Summary: | Physical habitat is important in determining the carrying capacity of juvenile brown
trout, and within freshwater management. Summer daytime physical habitat selection for the
parr lifestage (7–20 cm) juvenile brown trout (Salmo trutta) was assessed
in 6 small lowland streams. Habitat preference was determined for the four variables;
water velocity, water depth, substrate and cover, and the preferences for physical habitat
selection were expressed in terms of habitat suitability indices (HSI’s). The statistical
confidence of HSI’s was evaluated using power analysis. It was found that a minimum of 22
fish observations was needed to have statistical confidence in the HSIs for water depth,
and a minimum of 92 fish observations for water velocity during daytime summer conditions.
Generally parr were utilising the deeper habitats, indicating preference for deeper water.
Cover was also being selected for at all sites, but selection was inconsistent among sites
for the variables substrate and velocity. The results indicate that during daytime summer
conditions water depth is a significant variable for parr habitat selection in these small
lowland streams, with cover also being important. Therefore, daytime refugia may be a
critical limiting factor for parr in small lowland streams, and important for stream
management actions under the Water Framework Directive.
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