The vendace Coregonus albulu (L.) populations in the lakes Mjma and Osensj~en exhibited fluctuating year-class strength. In M j~s a , a strong year-class emerged every third year. except for the four year period between the strong year-classes 1969 and 1973. The ditrerence between the strong and weak year-classes decreased from the 1960s, through the 1970s to the 1980s. The Mjosa vendace matured sexually at age 2+. and more than ten sexually mature age-groups were present in the population. Growth ceased at maturation. and asymptotic length was 23.6 cm. In Osensjoen. one strong year-class (1969) dominated the population during the period 1976 1987. The Osensjoen vendace matured sexually at age 3, and more than I5 sexually mature age-groups wcrc present in the population. Growth ceased at maturation, and asymptotic length was 28.4 cm. In both lakes. vendace fed on crustacean zooplankton in the epilimnion throughout summer and autumn. Our data indicate that regular year-class oscillations occur as a result of the juvenile survival being negatively correlated to the number of adults.

Larvae (eleutheroembryos) of cisco Coregonus albula and whitefish C. larvaretus started to drift downstream in the Gudbrandsdalslågen River concurrently with the beginning of the spring freshets each year, 1981–1983. The duration of yearly outdrift lasted 2–5 weeks in late April and May. The yearly maximum out‐drift was positively correlated with the rate of increase in water discharge, but not with water temperature during spring. Most larvae drifted downstream during night.

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Vendace (Coregortus albula) in Lake Mjøsa lived in the pelagic zone and gillnet catches were correlated with water temperature (r2 = 0.457, p < 0.005). Whitefish (Coregonus lavaretus) exploited both epibenthic and pelagic areas. Whitefish with body lengths between 25 and 35 cm moved from the epibenthic to pelagic zone during summer; other length groups remained in epibenthic habitats. The pelagic fish returned to epibenthic areas in the autumn. Cladocerans were important food items for both fish species in the pelagic zone. The large copepod Limnocalanus macrurus was eaten only by vendace whereas surface insects were more important prey for whitefish. This suggests that vendace is the more specialized zooplanktivore. The pelagic gillnet catch of whitefish and the pelagic habitat overlap between whitefish and vendace increased with increasing zooplankton densities at depths between 0 and 50 m (r2 = 0.609 and 0.494, respectively, p < 0.01). During spring and autumn we observed a time lag between the change in zooplankton abundance and habitat switches of whitefish. The fish moved to the more profitable habitat several days after food availability had changed, indicating an element of learning in choice of habitat.

SynopsisThe whitefish, Coregonus lavaretus, in the lake Mjosa exhibited two niche changes during their life cycle. Juveniles (< 2.5 cm body length) were confined to the shallow (0-30m) epibenthic zone. Medium sized whitefish (25-35 cm body length) expanded their habitat use to include the deep (30-90 m) epibenthic zone as well as the pelagic zone. From a body length of 35 cm, habitat use was restricted to the deep epibenthic zone. Small fish in the shallow epibenthic zone ate small and medium-sized prey (zooplankton, insect larvae and surface insects). Medium-sized fish in this zone were in addition feeding on the larger amphipod, Pallasea quadrispinosa. In the pelagic zone, the diet of medium-sized whitefish was dominated by zooplankton, although some larger prey like surface insects and age-0 smelt, Osmerus eperlanus, were also eaten. In the deep epibenthic zone, the diet of both medium-sized and large (< 35 cm) whitefish consisted mainly of the large prey P. quadrispinosa.