Physical damage, behaviour and post-release mortality of Argyrosomus japonicus after barotrauma and treatment

Two experiments were conducted to quantify the mortality of, and clinical signs of barotrauma to, Argyrosomus japonicus after retrieval from 20 m following either 'no treatment', 'venting' (a needle inserted into the swim bladder) or 'recompression' (released with a weighted line). In Experiment 1, 10 fish were hauled from 20 m and from 5 m (controls) and euthanised for assessment. The only effect on controls was a distended coelomic cavity, but all fish retrieved from 20 m had this symptom and a prolapsed cloaca, and many had haemorrhaging (81%), gastric herniation (70%), swimbladder rupture (50%) and affected buoyancy (66%). Another 20 fish were subjected to the treatments and, along with controls (from 5 m), were released into 'bathy-cages' (2.5 m x 20 m) for three days. Only two fish retrieved from 20 m died (total mortality of 3.3%). In Experiment 2, six fish retrieved from 20 m and three control fish were tagged with acoustic transmitters, released following the abovementioned treatments into the wild and monitored for 214 d. All fish with barotrauma remained at shallower depths than controls for the first 10 d, after which behaviour was similar among groups. Our findings are positive, but further research is required to explore the relationship between retrieval depth and the severity of barotrauma.