The study area was heterogeneous with respect to bathymetry and other physical and geological characteristics (Table 1).
A coarse measure of the similarity among fish communities is apparent from the sets of species observed in the three deeper dives (Table 2), the composition and distribution of fish taxa being relatively similar in dives 22 and 34 over soft sediments (r=0.69) (Table 1), negligibly related to the set obtained over hard sediments and strong currents (dive 35). The fish assemblage encountered in the shallow central region (dive 37) differed clearly from those in deeper areas (Table 2).
The most frequent species in dive 22 were Coryphanoides rupestris, Lepidion eques and Synaphobranchus kaupii. Dive 34 revealed a high density of S. kaupii, while an enormous concentration of Hoplostethus atlanticus was found in dive 35. Helicolenus sp. was common along the upper slope (dive 37).
A global analysis of the four dives by means of CCA showed that a large part of the variation in the species-environment relationship could be explained, with partial percentages of 40.5% and 30.3% for the 1st (horizontal) and 2nd axes (vertical) axes, respectively. The 1st canonical axis was statistically significant as well as the relation between species and the environmental variables (p=0.005). The 2nd axis was not significant. A number of physical and geological variables, such as depth, slope, bottom structure, type of substrate, current, water temperature, and occurrence of ripple marks, as well as the prevalence of certain types of epifauna, such as scleractinians, sponges, pennatularians, hydroids and antipatharians, contributed significantly (at the 5% significance level) to the model. The CCA bi-plot illustrates the centres of distribution of the fish species and benthic fauna, and the directions of influence of the main physical and geological variables (Figure 2). The 1st axis, the axis carrying most information in the analysis, contrasted the fish-benthos associations in deep waters (on the left side) to those observed in shallower and warmer areas i.e. dive 37 (on the right side). Shallow and intermediate depths were often characterized by relatively high currents and coarse sediments or hard bottoms (high percentage of clast or rock on the bottom). In shallower areas, the bottom was either devoided of visible benthos or populated by sponges, scleractinians or hydroids. Molva molva, Helicolenus sp., Galeus melastomus and chimaerids were often found in association with these types of benthos.
Figure 2.Canonical correspondence analysis (CCA) ordination diagram of all the dives, showing associations between fish species (dots) and environmental variables (arrows) in the Bay of Biscay. Clusters of fish species showing similar association patterns are indicated by circles. Species names and environmental variables were coded for simplification, and their full names are given in Tables 1, 2 and 3.
Fish observed at intermediate and deep waters could not be sorted into discrete clusters. Instead, they were mostly found along a complex ecological gradient parallel to the 2nd plot axis. On or above soft bottoms, Anguilliformes, Moridae, Synaphobranchus kaupii, Lepidion eques and Mora moro were positively associated with Actinians and poorly structured habitat (Figure 2). At the centre of this gradient Coryphaenoides rupestris and other macrourids, mesopelagic fishes, and sharks were associated to pennatularians, asteroids, echinoderms and crinoids. Finally, Hoplostethus atlanticus formed a mono-specific cluster defined by the association with gorgonians, antipatharians and ripple seabed (Figures 2 and 3).
Figure 3. (A) Hoplostethus atlanticus and diverse deep sea coral fauna (dive 35); (B) Molva molva and deep-sea scleractinian corals (dive 37); (C) Chimaera and benthos (dive 22); (D) Coryphaenoides rupestris over desert bottom (dive 34).