The different organisms provided very useful information with regard to the current system prevailing in their habitats. In the case of gorgonians on the edge of hard substrate and rocks (van den Beld et al. 2017) and crinoids above the viscous sub layer of the benthic boundary layer were clear indicators of current flow (Tyler P & Zibrowius H 1992).
The deep-sea habitats investigated in the Bay of Biscay were highly diverse, and fish occurrence showed species-specific patterns in relation to physical, geological and biological characteristics (Table 1, Figure 2).
At the micro-scale level, there were important biological indicators of habitat and formation, particularly the presence of deep-water coral colonies of gorgonians and scleractinians (Figure 3). The occurrence of sponges and pennatularians were valuable indicators of distinct hydrological conditions. The most important physical characteristics determining fish occurrence were depth, temperature and current. Geomorphological factors were likely important, such as the substrate complexity, bottom structure and ripple marks.
Emphasis of the present study focused, however, on environmental heterogeneity as a determinant of fish occurrence at the micro-habitat level. The aim of the survey was multi-purpose. One of the limitative constraints in the investigation of the deep ocean is the inevitable bias imposed by the sampling instruments. The lights and sounds produced by the submergible can attract or scare fish in a selective way. Other factors that could have influenced data collection were sea floor relief, profusion of suspended particles and consequent visibility conditions, size of organisms, and occurrence of mimetic species.
Fish species analysed in the present study showed different distributions according to depth and temperature but this also reflects the different sampling characteristics of the four dives. Dive 37 was particularly shallow and warm (Table 1). It is well known that species inhabiting the deep-sea are zoned with depth (Uiblein et al. 1998). Diversity patterns of demersal fish assemblages can generally be comprehended in terms of interrelationships of predation, competition, environmental heterogeneity, and trophic level (Haedrich et al. 1980).
In the present study water current was qualitatively identified in two different ways, ‘current velocity’ and ‘current temporal characteristics’. Occasionally organisms typical of fast currents, such as gorgonians and antipatharians, were found in habitats showing relatively weak currents. Thus, caution was warranted in the utilization of current as explanatory variable of fish or micro-habitat distributions over larger scales in time and space.
Species-specific patterns and habitat use
Hoplostethus atlanticus was found in dive 35 (Table 2). However, most of these specimens were sampled only in a specific zone of the diving transect. This was a zone of high hydrological activity, rich in corals, like gorgonians and antipatharians, characterised by hard and complex bottoms. As pointed out by Baker et al. (2012) and Husebø A et al. (2002), these fish are probably associated with areas of high turbulence and mixing, and adopt calm areas when recovering between foraging trips. Sea fans and corals, fan-shaped gorgonians (Tyler & Zibrowius 1992) and other suspension feeders are common at sites of flow acceleration (Genin et al. 1986). The association of orange roughy to these organisms and associated habitat features suggests feeding concentrations of this benthopelagic fish (Rosecchi et al. 1988).
Synaphobranchus kaupii preferred areas associated to currents, low subtract complexity and soft bottoms populated by asteroids (Tables 1 and 2; Figure 2). This can probably be explained by its adaptive adjustment of habitat selection and foraging behaviour. The presence of currents in its habitat can probably improve the strategy of food search through the canalization of odour plumes from food sources (Uiblein et al. 2002). The diet of S. kaupii is broad and typical of scavenger species (Priede et al. 1994). However, there was no strong evidence of these types of prey in this study. It is also known that generalist predators show wider distribution ranges than restricted predators (Haedrich et al. 1980). In all the dives S. kaupii appeared associated to cooler and deeper habitats and this is in agreement with other studies for the depth range considered (Merrett & Domanski 1985; Uiblein et al. 2002).
Coryphaenoides rupestris and S. kaupii shared similar habitats, depth and temperature ranges. This species showed a general tendency to environments containing pennatularians, asteroidea, as well as, bottoms characterized by fine sediment and sponges (Table 1; Figure 2). There is, however, strong evidence related to its mode of life and foraging flexibility. C. rupestris feeds upon on zooplankton and small mesopelagic fishes (Mauchline & Gordon 1984). This species seems to be rather more influenced by hydrological and dietary factors than by bottom structure.
Other Macrouridae followed the same patterns of habitat use as Coryphaenoides rupestris, probably due their biological affinities. Macrourids are generally known to have a rhythmic and active feeding behaviour, affected by the tide and transport of food (Guennegan & Rannou 1979; Mauchline & Gordon 1980).
The scorpaenid species Helicolenus sp. (personal observation; Figure 2) were mainly found associated with sponges. Scorpaenid fishes were observed and documented associated to sponges by (Smith & Hamilton 1983) in the Santa Catalina Basin using the submersibles Alvin and Sea Cliff. On the continental shelf off south western Norway (Husebø et al. 2002) noted a similar association for Sebastes sp., fish being often observed close to large sponges, resting or hiding in their concavities, and among stones. This association seems to be explained by the sheltering provided by these organisms (Husebø et al. 2002). Hydroids were occasionally associated to the microhabitat of Helicolenus sp. (Husebø et al. 2002) (Figure 2), but due to their small sizes and forms, accurate measurement of coverage was particularly difficult.
Molva molva, Galeus melastomus and chimaerids dwelled in a similar habitat type of Helicolenus sp. (Figure 2). Within these fish groups, Chimaerids were less associated to complex bottoms, and this is probably explained by their feeding behaviour, benthic affinity and active behaviour (Figure 3) (Lorance et al. 2000).
Little is known about the ecology of Lepidion eques. It has a diet typical of a euryphagic predator, with a wide variety of prey as amphipods, decapods, copepods and mysids (Mauchline & Gordon 1980). Inhabiting a similar habitat (Fig. 2), Mora moro and L. eques as well as other Moridae are known to adopt station holding behaviours (Uiblein et al. 2003). These fish seemed to choose interstices of the substrate as shelter from predators, and at larger sizes tend to occur in different types of habitats. When compared with the other species analyzed in the same dive 37, Beryx decadactylus seemed to prefer areas of higher substrate complexity (Table 1), mainly formed by small reefs of scleractinians and small rocky formations. Similar reasons as the ones suggested above for H. atlanticus, could possibly explain its association to complex reefs (Baker et al. 2012; Lorance & Trenkel 2006). Neocyttus helgae was another species strongly associated with coral colonies (personal observation; Table 2) (Milligan et al. 2016).