SEASONAL AND SIZE-RELATED VARIATION IN MICROHABITAT USE BY A SOUTHERNVICTORIAN STREAM FISH ASSEMBLAGE

Microhabitat use by a stream fish assemblage was examined bimonthly at 51 sites along Armstrong Creek, Victoria, Australia, for 12 months. F ive species-river blackfish (Gadopsis marmoratus), short-finned eel (A nguilla austr alis), short-headed lamprey (Mordacia mordax), and the e xotic species brown trout (Salmo trutta) and roach (Rutilus rutilus)-w ere collected. Because blackfish were abundant, length-frequency data could be used to distinguish three size groups, corresponding approxim ately to cohorts of Years 0, 1 and 2+. Twenty-seven habitat variables were measured at each site, and these were reduced by principal compon ents analysis to eight major components. Densities of each blackfish s ize group and of eels, trout and lamprey showed significant correlatio ns with one or more components. The mean preferred water depth of blac kfish increased with fish size. Small blackfish could be found in wate r ranging from 10 to 50 cm deep, but large blackfish were restricted t o depths greater than 20 cm and could be found at depths greater than 50 cm. All species showed preferences for water velocities less than 2 0 cm s(-1). There was also a relationship between fish size and the si ze of shelter available among substratum interstices.