SEASONAL, ANNUAL AND REGIONAL VARIATIONS IN ICHTHYOFAUNAL COMPOSITIONIN THE INNER SEVERN ESTUARY AND INNER BRISTOL CHANNEL

Monthly samples of fish from the intake screens of power stations at O ldbury and Berkeley in the inner Severn Estuary and Hinkley Point in t he inner Bristol Channel, were used to analyse the community structure s of the ichthyofauna in these regions. Marine species that use the es tuary as a nursery area (marine estuarine-opportunists) were very abun dant in the shallow inshore waters at Oldbury. Diadromous species were more abundant in the offshore and deeper waters at Berkeley than at O ldbury. Only one of the two species that complete their life cycles in the estuary was even moderately abundant in the inner estuary and the 15 freshwater species were relatively rare. Bass and particularly the sand goby complex were more numerous in the protected, inshore waters than the more offshore waters of the estuary. With the yellow and sil ver stages of the European eel, the reverse situation pertained. Seaso nal changes in faunal composition were more pronounced in the inshore shallow than in more offshore deeper waters of the estuary. This large ly reflected the sequential immigration of large numbers of the juveni les of marine estuarine-opportunist species into the former area for r elatively short periods. Although the ichthyofaunal composition in the shallows at Oldbury underwent the same pattern of cyclical variation in each of Eve consecutive years, the degree of intra-annual variabili ty differed, reflecting interannual differences in the recruitment str engths of the 0+ age classes of the different marine estuarine-opportu nists. These cyclical changes were not correlated strongly with either salinity or water temperature. The faunal composition of the protecte d inshore, more marine waters of the inner Bristol Channel differed fr om those in both inshore and offshore regions of the inner estuary. Th e species which typified the fauna of the Channel were bib, poor cod, five-bearded rockling, sole and conger eel. Although the first four of these species were relatively more abundant in these waters than in t he estuary, their juveniles often made extensive use of the shallows a t Oldbury. This study emphasizes that, for some marine species, the pr otected inshore, and more marine, waters in the Bristol Channel can ac t as alternative nursery areas to those provided by the inshore shallo ws of the Severn Estuary.