Consistency of seasonal changes in an estuarine fish assemblage

Data on the abundance of all fish species collected at weekly intervals from the intake screens of the Oldbury-upon-Severn Power Station in the inner Severn Estuary, U. K., between 1972 and 1977, have been analysed using classification and multi-dimensional scaling (MDS) ordination techniques. The results show that in each year the structure of the fish community in the shallows of the estuary underwent similar cyclical changes. These changes were largely attributable to a sequential immigration and emigration of different species, particularly estuarine-dependent marine species, and were not driven directly by variations in water temperature, salinity or freshwater discharge from the river. However, comparisons between the data for years with the driest and wettest winters show that the pattern of change in faunal composition was modified under extreme environmental conditions.