Consistency of seasonal changes in an estuarine fish assemblage

Abstract

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 con~munity in the shallows of the estuary underwent simdar cycllcal 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 vanations in water temperature, sahnity 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 fauna1 composit~on was modified under extreme environmental conditions.