Variability in natural behaviour, and observed reactions to an ROV, by mid-slope fish species

The behaviour of eight large benthopelagic fish taxa was analysed using video records collected with an ROV on the mid-slope of the Bay of Biscay. The studied species were roundnose grenadier (Coryphaenoides rupestris), orange roughy (Hoplostethus atlanticus), deep-sea scorpionfish (Trachyscorpia cristulata echinata), and black scabbardfish (Aphanopus carbo), as well as individuals belonging to the families Alepocephalidae, Chimaeridae, and Scyliorhinidae, and to the order Squaliformes. Some of the observed fish were grouped at the family taxonomical level due to visual identification to species being unreliable. Assumed natural (undisturbed) behaviour was categorised in terms of (i) body position with respect to the bottom sea floor, (ii) locomotion and (iii) activity type. Reaction (disturbed) behaviour to the approaching ROV was categorised in terms of reaction type and distance. Environmental conditions (depth, temperature, current speed and direction) and observation conditions (ROV speed and altitude) were recorded simultaneously with fish observations in order to explain the variability in the observed reaction behaviour. A multivariate analysis identified three groups corresponding to a behaviour pattern of a sit and wait strategist (one species), an active bottom hunter (three taxa), and a group of species displaying little activity in their bottom habitat (three taxa). At species level the environmental and observation conditions had some explanatory power for individual behaviour variability. It is hypothesised that the varied behaviour of mid-slope benthopelagic fish contributes to maintain a high species diversity of large predators in an energy poor environment.