Population ecology of the grooved tiger prawn, Penaeus semisulcatus, in the north-western Gulf of Carpentaria, Australia: growth, movement, age structure and infestation by the bopyrid parasite Epipenaeon ingens

Abstract

Concurrent trawl surveys and tag-recapture studies carried out in the north-western Gulf of Carpentaria between August 1983 and March 1985 provided a detailed description of the growth, movement and age structure of the population of grooved tiger prawns, Penaeus semisulcatus. Growth curves based on the tag-recapture data were used to interpret the length-frequency data collected from the trawl surveys and to determine the number of year classes present. The length-frequency data pertaining to the 1984 year class provided the basis for an estimate of the longevity of the species and a description of the offshore movement patterns. The effect of infestation by the bopyrid parasite Epipenaeon ingens on growth and movement was also examined. Individuals of P. semisulcatus may live for about 2 years, but in the north-western Gulf of Carpentaria very few survive beyond 18 months. Subadults recruit to the offshore fishing grounds during summer and autumn (November-March) at sizes between 20 and 25 mm carapace length (CL) (between 4 and 6 months). By 18 months of age, males reach a size of about 39 mm CL and females about 50 mm CL. The growth rates of both sexes are affected by the presence of E. ingens: males grow faster and larger, the females slower and smaller, with both sexes attaining a size of around 43 mm CL by 18 months of age. The recruiting year class continues to disperse offshore during autumn (from March to May) in such a way that by winter (June and July), although highest abundance is in depths of 35-40 m, the population extends well beyond the commercial fishery into depths greater than 50 m. Although prawns infested with E. ingens show a similar offshore movement, they do not venture beyond depths greater than about 30 m. The sex ratio within the year class remains at around 1 : 1 from the time of recruitment to about 1 year of age. Thereafter, the percentage of females declines steadily; by about 18 months of age very few prawns remain in the population, and of these only about 25% are female. In contrast, the sex ratio for prawns infested with E. ingens remains at about 1 : 1 throughout.