1. The growth and the spawning and feeding habits of nine species of British freshwater fish are described. These species are:—Rutilus rutilus, Scardinius erythrophthalmus, Leuciscus leuciscus, Abramis brama, Blicca bjoerkna, Gobio gobio, Perca fluviatilis, Acerina cernua and Esox Lucius.
2. Growth has been studied by the use of scale-reading. Individual growth-rates vary greatly, the range in size of any age group nearly always exceeding the differences in average length between that age group and the next junior and senior age groups.
3. In the cyprinids, male and female fish are at first equal in numbers, but the males die off more swiftly than the females and are in the minority in the senior age groups.
4. Freshwater fish show a wide range of choice of foods; such species as Rutilus rutilus and Scardinius erythrophthalmus may be herbivores or carnivores as opportunity offers.
5. Two instances have been found of closely related species sharing an ecological niche. These species are:—Abramis brama with Blicca bjoerkna, and Perca fluviatilis with Acerina cernua.