Early formation of ice cover and low water level coupled with thick ice and snow cover are causing mass deaths of fish in Estonia. The Ministry of the Environment has received information concerning mass deaths of fish due to oxygen deficit from the Lääne, Harju, Lääne-Viru, Jõgeva, and Saare counties. In case the current weather conditions will continue, oxygen deficit may threaten a high number of additional water bodies. According to mr. Ain Soome, Head of Fish Resources Department of the Ministry of the Environment, the state of fish stock may especially suffer in water bodies used for commercial fishing. Possibilities for improving the situation are quite limited. "Nature takes its own. On the other hand, in water bodies not used for commercial fishing oxygen deficit will rather improve their ecological balance and increase the transparency of water," said mr. Soome. According to data of the Institute of Zoology and Botany of the Estonian Agricultural University, the situation is relatively more threatening in low entrophic lakes with slow water exchange. This category also includes lakes of fisheries importance such as Lake Võrtsjärv, lakes in Vooremaa , and lakes Maardu, Tamula, Ermistu, Tõhela, Harku, and Veisjärv. Oligotrophic deep lakes are also threatened, however, these are mostly smaller and have importance for fisheries at local level. The least threatened lakes are those deep and large in area. Oxygen deficit can be mitigated by cleaning the ice of snow and cutting of holes in the ice, especially near the water inflows. The cutting of ice holes over the whole lake is of relatively low effect. In case the lake has lush bottom vegetation or micro-algae, photosynthesis can occur in areas clean of snow, thus enabling production of oxygen. Cleaning of snow has more effect in the second half of winter when the days get longer. Artificial aeration of water is feasible only in smaller lakes. Pumping of water onto ice might be a technically least complicated and good solution. The pumped-out water can then be directed back into the lake through holes dug in a different area of ice. Only surface water layers may be pumped out; the pumping of bottom water layers may on the contrary cause further damage to aquatic wildlife. At the undertaking of such activities, cost-benefit analysis should first be carried out. It should also be considered that January is just the beginning of winter. According to scientific assessment, winter death of fish will cause changes in the lake ecosystem for a few years. Some species, such as the Crucian Carp and the Tench, have the ability to survive over winter in lakes with oxygen deficit. With serious reduction of stock of smaller fish such as the Roach and small Perch, the proportion between the different links of the food chain will change and the lake water should become more transparent and clean.This will be caused by increase in the number of zooplankton - one of the most important food objects of smaller fish -, zooplankton in its turn feeds on micro-algae which often causes low transparency of water. Thus, nature through giving us severe winter contributes to improving the quality of lake water.
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