Effects of predator and shelter conditioning on hatchery-reared white seabream Diplodus sargus (L., 1758) released at sea.

The behavioural deficit of hatchery reared (HR) fish used for stock enhancement is the main cause of their low survival in the wild. In this study the effects of predator and shelter conditioning on survival and dispersal of HR white seabream (Diplodus sargus) released at sea were investigated. The hypotheses were that conditioned white seabream would avoid predators more efficiently and would be more capable to shelter, showing higher survival and smaller dispersal than naïve fish. Six thousand HR white seabream (6.32±0.93 cm total length) were allocated in twelve plastic tanks and divided in four experimental groups: three groups were conditioned with a predator, a refuge or both, while one group was left unconditioned and used as a control. The conditioning phase lasted 30 days and was conducted using live conger eels as predators and pyramids of perforated bricks as refuges. Flight initiation distance (FID) and time to shelter (TS) were used as response variables to test the effects of conditioning in the arena, using a dummy conger eel. The effect of conditioning on post-release survival and dispersal was assessed through the monitoring of 1465 tagged seabream belonging to the four experimental groups, released at sea. Underwater visual census was used as monitoring technique. The sighting rate (SR) (sighted fish/ released fish×100) and the distance (D) of each sighted fish from the release site were used as proxies for post-release survival and dispersion, respectively. In the arena, conditioned seabream showed significantly higher FID and lower TS than naïve fish. At sea, the estimated post-release survival of conditioned seabreams (SR=9.4%) was almost twice as much as that of naïve individuals (SR=5.5%). The dispersal of HR seabream acclimated to refuges from the release site (D=2.4±3.1 km) resulted lower than in naïve fish (D=3.7±4 km). This study indicates that predator and shelter conditioning of HR white seabream is an effective practise to increase their post-release survival at sea, and our findings provide support for effective stock enhancement initiatives.