Aggregation dynamics in penaeid fisheries:: banana prawns (Penaeus merguiensis) in the Australian Northern Prawn Fishery

In the Gulf of Carpentaria, banana prawns (Penaeus merguiensis) form dense aggregations that are targeted by trawl vessels with the help of small aeroplanes and colour sounders. Such aggregating behaviour is rare in penaeids and may lead to a change in catchability when stock abundance changes. Commercial logbook data containing trawl-tow records have been used to identify the location, time of capture and biomass of over 600 banana prawn aggregations during 1991-92. The number of aggregations decreased by 83% in the first three weeks of the fishing season. The average biomass of an aggregation decreased even more rapidly than the number of aggregations and, after three weeks of fishing, it had been reduced by 93%. The apparent decrease in the biomass of aggregating prawns during the first three weeks of the season is, therefore, in the order of 99%. This is considerably greater than the 66% decrease that can be estimated from catch per unit of effort data. It also suggests that catchability is directly related to stock abundance. Assessment models that assume that catchability is independent of stock size may not be appropriate for the assessment of the status of common banana prawns in this fishery.