Northern pike (Esox lucius) in a warming lake: changes in population size and individual condition in relation to prey abundance

Climate change is anticipated to have differential effects on the salmonid and percid-cyprinid components of lake fish communities, which may in turn impact the foraging conditions and so population biology of top aquatic predators including northern pike (Esox lucius Linnaeus). Since the 1940s, extensive monitoring has been undertaken on the northern pike, the salmonid Arctic charr (Salvelinus alpinus) and the percid Eurasian perch (Perca fluviatilis) populations and environmental features of the mesotrophic north and eutrophic south basins of Windermere in the English Lake District, UK Analysis of data from 1982 to 2006 revealed basin-specific changes in the degree of eutrophication, but a common warming of the lake in the 1990s was accompanied by contrasting trends in the two basins’ northern pike populations. Following a common temporary increase in catch-per-unit-effort (CPUE) during the 1980s in response to a lake-wide decrease in fishing pressure, CPUE of the south basin subsequently decreased while that of the north basin showed a second increase in the late 1990s to reach a peak in 2000. In addition, the mean weight at length of female northern pike was initially similar between the two basins but then increased in the north basin from the early 1990s to the early 2000s before decreasing, while in the south basin it has shown a decrease since 2000. These changes are interpreted with respect to contrasting trends in potential prey populations, including an overall decrease in the abundance of the coldwater Arctic charr, particularly in the more eutrophic south basin where hypolimnetic oxygen levels have fallen to very low levels, and a recent marked increase in the abundance of the warmwater roach (Rutilus rutilus) in both basins. It is concluded that although indirect in nature, the recent warming of Windermere has had a significant impact on its northern pike populations.