The length growth was estimated, using the Dahl-Lea back-calculation method, in the vendace inhabiting two vendace-type lakes (Łańskie and Pluszne), similar in surface area (about 1000 hectares) and depth (more than 50 m), which, for many years, have been showing extensive fluctuations in vendace catches. The total length (LT.) of the vendace caught in the Pluszne was lower by more than 2 cm, on the average, compared to the total length of the vendace caught in Lake Łańskie, although gill nets of identical mesh sizes were used in both lakes. Analysis of length increments showed the Łańskie vendace to have grown at a faster rate. As from the second year of life on, the differences between mean lengths were very highly significant (Student’s t test, α<0.001). As demonstrated by comparing vendace growth between the early 21st century (samples collected during the 2003 spawning season and March 2004; in the case of Lake Pluszne, a sample obtained in July 2005 was included into the analysis) and the 1960s and 1980s, the present (early 21st century) growth rate in the Pluszne is slower than it used to be. In contrast, the vendace growth rate in the Łańskie has not changed. The Pluszne vendace grow rate shows a pattern similar to that reported from the vendace of Lake Narie, known in past decades for its slow growth. Most probably, the unfavourable conditions of life the Pluszne offers the older vendace at present (the fish stay mainly in the epilimnion during the summer), manifested as intensified competition for food, result in periodic food deficiencies and retard the individual growth rate.

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