Seabird Abstracts

Seasonal changes in movements, abundance, size composition and diversity of

the fish fauna of the Severn Estuary

Claridge PN, Potter IC & Hardisty MW (1986) J Mar Biol Ass UK 66:229-258

Extensive sampling of the intake screens of power stations in the Severn Estuary (Berkeley, Oldbury-upon-Severn and Uskmouth) and Bristol Channel (Hinkley Point) yielded a total of 97 species of lampreys, elasmobranchs and teleosts. Data were most comprehensive for Oldbury in the inner estuary where samples of all the fish collected over 24 h were obtained on four occasions in each month between July 1972 and June 1977. The Gadidae was the most abundant family at Oldbury, both in terms of numbers of individuals (51934) and species (13). The fifteen most abundant species at Oldbury included two anadromous (river lamprey (Lampetra fluviatilis), twaite shad (Alosa fallax)), one catadromous (European eel (Anguilla anguilla)), one estuarine (common goby (Pomatoschistus microps)) and one freshwater species (3-spined stickleback (Gasterosteus aculeatus)). The remaining ten species, which fall within the broad category of estuarine-dependent marine species, contained a large proportion of O+ individuals. This group comprised a species complex consisting of two morphologically very similar sand gobies (Pomatoschistus minutus and Pomatoschistus lozanoi), which were only separated during one year of the study, and the whiting (Merlangius merlangus), flounder (Platichthys flesus), bass (Dicentrarchus labrax), sea snail (Liparis liparis), poor cod (Trisopterus minutus), thin-lipped grey mullet (Liza ramada), herring (Clupea harengus), sprat (Sprattus sprattus) and bib (Trisopterus luscus). Juveniles of the last nine species took on average between11-15 and 38-42 weeks to enter the shallows in the middle of the inner estuary from their spawning grounds, often having previously passed further up the estuary as postlarvae. These species showed a markedly seasonal pattern of occurrence at Oldbury, with the majority of each usually being collected within distinct two month periods. The number of species, and to an even greater extent the total number of fish, underwent consistent seasonal trends, with maximum and minimum values for the latter occurring between September and January and between March and May respectively. The seasonal trends for species richness (D), Shannon-Wiener (H') and Evenness indices (J) were similar, with maximum and minimum values generally occurring in the winter and summer respectively. A comparison between our data and those of earlier workers indicates that no major change has occurred in the composition of the fish fauna of the Severn Estuary during this century, except for the establishment of two 'northern' species, northern rockling (Ciliata septentrionalis) and Norway pout (Trisopterus esmarkii), during recent times.



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