Assessment of sea bass swimming activity and preferential space use in sea cages using acoustic telemetry and archival tags

Swimming activity rhythms and levels as well as space occupation by sea bass, Dicentrarchus labrax, in a sea cage (6.5 x 6.5 x 8 m, ca. 25000 fish of 400 g stocked at 30 kg m-3) were recorded using acoustic telemetry (9 fish, IBDT Sonotronics) and archival tags (11, DSTs STAR ODDI). Acoustic tracking was realized over a 13 days period and successful for 3 individuals (one fix every 2 min on average). For archival tags, only 4 tags were retrieved and 3 had been recording swimming depth and temperature for 9 mo (Nov 05-June 06). Telemetry results showed an even day/night swimming activity levels in November with fish staying in close association with the bottom. Archival tags recordings revealed a preferential distribution in the water column between mid-water and bottom of the net cage in winter, over the whole water column in spring and near the surface zone in early-summer. In addition, on a 24 hrs period basis, sea bass presented a vertical movement rhythm: they swam closer to the surface during the day, specially around feeding events, and near the bottom at night. That vertical movement was pronounced during winter, faded during spring and almost disappeared in summer.