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

Abstract

Commercially exploited tench, Tinca tinca, (L.) populations in 480 lakes in the northeastern Poland were analyzed on the basis of the fishery data from 1949 to 1994. The long-term changes in tench catch were studied in four categories of lakes with respective proportions of littoral zones (<30, 31–60, 61–90 and <90% of total lake area). The analyses were performed with using relative catch (kg ha⁻¹ month⁻¹) of tench. Mean relative catch of tench for 1949–1994 ranged from 0.36 kg ha⁻¹ month⁻¹ in lakes with smallest littoral zone (>30%) to 0.93 kg ha⁻¹ month⁻¹ in lakes with the largest littoral (<90%). Mean relative catch increased with increasing proportions of littoral zones in lakes. However, long-term trends in mean relative catch of tench showed decreasing tendencies in all lakes. The beginning of these tendencies was found about ten years earlier in lakes with smaller littoral (<60%) in comparison to lakes with larger littoral zones (>60%). Mean stocking rates of tench (number of 1-year-old fish per ha of littoral) for 1949–1994 were not significantly different in lakes of studied categories. However, lakes with proportionally less littoral zone had reduced stocking efficiency estimated as number of stocked fish per ha of littoral to require 1 kg ha⁻¹ month⁻¹ of tench.