Temporal abundance patterns and growth of juvenile herring and sprat from the Thames estuary 1977-1992

Most herring Clupea harengus and sprat Sprattus sprattus sampled from West Thurrock power station intake screens, middle Thames estuary, between 1977 and 1992 were age-0 and followed regular patterns of seasonal occurrence. J uvenile herring entered the estuary in July, peaked in abundance November t o March, and then declined. Juvenile sprat first appeared in September and peaked in abundance in January. Neither species was abundant in summer samp les. While in the estuary, herring and sprat increased in length an average of 4.0 and 0.33 cm respectively. Abundance of both species was significant ly affected by temperature, temporal trend, shoaling behaviour and seasonal variables, and of herring by suspended solids, interactions between enviro nmental variates did not influence the abundance of either species. As estu arine clupeids were influenced by a complex set of events within and outsid e the estuary, estuarine monitoring studies alone will not be sufficient fo r understanding the changes in estuarine fish communities resulting from fu ture human activity. (C) 2000 The Fisheries Society ef the British Isles.