The UK has some important natural fish communities, which require active conservation via habitat protection and ensuring that other fish species that may upset the existing ecological balance are not introduced to these key sites.

Where sites contain Lampreys, Vendace, Shad, whitefish, Smelt, Charr Salvelinus alpinus, Bullhead or other fish species of conservation concern, information on the status of stocks is particularly important.

Whether a fish stock is self-sustaining in the long term is an important attribute: some exploited freshwater fish (e.g. Brown Trout Salmo trutta) are now routinely stocked with hatchery-reared individuals. Stocking can have impacts on locally adapted fish populations, and stocked fish can give the impression that a population is abundant when, in fact, it is not self-sustaining. The distribution of successfully reproducing fish is a valuable measure of the ecological condition of a given river system. Tributaries or main river stretches in which fish are unable to spawn successfully may indicate, for instance, habitat degradation of various forms or barriers to migration. Fisheries surveys can therefore produce important insights into the health of the overall aquatic environment.

All natural fish stocks fluctuate in abundance in response to changing environmental conditions and degrees of exploitation. Any single measure of abundance is therefore of limited value unless it is viewed in the context of historical change. Long-term catch records, for instance for Atlantic salmon Salmo salar, can be particularly valuable in helping to determine likely trends in population abundance.