Search

The eastern king prawn, P. plebejus, has been commercially exploited for over 50 years. Controversy has arisen over the presumed effect of increasing fishing effort on reported declining catch per vessel and the need for further management of the fishery. The present study develops the information necessary to evaluate this presumption by analysing landing statistics, data developed from a log-book programme, commercial catch composition and data from a series of mark-recapture experiments conducted on part of the fishing grounds. Population parameters (growth, mortalities and other losses) were calculated via a series of tagging experiments. Recapture rates of tagged prawns were examined for each point release (local recapture) and for the state as a whole (sequentially applied effort), the difference providing estimates of emigration rate. Weekly values chosen for these parameters allowed calculation of yield-per-recruit estimates. For 1979 and 1980, respectively, fishing mortality estimates ranged between 0.04 and 0.08 per week, depending upon number of vessels per port; emigration was estimated to be 0.13 and 0.18 per week for the two years, tagging mortality (0.08 per week), and all other losses other than by tagging and emigration at 0.06 and 0.08 per week. A compartmented, time-lagged model of the fishery was used to predict changes in yield per recruit as a consequence of changes in management strategy and to describe the relationship between fishing effort and yield per recruit. The analyses suggested that limited benefits in terms of yield per recruit would eventuate from substantial increases in effort but that substantial decreases in catch per effort were likely.

Relationships between river discharge and production (catch) of the school prawn M. macleayi were examined as part of an ecological study of this species in the Clarence River region. Schooling behaviour of juvenile M. macleayi exhibited a lunar periodicity and mean daily abundance peaked 5 days after full moon. This was followed 7 days later by a peak in mean daily abundance of emigrating juvenile M. macleayi. Fluctuations in the magnitude of the oceanic component of the total annual catch were found to determine the difference between high and low production seasons. During the period examined there was a direct relationship between Clarence River discharge and the oceanic component of total production of M. macleayi, for various time periods. The absence of a persistent trend in total annual production and annual catch per unit effort of M. macleayi in the Clarence River region suggests that the stock has not been overfished. Common peaks in mean monthly oceanic production of M. macleayi off the Clarence, Evans and Richmond Rivers suggest a common enhancement of the seasonal emigration of M. macleayi from the three estuaries in November due to increased river discharges. Successive troughs and peaks in mean monthly oceanic production suggest a northwards migration of M. macleayi from the Clarence and Evans Rivers and subsequent recruitment to the Evans and Richmond ocean fisheries respectively. Results indicate the importance of river discharge to production of M. macleayi and suggest that modification of discharges by restriction of freshwater flow could adversely effect production.

Growth rates and migration of P. plebejus were investigated by a series of tagging experiments. Growth records of 157 recaptured, streamer-tagged prawns from 2450 releases were analysed for estimates of von Bertalanffy growth parameters by Fabens method. The size-age relationship differed between sexes with males attaining only 75% of the maximum size of female prawns. There was no appreciable size-related mortality of tagged individuals. Recaptured tagged prawns confirmed the previously determined northerly movement and showed little movement toward deeper water off New South Wales. Once past the easternmost point of the continent dispersal into a range of depths occurred. There was no apparent relationship between rates of movement and size, suggesting that fluctuations in the intensity of the East Australian Current were responsible for variations in these rates. Based on the results of this and other tagging studies, a two-substock hypothesis, defined by the origins of the bulk of recruits, is postulated.

Movement and growth of M. macleayi in the Clarence River region were investigated during tagging studies using Atkins-type tags. Although some tagged prawns released into the estuary showed a marked susceptibility to developing lesions and skeletal necroses at the tag wound site, the majority were able to moult, bury and behave normally. Movement of tagged prawns suggested that M. macleayi from the Clarence River undertake limited northwards migrations which are possibly assisted by currents. Such movement may therefore contribute to the Evans Head fishery. Growth of tagged prawns appeared to be unusually rapid in the first 2 weeks after release but in subsequent weeks was found to be comparable with that recorded in other similar studies.