The biology and conservation of the fish assemblage of the Severn Estuary (cSAC)

The Severn Estuary is the second largest estuary in the UK and has been designated a cSAC because it contains habitat types and species that are rare or threatened in Europe, including the anadromous river and sea lampreys and the twaite and allis shads. Extensive intertidal mud and sandflats support vast numbers of benthic invertebrates, fish and internationally important populations of wading birds. More than a hundred species of fish have been identified from the Severn Estuary and its seaward extension, the Bristol Channel. Most of our knowledge of the estuary’s fish community comes from individuals entrained on the cooling water-intake screens used at power stations sited along the English and Welsh shores. Fish abundance at Hinkley Point power station situated at the seaward margin of the estuary in Bridgewater Bay, has been monitored for more than 25 years and similar records are available from Oldbury power station from the 1970s and 1990s. These data have been used to describe the biology and ecology of the more abundant species of fish and crustaceans in the estuary.Power station samples have enabled factors that affect seasonal and annual variations in the abundance of individual species to be elucidated. The pattern of fish abundance that emerges is one in which sequential waves of different species enter the estuary at specific times of year. These seasonal changes in the fish assemblage are remarkably consistent, but the abundance of individual species can fluctuate markedly from year to year depending on breeding success, juvenile recruitment, changing temperatures, salinity and freshwater discharge. The most abundant species of fish consist primarily of the juveniles of marine species that utilise the estuary as a nursery that are termed marine estuarine-opportunists. Other life-cycle categories include marine stragglers and freshwater species that occasionally stray into estuarine waters, and anadromous and catadromous species that use the estuary as a migratory corridor. The report reviews the biology and ecology of 27 species that are dependant on the estuary for at least some part of their life-cycle and considers factors affecting their seasonal abundance, growth, behaviour and conservation status.Possible reasons for the marked increase in fish abundance that has occurred gradually since the 1970s and more rapidly since 2002 are discussed. The fact that sediment and water quality in the estuary has improved, particularly with regard to heavy metal contamination, may partly explain the increase in fish numbers, but increases in water temperature linked to climate change are likely to be very significant. Other factors that may adversely affect the fish community and threaten its stability include the proposal to construct a barrage across the estuary that will affect the distribution of sediment and interfere with the migratory activity of protected species. Of particular concern is the potential for turbine-induced mortality that is likely to be very significant for some migratory species and also for those marine estuarine-opportunists that show seasonal movements in and out of the estuary. The report concludes by summarising the health of the estuarine fish assemblage and makes recommendations for its future management and conservation