Daniel Latrouite

Variation in locomotion behaviour in northern cutthroat eel (Synaphobranchus kaupi) on the Bay of Biscay continental slope

Based on the analysis of video sequences recorded during four dives with a manned submersible in the Bay of Biscay, NE Atlantic, evidence is provided that the northern cutthroat eel Synaphobranchus kaupi shows marked variations in abundance and in the adoption of three locomotion behaviours—forward movement, station holding, and drifting—among six ecologically different habitats. Furthermore, significant variations in association with the bottom and disturbance responses to the submersible were found. Two swimming velocity measures taken from eels during forward movement were significantly correlated with both depth and temperature. The behavioural variations observed partly occurred also between adjacent habitats that were crossed during the same dive. These results and additional observations of instantaneous fluctuations in hydrological conditions suggest that these deep-sea eels are able to adjust to ecological variability at small spatial and temporal scales in a flexible and adaptive way.

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.