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Biomechanics of rheotaxis in hill stream fish Macdonnell, John Andrew


Behaviour to increased water velocity is examined in fast stream fish (Otocinclus, Hypostomus, Pterygoplichthys, Chaetostoma and Gyrinocheilus) and a slow water form (Farlowella). Behaviour can be divided into two stages; resting and adhesion (Chapter I). In Otocinclus a third fin extension stage is apparent. Based on the slipping velocity of live and dead fish it is determined that Gyrinocheilus has the greatest station holding ability on a smooth perspex surface. This is attributed to a complete seal produced by its oral sucker lips (closed sucker). Station-holding ability is also examined on rougher surfaces. Slipping does not occur in any of the genera at water velocities up to 90 cm s⁻¹. Morphological adaptations (eg. oral sucker, pectoral fins, frictional pad and odontodes) that may contribute to increasing slipping velocity are examined. In Otocinclus these structures are analyzed using a Scanning Electron Microscope. Otocinclus is the only genera with the ventral dermal plates between the pelvic and pectoral fins organized laterally into a frictional pad. Drag on fish is directly measured with strain gauges and used to calculate drag coefficients (0.10 - 0.94; Chapter II). Drag coefficients for low fineness ratio (length/height < 10) forms at Reynold's numbers below 10⁴ compare poorly with literature values for technical bodies. Drag coefficients determined for fish are high due to roughness and interference drag produced by the fins. Using morphological measurements, dead slipping velocities, drag coefficients, static frictional coefficients and submerged body weight, lift coefficients (-0.55 - 1.23) calculated. Fast stream fish maximize slipping speed by having high frictional coefficients (0.67 - 0.95, on a smooth perspex surface), density (1.03 -1.10 g cm⁻³), rheotactic suction pressure (13 - 173 N m⁻²) and negative lift Although Farlowella has high density (1.129 g cm⁻³) and a low drag coefficient (0.23), its lift to drag ratio is high (6.71) and rheotactic suction pressures (2 - 27 N m⁻²) are low. In general Farlowella does not exhibit hydrodynamic, behavioural or morphological characteristics that enhance station-holding.

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