; the logbook data are therefore generally reliable. Monthly CPUE from the trawl surveys for the individual species showed distinct peaks in late summer, which were apparently related to recruitment of small prawns into the fishery. CPUE for P. semisulcatus peaked in February and April (juvenile abundance had peaked in January and March); CPUE for P. esculentus peaked in January and May (juvenile abundance had peaked in November and March). However, a distinct CPUE peak in spring (August/September) for P. semisulcatus could not be related to a previous peak in juvenile abundance; this was presumably a result of an increase in catchability. Although there was evidence linking changes in the catchability of P. semisulcatus to changes in water temperature, a similar link was not as evident for P. esculentus.
The reproductive cycle of female P. esculentus was investigated in the Groote Eylandt region of the Gulf of Carpentaria from August 1983 to March 1985. The minimum size at maturity for P. esculentus was 25 mm carapace length (CL), and 50% of the population were mature at 32 mm CL. The proportion of females which had mated increased sharply above 28 mm CL to a maximum of 80% inseminated in the size range 32-50 mm CL. An index of population egg production, calculated from female abundance, the proportion of females spawning and fecundity with size, was used as an indicator of reproductive output. Egg production tended to be spread throughout the year, but with eggs being produced most consistently in late winter and early spring. Spawning occurred in a limited area within the study area. A comparison of P. esculentus and P. semisulcatus showed that P. esculentus matures at a smaller size (50% at 32 mm CL) than P. semisulcatus (50% at 39 mm CL), fecundity is lower, spawning is nearer inshore and egg production is less strongly seasonal.
The reproductive cycle of female P. semisulcatus was investigated in the region north of Groote Eylandt in the Gulf of Carpentaria from August 1983 to March 1985. Approximately 1750 trawls were carried out over 21 monthly sampling cruises, and 13 748 females were examined. The minimum size at maturity was 29 mm carapace length (CL), and 50% of the population were mature at 39 mm CL. The proportion of females which had mated increased sharply above 34 mm CL with a maximum of 80% of females inseminated in the size range 38-54 mm CL. An index of population egg production, calculated from female abundance, the proportion of females spawning and fecundity according to size, was used as an indicator of reproductive output. Egg production was markedly seasonal, with a major spawning peak in August-September, and a minor one in February. Spawning occurred in a limited area within rhe study area. The spawning stock of P. semisulcatus is likely to be vulnerable to fishing pressure because the area and time of major spawning coincides with the major fishing effort in the region.
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