Seasonal, annual and regional variations in ichthyofaunal composition in the inner Severn Estuary and inner Bristol Channel

Abstract

Monthly samples of fish from the intake screens of power stations at Oldbury and Berkeley in the inner Severn Estuary and Hinkley Point in the inner Bristol Channel, were used to analyse the community structures of the ichthyofauna in these regions. Marine species that use the estuary as a nursery area (marine estuarine-opportunists) were very abundant in the shallow inshore waters at Oldbury. Diadromous species were more abundant in the offshore and deeper waters at Berkeley than at Oldbury. 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 silver stages of the European eel, the reverse situation pertained. Seasonal changes in faunal composition were more pronounced in the inshore shallow than in more offshore deeper waters of the estuary. This largely reflected the sequential immigration of large numbers of the juveniles of marine estuarine-opportunist species into the former area for relatively short periods. Although the ichthyofaunal composition in the shallows at Oldbury underwent the same pattern of cyclical variation in each of five consecutive years, the degree of intra-annual variability differed, reflecting interannual differences in the recruitment strengths of the 0+ age classes of the different marine estuarine-opportunists. These cyclical changes were not correlated strongly with either salinity or water temperature. The faunal composition of the protected inshore, more marine waters of the inner Bristol Channel differed from those in both inshore and offshore regions of the inner estuary. The 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 the estuary, their juveniles often made extensive use of the shallows at Oldbury. This study emphasizes that, for some marine species, the protected inshore, and more marine, waters in the Bristol Channel can act as alternative nursery areas to those provided by the inshore shallows of the Severn Estuary.

Fish communities in temporarily open/closed estuaries from the warm- and cool-temperate regions of South Africa: A review

“…The dominance of marinespawning teleosts in temperate estuaries has been reported from TOCEs and permanently open estuaries worldwide (Loneragan et al 1989;Whitfield et al 1994;Potter et al 1997;Griffiths 2001;CastilloRivera et al 2002;Harrison 2003). In warmtemperate estuaries, Mugilidae and Sparidae were represented by the greatest number of species.…”

Temporal abundance patterns and growth of juvenile herring and sprat from the Thames estuary 1977–1992

“…Seasonal migrations further determine temporal patterns (van den Broek Potter et al, 1986Potter et al, , 1997Maes et al, 1998). Turbidity (Blaber & Blaber, 1980) and interactions between physico-chemical variates, most notably dissolved oxygen and temperature (Pomfret et al, 1991), are also thought to influence species abundance.…”

“…Studies of fish community structure in the Severn (Potter et al, , 1997 attributed changes in community structure to the sequential immigration and emigration of species from the estuary, noting that variations in flow, salinity and temperature acted only indirectly under extreme environmental conditions to influence abundance. However, standardized temperature regression coefficients in the Thames models indicated temperature was a more important determinant of abundance than was season.…”

The Decapod Custacean Community of the Guadalquivir Estuary (SW Spain): Seasonal and Inter-Year Changes in Community Structure

“…A seasonal pattern showing well-defined temporal changes in the species' composition has been previously found in aquatic communities in temperate estuaries (Potter et al, 1997;Maes et al, 1998;Thiel & Potter, 2001). It is clear that in temperate latitudes with four different well-defined seasons, the associated changes in the light-time/night-time pattern and water temperature play an important role in the timing of the reproductive activities of aquatic species and, as a result, in the seasonal succession of these species in estuarine communities.…”