Changes in commercially exploited populations of tench, Tinca tinca (L.) in lakes of Northeastern Poland

Commercially exploited tench populations in 755 lakes in the Northeastern part of Poland were observed on the basis of the fishing data from 1951 to 1994. The observations were analyzed in five size categories of water reservoirs, whose total area reached over 123,900 ha. The analyses were performed according to the parameter of a commercial fish catch per area unit, taking into consideration the number of months per year when the lakes were exploited. Statistically significant differences were determined in the mean value of the tench catch parameter between the different size categories of lakes. Similar time-related differences were observed. In the lakes >100 ha the relative tench catch gradually declined in the successive time intervals. Tench was the rarest in catches from lakes <50 ha. The complete disappearance of tench from commercial catches occurred first as early as the late 1960s in lakes <200 ha. The largest quantities of tench stocking material were introduced to the lakes from 1951 to 1972. The amount of stocking material per 1 ha of stocked lakes decreased for larger lakes. The average frequency of tench stocking in the analyzed lakes was higher in larger lake size categories. On the other hand, in each lake size group the frequency of stocking was found to have declined along with the average volume of the stocking material in the subsequent 11-year-long periods.