Gergely Boros, a senior researcher at the Balaton Limnological Research Institute (BLKI) of the Eötvös Loránd Research Network, has studied the extent to which recreational angling and other methods of fish removal, such as fishing or carcass removal, result in nutrient outflows from Lake Balaton, and how the amount of nutrient removed relates to the external nutrient load found in the lake. A study presenting the results of research based on data from 2017 to 2019 has been published in the journal Inland Waters.

It has long been known that fish stocks can have a significant impact on metabolic processes in aquatic ecosystems. Fish dig up the sediment in search of food. They can mobilize the nutrients in it and release significant amounts of nutrients into the environment through their metabolic by-products. However, they can also act as a ‘nutrient trap’, as they bind a relatively significant amount of nitrogen and phosphorus in their bodies, making these nutrients temporarily unavailable to other aquatic organisms. In general, these two biogenic elements play a key role in water quality and the evolution of eutrophication processes, as their available quantity can fundamentally determine algae growth.

Because fish biomass stores large amounts of nitrogen and phosphorus, significant nutrient extraction from the internal stock of aquatic ecosystems can be achieved through recreational angling and fishing. This can ultimately lead to an improvement in water quality and partially offset the external nutrient load. Numerous domestic and international examples support the success of such interventions, but no scientific analysis of the impact of fishing and fishing had been carried out in the case of Lake Balaton.

According to a study by a BLKI researcher, between 2017 and 2019, an average of 3.1 tonnes of phosphorus and 10.2 tonnes of nitrogen left Lake Balaton per year as a result of net fish removal, i.e. the difference between fish stocks and catches. These amounts are difficult to interpret on their own, so it is advisable to compare them to external sources of nutrient load. This comparison shows that fish removal accounted for an average of 1.4 percent of the nitrogen load from the inflow and 7.5 percent of the phosphorus load from such a source.

As a result, it can be stated that fishing and fishing catches can only moderately compensate for Lake Balaton’s external nutrient load. However, in the case of fishing exploitation, the net nutrient balance, in addition to the amount of fish caught, also depends to a large extent on the installations carried out for successful fishing and the amount of bait used. Researchers will now try to determine the quantity of nutrients that can be brought into Lake Balaton each year through bait.