Last fall we conducted scientific fishing in Velnezers lake in order to assess the health of the fish community before the lake treatment
Results of the scientific fishing: there are few fish in Velnezers and they are small and malnourished
Last fall more than 80 local residents and media representatives gathered at lake Velnezers in Riga to observe scientific fishing for demonstrating and assessing the environmental status and fish community in the lake before the treatment. The event gained wide interest and was led by the research team of Latvian Institute of Aquatic Ecology. The amount of fish caught was small, slightly below 3.5 kilograms. Fishing results showed that the fish community of Velnezers could be more depleted than was believed at the beginning of the research.
Six species of fish found in Latvia were caught: perch, bleak, common roach, bream, white bream and ruffle. Catches were low for all fish species, but of additional concern was the fact that only 8% of the fish were predators, typical of lakes with high phosphorus loads and eutrophication. When conducting an in-depth analysis of fish in the laboratory, it was found that omnivorous fish are very thin and 57% of the time their stomachs were empty, while benthic feeding fish are closer to the optimal weight and only 27% of their stomachs were empty .
In summary, 1/3 of the fish had empty stomachs. Most of the fish caught were young and small in size. For example, the average length of perch was only 12 cm, which indicates age of one and a half years. For white bream significantly more young fish (one-year age class) were found, and their number decreased significantly with each subsequent year of the age class. The oldest fish caught were 5-year-old perch and roach, as well as 4-year-old bream and white bream. Although the maximum age of species in water bodies can vary significantly, all these species can live at least twice as long as was found in Velnezers.
Over a 10 month period the researchers have been monitoring the lake and have concluded that oxygen concentration in the water in summer at a depth of 2 meters is already below the critical level for fish survival, while in the winter, under the influence of the long-term ice cover, the oxygen concentration decreased even more and was well below the necessary level for fish survival already at the surface layer of the water (at a depth of 0.5 m). As a result, there was a mass fish die-off in the lake, and our researchers received alarming reports from local residents that: “The lake is full of dead fish!”. When the researchers surveyed the lake, it was found that many dead fish were frozen in the ice on its eastern shore. When studying the fish in more detail, the researchers found that certain fish species and age groups were affected more than others, which may be due to several factors.
The research and data collection will continue until October, when the treatment of the lake with a method developed in Sweden will take place. Due to technical challenges the treatment has been moved from May to October.
