Abstract
Riverine fish are subjected to heterogeneous flow velocities and turbulence and may use this to their advantage by selecting regions that balance energy expenditure for station holding while maximizing energy gain through feeding opportunities. This study investigated microhabitat selection by guppies Poecilia reticulata in terms of flow characteristics generated by hemisphere boulders in an open channel flume. Velocity and turbulence influenced the variation in swimming behaviour with respect to size, sex and parasite intensity. With increasing body length, fish swam further and more frequently between boulder regions. Larger guppies spent more time in the areas of high-velocity and low-turbulence regions beside the boulders, whereas smaller guppies frequented the low-velocity and high-turbulence regions directly behind the boulders. Male guppies selected the regions of low velocity, indicating possible reduced swimming ability owing to hydrodynamic drag imposed by their fins. With increasing Gyrodactylus turnbulli burden, fish spent more time in regions with moderate velocity and lowest turbulent kinetic energy which were the most spatially and temporally homogeneous in terms of velocity and turbulence. These findings highlight the importance of heterogeneous flow conditions in river channel design owing to the behavioural variability within a species in response to velocity and turbulence.
Highlights
Rivers differ from oceanic and estuarine habitats in that flow is primarily in the longitudinal direction and subjected to disturbance events, for example flooding as a result of heavy rainfall
An open channel flume was used to quantify guppy swimming behaviour according to fish size, sex and parasite load in relation to microhabitat variation around hemispherical boulders placed at 0.5 m intervals
While it was difficult to precisely interpret the type of coherent flow structure, it was clear that this recirculating region had the highest kinetic energy, the highest relative turbulence intensity and the strongest shear stresses in the horizontal, longitudinal and vertical planes
Summary
Rivers differ from oceanic and estuarine habitats in that flow is primarily in the longitudinal direction and subjected to disturbance events, for example flooding as a result of heavy rainfall. Natural and man-made structures, such as boulders, woody debris and bridge piers, create physical obstructions to the water flow and generate localized regions where velocity magnitudes and turbulence levels are spatially heterogeneous. Placement of in-stream structures, such as boulders and woody debris, to improve connectivity and complexity has been increasingly employed in river rehabilitation programmes [2]. Improvement in fish species richness and abundance as a result of these restoration projects is variable, with increased densities reported for some species, but not for others, depending on species habitat requirements [3]. The placement of boulders in watersheds in southwest Oregon caused an increase in the number of pools, providing more suitable habitat for coho salmon Oncorhynchus kisutch (Walbaum 1792) and trout O. mykiss (Walbaum 1792), but saw a decrease in dace Rhinichthys spp. Successful restoration projects need to be both species and site specific [2]
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