Microhabitat use of fish larvae and 0+ juveniles in a highly regulated section of the River Great Ouse

To address the scarcity of information on species-environment relationships of fish larvae and 0+ juveniles in regulated rivers, point abundance sampling by electrofishing was used to compare the microhabitat use of seven species of fish in a highly regulated stretch of the River Great Ouse and associated off-channel water bodies during summer 1989. The larvae of dace (Leucisus leuciscus) showed no enviromental associations, but 0+ juveniles preferred cooler than average, shallow waters at weak/medium sloped banks where gravel substrates dominated, avoiding dense duckweed, milfoil (Myriophyllum sp.)/Canadian pondweed (Elodea canadensis) and other submerged vegetation. Both larvae and 0+ juveniles of gudgeon (Gobio gobio) preferred similar conditions to the dace juveniles, but were found more often than expected in filamentous algae. Juvenile perch (Perca fluviatilis) demonstrated only one preference, the littoral interface between the water and bankside vegetation. Roach (Rutilus rutilus) larvae preferred dense milfoil/pondweed in warmer than average waters over silty bottoms, avoiding the littoral interface, wheras 0+ juveniles preferred cooler than average waters over sand/gravel bottoms at the littoral interface, but avoiding filamentous algae, milfoil/pondweed and other submerged vegetation. The preferences of 0+ juvenile bream (Abramis brama) were weak but resembled those of juvenile roach. Larvae of silver bream (Blicca bjoerkna) preferred mid-channel areas with dense filamentous algae and milfoil/pondweed, warmer than average waters and silty bottoms, wheras juveniles preferred cooler than average waters at the littoral interface. Silver bream was the only species to show a preference for sunlit locations. The microhabitats of tench (Tinca tinca) larvae and juveniles were nearly identical, preferring the silty bottoms and dense milfoil/pondweed, duckweed and filamentous algae of an off-channel site. The microhabitat of young roach, bream and tench resembled that of these species in a French oxbow lake of the upper River Rhóne in France.