Reducing the short-term mortality of juvenile school prawns ('Metapenaeus macleayi') discarded during trawling

A field experiment was carried out in southeastern Australia to assess the short-term mortalityand stress incurred by juvenile school prawns ('Metapenaeus macleayi') discarded froman estuarine trawler. Some 35% of the prawns died up to 72 h after being caught in a trawl,exposed to air during sorting and separation from the retained catch (as per normal commercialprocedures), then discarded into replicate cages. Total mortality was partitioned intothat caused by trawling (about 16% of mortalities), and by subsequent sorting and grading(about 19%). Assuming that the majority of the non-penaeid bycatch is excluded fromtrawls (by the use of bycatch reduction devices), the latter mortalities could be almost eliminatedby sorting and separating unwanted school prawns in water-filled compartments.Emersion stress was measured as concentrations of L-lactate in the haemolymph, whichwere elevated for at least 40 min following capture, but similar among all trawled treatments.L-lactate levels decreased within the first 24 h post-capture, then remained constantover at least the next 48 h, and were greater than baseline levels. The potential benefits associatedwith subtle changes to handling practices onboard estuarine trawlers are discussed.