Restoring water resources by managing fish stocks

Finland has a history of seine-fishing that dates back in 1200s. Back then people were fishing for domestic use and they were also selling fish. It was time of the Catholic Church, so people were eating fish at the time of fasting. In the Middle Ages Finland was exporting dry fish, but also soured barrel fish to England and Middle Europe. Vatican among others was significant buyer of Finnish dried fish. Fish trading was very important to Finnish economy and wealth of the nation.

Hansa Traders bought 160 - 200 tons of dried pike fish annually from Lake Kemijärvi in Southern Lapland. Professional fishing and especially fishing control methods were very affective in the Middle Ages in Finland. People were able to control and increase productivity of valuable fish by managing fish stocks. Method, which we use today in managing fish stocks, is based to that old knowledge that we have now applied to cyprinids. This method also has a positive impact to overall condition of water resources.

As the condition of water resources is getting worse, there is a need to restore them in many different ways. As a result of eutrophication, the overgrowth of lakes and annual blue-green algae bloom has reduced the overall use of water resources as seen from a point of recreational and leisure use. Also the fish stocks have changed so that less valuable fish like cyprinids (roach etc.) dominate at the cost of valuable fish like salmon and trout. Selective fishing is an effective way to reduce overgrowth of less valuable fish populations and at the same time reduce eutrophication of water bodies.

Controlling fish populations is the most important work stage in managing fish stocks. Work begins by reducing overpopulated fish stocks by intensive fishing (seining, trap fishing). Then after a period of time work continues with fish stock management that is mainly follow-up work, and removing fish if needed. It usually takes about five years to balance internal loading. In intensive fishing  fish catch should be 20-80 kg / ha and in basic management of fish stocks about 10-40 kg /ha.

Eutrophication of water bodies is caused by external loading to lakes and also internal loading, which is caused by fish populations. External loading is caused by agricultural run-offs and forestry, natural inflow of nutrients and human activities (cultural eutrophication).

There are two reasons, which cause internal loading:

1) External loading, that triggers internal loading by using fish stocks. Eutrophication increases fish populations and distorts the natural balance of fish stocks.

2) Intensive fishing distorts the structure of fish stocks and this triggers increased internal loading.

There are eutrophicated lakes even in wilderness of Lapland. In those lakes eutrophication is caused by internal loading witch is caused due to distorted structure of fish stocks. The basic size of fish stocks is too big and the size of the fish is small. Fish population is causing water murkiness and nutrient distribution by digging sediment from the bottom of the lakes, and defecating. They also eat a lot of zooplankton. This leads to increased algae growth, and among others blue-green algae also grows rapidly. So algae, that was not eaten by zooplankton rots in the bottom of the lakes and consumes oxygen from the sediment. Loss of oxygen in bottom sediments causes distribution of nutrients to increase rapidly. Phosphorus is one of the worst nutrients that cause eutrophication.  Phosphorus distribution to the lakes can be very heavy. It is well known, that in lakes in Southern Finland, the internal loading to lakes at its worst can be about 10 times heavier in comparison with outside loading.

Among others, lakes before restoration were: Lake Tuusula, Lake Vesijärvi in Lahti and Lake Enäjärvi in Vihti. Nutrients which cause eutrophication, mainly phosphorus, come from nutrient reserves, which have accumulated over thousands of years in the sediment layers of the lakes. These reserves are practically inexhaustible. The only way to stop nutrient distribution of sediments is to restore the natural state of  sediments by using management of fish stocks.  Airing/oxygenation of water column also speeds up the recovery of sediments.

Managing fish stocks is very powerful. It affects fish populations and further to the whole delicate water ecosystem very much. If management of fish stocks is not carried out correctly it increases the inner loading to the lakes. Healthy structure of fish stocks is ruff and well balanced according to species of the fish. Overall contingent of 10 to 12 cm. cyprinids is only few percents of the whole fish population. Management of fish stocks plays a significant role of repairing water bodies and to restore good ecological condition of water resources.

To increase the overall condition of the water bodies, there are several ways to catch fish. Most efficient ways however are traditional seine fishing, fish trap and trawling. Fishing takes place usually at the open water season, but there are also good experiences of winter-seining.  Different ways of fishing are suitable in different types of lakes according to profile of lake and also how fish moves.