Three species of penaeid prawn, Penaeus plebejus Hesse, Metapenaeus bennettae Racek and Dall, and Metapenaeus macleayi (Haswell) were sampled using a beam trawl in the Noosa River, Queensland, Australia. The studies' aims were to determine the efficiency of a beam trawl as a sampling device, to make inferences on the behaviour and changes in catchability of prawns, and to study seasonal abundance, providing information on life histories and timing of migrations.
The 1.5m wide, 0.5m high beam trawl with a 1.2mm mesh, used for sampling, caught 45.7% and 52.1% respectively of the number of M. bennettae and M. macleayi estimated using a unit area sample of the bottom and the volume of water above it. The beam trawl was most efficient at night, and at catching prawns in a l.l-5mm carapace length size class. Towing method did not significantly affect the total number of prawns caught, M. macleayi are more easily disturbed from the substratum than the other species as evidenced by results from a beam trawl net divided into three separate nets, one on top of the other.
Multivariate analyses of data from estuarine catches show differing responses among the three species. Tidal current condition was not significant as a main effect for P. plebejus. Interaction with daytime and nighttime periods indicates that catches taken during nighttime flooding tides and slack-water, were larger than daytime catches during any tidal current condition. Tidal current condition had a significant effect on catches of M. bennettae. The largest catches were during nighttime slack waters followed by those on daytime slack waters. Catches during ebbing and flooding tidal currents were similar. Tidal current condition had a significant effect on catches of M. macleayi with daytime and nighttime catches at slack water being larger than catches at all other times. Alignment of trawl direction with the current did not have a significant affect on catch size. The behaviour of the three species and the concept of temporal partitioning are used to explain the differences in catch size between the species under different tidal current conditions.
A beam trawl divided into three separate nets was used to collect data in a study of the distribution of prawns in the water column. The vertical distribution at two sites, one influenced by tidal currents and the other not, was similar for P. plebejus and M. bennettae. M. macleayi of l.l-9mm carapace length were caught in significantly larger numbers in the bottom net at both sites. M. macleayi of > 9.1mm carapace length were more likely to be caught in the bottom net when there was not a tidal current.
P. plebejus postlarvae were recruited to the river throughout the year, M. bennettae between March and June, and M. macleayi between April and July. P. plebejus juveniles remained only briefly in the river and were most numerous at sites near the river mouth. There was no distinct period of migration of this species from the river. M. bennettae and M. macleayi juveniles remained in the river until December and March respectively, when, despite the absence of obvious abiotic stimuli there was a marked egress from the river. Both of these species were caught throughout the zone of salt water penetration up to 35 km from the sea, and both were numerous in the southern half of Lake Cootharaba, Similarity between the pattern of prawn distribution in the Noosa River, and those in topographically different rivers suggests salinity is an important factor in determining the distribution of prawns during the estuarine phase of their life cycle.
All three species were caught at night. The catchability of M. bennettae and M. macleayi changed remarkably during the year. They were only present in large numbers in catches during the daytime between March and May and October and May respectively. Although differences were found between the number of prawns caught in trawl samples at different stages of the lunar cycle, moon phase did not appear to have a major effect. Changes in the catchability of prawns are related to a range of physical and biological parameters. Several aspects of the relationship between prawn abundance, catchability, and behaviour are examined in detail. The conclusions of this thesis indicate the importance of the results for commercial prawn fisheries management in the Noosa River.