Fish biomass and density data for 3 intensively studied Wessex chalkstream sites, England, UK

Samples were collected on five separate occasions from the three sites. Sampling did not occur during the salmonid breeding season, December to March, so as not to disturb redds. Protocols and standard procedures were strictly followed for benthic fish sampling and electrofishing but there was no formal quality assurance procedure, such as repeat surveying of the same reach on a different day within the same sampling period. Electrofishing was undertaken by fully qualified and very experienced fish biologists (Bill Bellamy and Luke Scott) from the Game and Wildlife Conservation Trust.

Fish populations were assessed by multi-pass electrofishing. Population density estimates were derived using the the Zippin maximum likelihood method (Zippin, 1958). As electrofishing is not particularly effective at catching certain benthic species e.g. bullheads, stone loach, we used a modified Hess sampler (0.12 m2) to assess populations of these species, deployed 20 times on each date, using a stratified-random regime. The mean biomass of individuals of each taxa captured at each site on each occassion was determined using length-mass regressions, and multiplied by the density estimate to derive a biomass estimate.