Nitrogen and phosphorus removal by fishing in a large freshwater lake (Lake Balaton, Hungary)

ABSTRACT Fish biomass can serve as a large temporary sink of limiting nutrients, and thus fishing may represent an important anthropogenic nutrient efflux from aquatic ecosystems. However, the significance of this type of nutrient removal has rarely been evaluated for freshwater lakes. The aim of this study was to reveal how fishing contributed to the nutrient output from Lake Balaton (Hungary), the largest lake in Central Europe. The results show that net fish removal (the difference between the removed and stocked fish biomass) returned to land on average (standard deviation) 3.1 (0.3) tonnes of phosphorus (P) and 10.2 (1.1) tonnes of nitrogen (N) per year from Lake Balaton between 2017 and 2019, which is equivalent to a removal rate of 0.05 (0) kg ha−1 yr−1 P and 0.17 (0.02) kg ha−1 yr−1 N. These rates corresponded to 7.5% (1.5%) of the annual external P and 1.4% (0.6%) of the annual N loads from the inflowing watercourses. These findings suggest that fish harvest was moderately effective at offsetting external loading during the observed period. To mitigate the ongoing within-lake nutrient accumulation, more intensive selective fishing should be promoted in Lake Balaton, along with reconsideration of regulations on fishing practices.