Lake Ohrid - Human Influence

Human Influence

Shore habitats are under particular pressure from human activities. Particular threats are the building of tourist facilities directly at the shore, destroying of reed belts to gain agricultural land and intense pollution close to the mouth of tributaries. Although the effects of these human impacts have not been evaluated in detail they are of great concern, as the shallow water sites are particularly rich in endemic bottom fauna and form important spawning grounds for several endemic fish species. Moreover reed belts have great importance for water birds.

Commercial fish yield, i.e. the two endemic trout species, has dropped significantly over the past decades, both in Macedonia and in Albania. The most probable reason is overfishing and possibly destruction of spawning grounds. Although there are regulations regarding fishing practice (e.g., minimal mesh size) and only a limited number of licensed fishermen, these rules are always not obeyed, as a result of the high market value of the endemic trout. As a reaction to the situation, fishing has been banned for the past two years, to help the fish population recover and to allow scientists collect further data. While most of the endemic fish species are non-migratory, the European eel spawns in the distant Sargasso Sea while its offspring return to the lake. Unfortunately, like in many European lakes, it is very unlikely today that eels can reach Lake Ohrid naturally and return to the Sargasso Sea, as a result of several hydropower dams on the Black Drin and the Drin River, both in Macedonia and Albania. As a result, eel found in Lake Ohrid are stocked populations.

Given the population growth over the past 50 years, a particular concern is the potential eutrophication of currently oligotrophic Lake Ohrid from increased pollution. Indeed, sediment cores show a ~3.5 fold increase in phosphorus concentration over the past century. On the one hand, shifts from endemic to common European species, which are better adapted to higher nutrient conditions, have already been observed close to polluted inflows. On the other hand, higher nutrient levels have reduced the water transparency, as well as the oxygen availability in the deep water and at the lake bottom, two properties which are requisite for the endemic flora and fauna. Still, the lake is in a comparably good state at the moment. However it may take more than a decade to see the effects of today's pollution level in the lake, because of its long water residence time. Moreover it was shown that the negative effects from eutrophication would be significantly amplified by global warming. Although there is time to react, computer simulations indicate that at least a 50% reduction in phosphorus input must be reached to keep the deep water oxygenated for the next 50 years at predicted atmospheric warming. This aim could be reached by controlling household wastewater, which is by far the biggest phosphorus source at the moment. First steps in that direction have been taken by extending and improving the existing sewage system in Macedonia in the framework of a GEF (Global Environment Facility) program. The most important next task would be a solution for three remaining, severely polluted tributaries, one in Macedonia and two in Albania.