A protocol for stocking hatchery reared freshwater pearl musselMargaritifera margaritifera

Freshwater pearl mussel (Margaritifera margaritiferaL.) populations are under serious threat of extinction throughout their geographical range and only a few remnant populations are recruiting to adulthood. Consequently,M. margaritiferais classified as endangered on the International Union for Conservation of Nature Red List. Several institutions across many countries have set up Ark sites at hatcheries to culture and rear youngM. margaritiferafrom population remnants, with the intention of stocking these juveniles into rivers. The release location must fulfill the habitat requirements for the full life‐cycle of the species, so they can contribute to the next generation and thus the long‐term recovery of the species. However, little research or advice exists about how to decide if river environments are suitable for stocking. A protocol is presented for determining whether aM. margaritiferapopulation will benefit from stocking hatchery reared juveniles and how to identify suitable areas. Stocking locations are considered from catchment scale to microscale using water quality (reach), macrohabitat (site) and microhabitat, including physicochemical properties of the substratum (spots). A case study of the River Esk in north‐east England, is incorporated to exemplify the myriad of considerations surrounding attempts to conserveM. margaritifera, and describes how implementation of the protocol can structure and assist stocking programmes.