MICROHABITAT USE AND DIET OF 0-GREAT-OUSE, ENGLAND( CYPRINID FISHES IN A LENTIC, REGULATED REACH OF THE RIVER)

In the River Great Ouse, 0+ roach, bream, chub, bleak and gudgeon were each divided into three 'ecospecies' on the basis of their morphology : young larvae, old larvae and juveniles. Ecologically similar species were positively associated and dissimilar species were negatively ass ociated. All ecospecies, except juvenile bleak preferred water <1 m de ep, within 6 m of the bank with some plant cover. Habitat use was simi lar for all species. It is suggested that this habitat offers increase d growth rate, reduced predation risk and increased food abundance to all species, and that species overlap is due to complementary habitat use rather than aggregation. During the early larval period all specie s fed predominantly upon rotifers and diatoms. During the late larval period there was a switch to Cladocera, primarily Chydoridae and Cerio daphnia sp., and chironomid larvae. Species-specific dietary selection was evident during the juvenile period, with roach feeding primarily upon aufwuchs, bream upon Cladocera, chub upon Diptera, and gudgeon up on Copepoda. (C) 1996 The Fisheries Society of the British Isles