Whitefish is the most abundant fish species in the Pasvik River System. Two different forms can be distinguished, based on a number of morphological features. First of all these two forms of whitefish have various number and form of gill rakers: densely rakered whitefish (densely rakered whitefish) had 28 - 41 oblong gill rakers, whereas sparsely rakered whitefish (sparsely rakered whitefish) had 16 - 30 shorter and thicker gill rakers (Amundsen et al., 1983). These forms differed in habitat and food selection: sparsely rakered whitefish basically dwelled in benthic habitats feeding on benthic invertebrates, whereas densely rakered whitefish dominated in the pelagic feeding on zooplankton. Densely rakered whitefish were the most numerous (ratio approx. 3 : 1 in total catch).

Densely rakered whitefish had average body lengths of 15.4 - 19.7 cm, and average weights of 53.5 - 99.7 g in catches from the studied localities. The mean size of these fish was reduced in localities close to the main source of pollution, and had negative correlation with Ni concentration in sediments.

Average size of densely and sparsely rakered whitefish and Ni concentration in sediments in the three studied lake localities and one control lake (Chunozero Lake; Reshetnikov, 1980).

Densely rakered whitefish aged 1+ - 3+ prevailed in the catches, whereas fish older then 4+ were rare. The densely rakered whitefish had an early maturation, and size at first maturation decreased with decreasing distance to Nikel. In Kuetsijarvi, fish ready to spawn were found with body length 6 -9 cm and age 1+.

The minimum size of male and female densely rakered whitefish from Pasvik River system at first time of spawning (control - Chunozero Lake) (Reshetnikov, 1980).

Sparsely rakered whitefish were of larger size and included a greater number of age groups than densely rakered whitefish. In our catches, these fish had maximum age of 8+ - 9+ ( in the control lake - 15+ - 18+ ). However, the basic population of sparsely rakered whitefish were of the age 2+ - 4+. Sparsely rakered whitefish were of smaller sizes near Nikel compared to more distant localities.

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