Habitat, behaviour and colour patterns of orange roughy Hoplostethus atlanticus (Pisces: Trachichthyidae) in the Bay of Biscay

Analyses of 13 submersible dives on the continental slope of the Bay of Biscay (north-east Atlantic) showed that: (i) orange roughy formed dense aggregation of more than 4000 individuals per ha close to the bottom of a small underwater canyon. In all other study areas only a few individuals were observed. Orange roughy aggregations are probably associated with areas of high water mass movements and mixing; (ii) most individuals observed within this aggregation or in its peripheral areas were behaviourally inactive and could be approached with the submersible at close distance; (iii) a large number of these orange roughy showed a completely pigmentless white, or weak red coloration, never observed from specimens caught in trawls. Two individuals which were disturbed by the submarine changed from white to the ‘typical’ red coloration.

These observations suggest fine tuning of the foraging strategy and life cycle of the species. It is speculated that, as an active predator of a sparse food resource, orange roughy has developed adaptations to exploit areas with specific hydrological conditions which offer high prey encounter rates and shelter during metabolic relaxation phases between foraging trips.