Seasonal and size-related variation in microhabitat use by a southern Victorian stream fish assemblage

Microhabitat use by a stream fish assemblage was examined bimonthly at 51 sites along Armstrong Creek, Victoria, Australia, for 12 months. Five species-river blackfish (Gadopsis marmoratus), short-finned eel (Anguilla australis), short-headed lamprey (Mordacia mordax), and the exotic species brown trout (Salmo trutta) and roach (Rutilus rutilus)-were collected. Because blackfish were abundant, length-frequency data could be used to distinguish three size groups, corresponding approximately to cohorts of Years 0, 1 and 2+. Twenty-seven habitat variables were measured at each site, and these were reduced by principalcomponents analysis to eight major components. Densities of each blackfish size group and of eels, trout and lamprey showed significant correlations with one or more components. The mean preferred water depth of blackfish increased with fish size. Small blackfish could be found in water ranging from 10 to 50 cm deep, but large blackfish were restricted to depths greater than 20 cm and could be found at depths greater than 50 cm. All species showed preferences for water velocities less than 20 cm s-1. There was also a relationship between fish size and the size of shelter available among substratum interstices.

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