Environmental influences on the long-term fluctuations in the abundance of gadoid species during estuarine residence

Abstract

Samples taken regularly from the intake screens of West Thurrock power station between January 1977 and November 1992 were used to examine physico-chemical factors influencing the abundance of Gadidae species (poor cod, Trisopterus minutus; pouting, Trisopterus luscus; and whiting, Merlanguis merlangus) in the Thames estuary. Most sampled fish were age-0 and followed a dominant pattern of seasonal occurrence. Multiple regression was used to model variations in sample abundance in relation to fluctuations in estuarine environmental variables, interactions between environmental variables and seasonal factors. Model results were used to examine hypotheses concerning the relative importance of temperature, salinity, prey availability and seasonal factors as determinants of estuarine gadoid abundance. Temperature was the most important determinant of species sample abundance and negatively related to sample abundance in all cases. Salinity was a major determinant of pouting abundance and a minor determinant of whiting abundance, with seasonal factors significantly influencing the occurrence of all species. Crangon abundance was a minor determinant of whiting and poor cod abundance. Interactions between environmental variables played a minor role in determining the sample abundance of a single species, pouting. Determinants of Gadidae species abundances in the Thames appear to be a complex mix of seasonal and environmental influences, with seasonal influences determining the dominant cyclical pattern and influences of temperature having the greatest effects on short-term variations in the pattern.