Biometric data are presented for common freshwater fish species in England and Wales. These enable the lengths and weights of fish in the diet of piscivorous fauna to be estimated from measurements of bones and scales that are recovered from faeces and stomach contents. Use of these data was demonstrated by reconstructing the length composition of species in the diet of cormorants from their faeces and an otter from its stomach contents. Both species were shown to be exploiting common size classes of abundant fish species in their home range.