Sampling strategy related to fish distribution, with particular reference to the Norfolk Broads

Abstract

Large‐scale movements of fish within a complex freshwater system such as the Norfolk Broads can give rise to erroneous sampling data if the extent of these movements is not fully appreciated. The Norfolk Broads habitat is briefly described, and data are presented showing that mean estimates of standing crop in the broads vary from <1gm ‐2 in the winter to 9.4 gm ‐2 in the summer. The widespread nature of the phenomenon is demonstrated by winter and summer data from five broads, and a more detailed picture of the seasonal changes is shown by an intensive 2‐year study on one of these broads. Winter aggregations of fish have been found in sites adjacent to rivers connected with the broads, with densities of up to 36.7 fish m ‐2 and a biomass of 1787 gm ‐2 . Accurate overall estimates of fish standing crop can only be obtained when fish are maximally dispersed during the summer months.