20+ Years of Hydroacoustic Fish Monitoring at Windermere, U.K.

Regular day and night hydroacoustic surveys have been undertaken on the fish communities of the north and south basins of Windermere, U.K., at monthly intervals since 1990.  The resulting dataset of over 20 years duration has encompassed a period of great change in several of the fish populations of this England’s largest natural lake.  These changes have also been monitored by more traditional netting and sampling techniques since the 1940s and complemented by the collection of a range of environmental data over the same period.  Most notably, since the early 1990s the native salmonid Arctic charr (Salvelinus alpinus) has shown a marked decline in abundance, while the introduced cyprinid roach (Rutilus rutilus) has undergone a population explosion.  The contributions from hydroacoustic surveys to our understanding of these trends, including in particular changes in overall fish abundance and seasonal shifts in the vertical distribution of Arctic charr in response to deteriorating oxygen conditions, are illustrated.  Both of these changes are illuminated by comparisons between observations from the lake’s north and south basins, which although they share a common recent temperature increase have experienced contrasting histories of eutrophication.  Some initial results from the exploration of data from the north basin to investigate the population dynamics of young-of-the-year fishes are also presented.  Finally, the implications of the seasonal and diel patterns evident in these hydroacoustic data for the effective design of hydroacoustic surveys in temperate lakes are briefly considered.