Planning and execution of the fish reduction in Lake Ringsjön

Abstract

Lake Ringsjön, a naturally productive lake in southern Sweden, was subjected to high external phosphorus loading during the 20th century and became highly productive. In spite of effective waste water treatment which reduced external phosphorus loading, the improvements in lake water quality were moderate and proceeded at low rate. Consequently cyprinid reduction, which was expected to affect water clarity via several intertrophic mechanisms, was implemented as a measure for restoring the lake. The fish removal was performed using a board trawler equipped with a herring trawl modified for bottom trawling. The bottom rope of the trawl was 11 m and the total length of the trawl was 20 m, the cod end mesh size was 11 mm (knot to knot). In the beginning the catch amounted to 1 ton of fish in less than 2 hours, logistic problems bringing the catch to shore and replacing all piscivorous fish into the lake reduced the initial daily catch to 2–3 tons. A grand total of 240 tons of fish were removed by trawling Lake Ringsjön over a 3 year period. The catch was either fermented with myric acid or deep-frozen before being used as mink and pelican food respectively. Some of the catch was also used for gas production. The cost for boats and equipment was approximately 60 000 US$. The three persons needed for fishing and sorting as well as the costs of taking care of the catch on land are not included in this sum.