ABSTRACT
Benthic fish experience water currents caused by tides, wave action and stream flow. Orientation to flow and associated behaviour is termed rheotaxis (Frankel and Gunn, 1961). Avoiding displacement allows fish to remain in their habitats for feeding, finding mates and protection. Adaptations associated with station-holding on the bottom that maximize current induced slipping and lift-off involve morphology (e.g., overall body form; frictional devices: frictional pads, fin spines, odontodes; adhesive structures: oral suckers, thoracic suction discs; high density; reduced or absent swimbladder) and behaviour (e.g., burying, heading into the stream direction, adhesion through marginal fins, active fin beating, negative lift production from paired fins) (e.g., Hora, 1922a, b; Keenleyside, 1962; Arnold, 1969; Arnold and Weihs, 1978; Matthews, 1985; Webb, 1989; Macdonnell and Blake, 1990; Blake, 2006). Arnold (1974) gives a comprehensive review of rheotropism in fishes including rheotaxis per se, optomotor responses and ecological aspects. Arnold and Weihs (1978) give a thorough analysis of the mechanics and behaviour of rheotaxis in plaice Pleuronectes platessa which appears to be designed for 41minimum drag when heading into a current and maximizes lift-off speeds through behavioural adaptations associated with posture and marginal fin movements. Webb (1989) showed that compressed and depressed forms (P. platessa and Raja clavata, respectively) are characterized by high lift, low drag designs and behaviours that reduce lift. Benthic Atlantic salmon parr Salmo salar place their pectoral and anal fins into the interstices between gravel particles to increase friction and produce negative lift from their pectoral fins (Arnold et al., 1991).
