Observations of the Behaviour of European Sea Bass (Dicentrarchus labrax) in the North Sea

The distribution of European sea bass (Dicentrarchus labrax) changes seasonally as they migrate between feeding and spawning grounds, yet their behaviour during these migrations remains largely unexplored. We used miniature electronic data storage tags (DSTs) that record and store depth and temperature to identify characteristic behaviours of sea bass at a fine temporal scale during migrations from and to spawning grounds. A total 89 sea bass in the North Sea and English Channel were tagged with DSTs during 2005-2006. To date 11 tagged bass have been recaptured. Five bass showed evidence of long distance (> 100 km) migration, while the remaining six were recaptured relatively quickly in proximity to their release locations. Recovered tags yielded a total of 422 days of depth data, from which we could identify three characteristic behaviours. The most commonly observed behaviour was characterised by bass maintaining position in mid-water and making frequent ascents and descents of between two and ten metres. This behaviour was often supplanted by a diving behaviour when individuals would make many large descents (< 120 m) away from surface waters. Bass that remained close to their point of release exhibited inshore behaviour for the majority of their time at liberty, and only rarely broke a sequence of inshore behaviour. Whereas bass migrated long distances showed deep-diving and mid-water behaviour for extended periods, with relatively little inshore behaviour. Shoaling behaviour was also evident in two bass that mirrored their diurnal variability (being shallower at night than during the day) for two weeks before separating.