Hydroacoustics and concurrent experimental trawling reveal extreme annual variation in the density of 0+ pikeperch in late summer

Abstract

The density and mean size of 0 + pikeperch (Sander lucioperca) were studied in late summer in 2004–2017 in clay-turbid and eutrophic Lake Tuusulanjärvi in southern Finland. Hydroacoustics and simultaneous experimental trawling were used to estimate the density of 0 + pikeperch. In some autumns, 0 + pikeperch was the most abundant species in the pelagic fish assemblage. However, the annual amplitude of pikeperch density was extremely high (1,300 - 19,900 individuals ha−1). The density and size variations of 0 + pikeperch were analysed in relation to air temperature sums and degree days. Unexpectedly, the density of 0 + pikeperch showed no correlation with the air temperature sums or degree days, but both the mean length and weight of 0 + pikeperch correlated positively with these temperature variables. The highest correlation coefficients between mean size and temperature were found with temperature sums over 10 °C and degree days over 5 °C. The correlation between density and condition of 0 + pikeperch was negative but non-significant. The high density of 0 + pikeperch in some autumns suggests that pikeperch may play a central role in the pelagic food web in eutrophic lakes – not only as a predator of planktivores but also by its own planktivory.