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Several freshwater species use the Kyrönjoki River estuary as a spawning and nursery area. The main reasons for this seem to be the morphology of the estuary, the abundance of shelter provided by aquatic macrophytes, high food production and favourable temperature conditions. Acidification of the estuary due to drainage from acidic soils has made part of the estuary unsuitable for fish reproduction. In addition, year to year fluctuations in the acidity of the estuarine water have affected the reproductive success of several species. The severity of the effects of the acidification at the population level is determined by the spatial and temporal distribution of the larvae and juveniles.