The barbel was originally a fish with a relatively restricted range in the British Isles. However, despite the general environmental degradation of many British rivers, and in contrast with some native freshwater fishes, the barbel is now more widely distributed than it was a century ago. It may even be more abundant numerically.

The occurrence of post-F1 generation hybrids between common bream and roach in natural populations has still to be qualified. To do this, common bream, roach, F1 and F2 hybrid progeny were produced under controlled conditions. All progeny groups exhibited overlaps in the ranges of their meristic characters. Discriminant analysis of meristic data combined with allozymes allowed identification of post-F1 hybrids. Enzyme electrophoresis showed F1 progeny to be heterozygous at diagnostic loci. Back-crossed F2 progeny had a mixture of homozygosity and heterozygosity at diagnostic loci. Meristic and genetic analysis of fish from a natural population indicated that they were all F1 hybrids. Post-F1 hybridization cannot be ruled out from natural populations because F1 hybrids are known to be fertile. It is suggested that post-F1 hybrids were not detected either because of their low incidence of occurrence or due to biological phenomenon such as inferior fitness. 1997 The Fisheries Society of the British Isles 1997 The Fisheries Society of the British Isles

Adult roach, bream and their presumed F1 hybrid from an Anglian Water reservoir were identified on the basis of morphological and meristic characteristics. The hybrid was clearly intermediate. Four hybrid breeding crosses were induced to spawn by hypophysis. A bream × roach cross (female named first) failed to produce fertile eggs, whereas F1 hybrid × roach, roach × F1 hybrid and F1 hybrid × F1 hybrid all produced fry. Fertility (defined as survival of eggs to hatching) was high for the F1 hybrid × roach back‐cross (56%) but low for the others (<2%), in comparison to the pure species controls (roach 69%, bream 76%). Progeny from these crosses were reared until anal fin rays could be counted. These counts indicated intermediacy between the parents and back‐crossed individuals, and similarity between F1 hybrids and their F2 progeny.

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 arc presented showing that mean estimates of standing crop in the broads vary from < 1 gm-' in the winter to 9.4 g m-' 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 g m-'. Accurate overall estimates of fish standing crop can only be obtained when fish are maximally dispersed during the summer months.