Feeding of early- and late-hatched whitefish in illuminated cages

Abstract

In 1992 and 1993 tank-cage rearing of whitefish larvae was carried out. The fish hatched in two different periods: in 1992 larvae hatching took place at the beginning of March (early-hatched whitefish), and in 1993 - in the second half of April (late-hatched whitefish). Each year the larvae were kept in tanks for the first 3 weeks of their life. The tanks were fed with water of 140C (group A) and 180C (group B). During this period the fish were given artificial feeds. Fish rearing in the next 6 weeks was carried out in the illuminated lake cages (placed in Lake Legińskie), maintaining the division into two groups. While in cages, the fish fed exclusively on the zooplankton. In 1992 cage rearing was quite early in the season (1 April - 17 May), while in 1993 it was rather late (11 May - 20 June).Feeding activity of whitefish larvae was strictly related to the dynamics of energy content in crustacean zooplankton occurring in the lake. Copepods were the main energetic component of whitefish food during „early” rearing (1992): 0.7 cal. per individual in variant A and about 1 cal. in variant B. In the case of „late” rearing (1993), cladocerans dominated with respect to the energetic value: about 5 cal. per individual in variant A and 3.5 cal. in variant B.