Growth and abundance of O-group herrings, Clupea harengus L., in the Severn Estuary

Collections made from the cooling water intake screens of Power Stations in the Severn Estuary and the Bristol Channel have been used to investigate the biology of O-group herrings in this region. The abundance of herrings at Oldbury, the principal sampling site, was low in most years and in five successive seasons between July 1972 and December 1976, large numbers were caught only between the summer of 1975 and the spring of 1976. The herrings were first observed in July, when many had still not completed metamorphosis, and reached peak numbers in September and October during which period they were clearly growing. Numbers declined in December before rising dramatically in January at which time the length-frequency curves displayed a pronounced bimodality. On the basis of data from other parts of the Bristol Channel and Severn Estuary, it is suggested that this bimodality is due to the influx of some animals from other regions where the growth rate was greater. Although movement is apparently taking place at this time and the numbers decline markedly in subsequent months, the last remnants of the population do not leave the estuary until early May. Post-pelvic scute (K₂) and vertebral counts (VS) of 13.82 and 55.24 respectively, indicate that the Severn Estuary herrings are the product of a spring spawning stock, a view entirely consistent with length and field data.