Non Technical Summary

Some of the regulations governing the prawn fisheries in NSW concern the fishing gears used and the size of mesh allowed in nets. It is apparent that many small individuals of the targeted prawns contact the nets, pass through the meshes, and escape capture. To determine the benefits that minimum mesh sizes have on prawn stocks, it is necessary to assess the damage and mortality sustained by these escapees. Such an assessment also needs to consider the fact that, in fisheries with a high density of trawling effort like in the estuarine prawn-trawl fisheries in NSW, individual prawns may contact and escape trawls repeatedly.

We addressed this issue in two laboratory experiments which examined the stress, physical damage and mortality of juvenile school prawns after simulated capture and escape from trawls. In the first experiment, prawns that were trawled and escaped once, 5 and 10 times sustained some physical damage (mostly just loss of their antennae), but this was not significantly different from control prawns that had not been trawled. There were also no significant differences in stress between treated and control prawns. Levels of L-lactate (a measure of stress) were greatest in all prawns immediately after the experiment started and then significantly reduced after 24 and 48 hours. In the second experiment, prawns were trawled and escaped 10 times and then monitored for mortalities over two weeks. Compared to control prawns (that were not trawled), more treated prawns died at the end of the two weeks, although the overall survival rate was greater than 89%.

We conclude that the multiple contact and escape of juvenile school prawns from trawls had minimal effect on their overall condition and that minimum mesh sizes in prawn trawls are an appropriate management tool for this species. Further work will be done in the laboratory and field to assess the fate of prawns that are discarded from vessels. This information will be used to help develop fishing gears and practices for NSW prawn fisheries that reduce unwanted mortalities of juvenile prawns.