Fish fauna of the Severn Estuary. Are there long-term changes in abundanceand species composition and are the recruitment patterns of the main marine species correlated?

Fish were collected from the intake screens of the Oldbury Power Station in the Severn Estuary in each week between early July 1972 and late June 1977 and at least twice monthly between early January 1996 and late June 1999. The annual catches, after adjustment to a common sampling effort, demonstra te that the abundance of fish at Oldbury was far greater in the 1990s than 1970s, mainly due to marked increases in the numbers of certain marine spec ies, such as sand goby, whiting, bass, thin-lipped grey mullet, herring, sp rat and Norway pout. These increases may reflect the great improvement that occurred in the water quality of the Severn Estuary between these decades. The only species that declined markedly in abundance was poor cod. Modest declines in flounder and River lamprey paralleled those occurring elsewhere in the UK. The species composition in the two decades also differed, refle cting changes not only in the relative abundances of the various marine est uarine-opportunistic species, which dominated the ichthyofauna, but also in those of the suite of less abundant species in the estuary. The cyclical c hanges undergone each year by the species composition of the fish fauna of the Severn Estuary reflect sequential intra-annual changes in the relative abundances of species representing each of the marine, diadromous and fresh water categories. New approaches have been developed to test whether or not large sets of correlations between patterns of recruitment amongst abundan t marine species (internal correlations), and between those patterns and sa linity and water temperature within the estuary (cross-correlations), were significant. The correlation profile analyses found no evidence that the an nual recruitment strengths of these species were either intercorrelated, or correlated with either one or a combination of both of the above environme ntal variables. Yet, the timings of the recruitment of these species into t he estuary were intercorrelated, i.e. a slightly earlier or later than norm al immigration by one species in a given year was paralleled by the same tr end in other species. However, this association in recruitment times could be linked neither to salinity nor water temperature within the estuary, nor to a combination of these two variables. These results indicate that, whil e the factors that influence the annual recruitment strengths of the juveni les of different marine species vary, inter-annual differences in the phasi ng of events that regulate spawning times and/or larval dispersal influence , in the same direction, the times when marine species are recruited into t he estuary. (C) 2001 Elsevier Science B.V. All rights reserved.