This document provides the information needed to assess the condition over time of a unionid population, and the level of information for assessing whether a plan or project may be detrimental to their future prospects. It provides guidance on methods for survey and monitoring unionid mussel populations and the environmental characteristics important for maintaining populations in favourable condition. The document is based on best practice developed and used by unionid mussel experts in Europe, and describes approaches that individual countries have adopted for survey, data analysis and condition assessment.

Standard methods for restoring populations are not within the scope of this document.