Feeding of early larval pike Esox lucius L. reared in illuminated cages

The paper presents the results of investigations on the feeding of pike larvae reared in illuminated cages in a mesotrophic lake over a period of 19 days. The same zooplankton species were found both in the lake and illuminated cages. At night, the biomass of all zooplankton groups was higher in illuminated cages than in the lake (with Copepoda prevailing). The first exogenous food of pike larvae (with 13.1 mm average standard length) was cladocerans (Bosmina, Chydorus) and small copepods (Cyclops). Pike larvae were feeding throughout the whole period of 24 h, with the greatest intensity at night when illumination was switched on. The most popular zooplankton groups among pike larvae were Copepoda (up to 94.7%) with less numerous Cladocera (up to 28.8%) and other organisms (up to 0.9%). The indices of alimentary tracts filling (I) of larvae feeding on zooplankton reached up to 1220.5‱. With the age of pike larvae, Ivlev's selectivity indices (E) with respect to different food components changed—for Chydorus sphaericus and Bosmina sp. they were decreasing, and for Copepoda increasing. Rotifers usually made an insignificant part of the diet. Pike larvae willingly fed on Chironomus plumosus and roach larvae incidentally found in the cages. Cannibalism among pike larvae started when their standard length (SL) was 16.0–22.3 mm (18.7 mm on average). The size of a prey item constituted up to 90.9% of the predator's standard length. After passing over to predatory feeding, gut-filling indices in pike larvae reached up to 2859.6‱. Cannibalism occurred even in case of very low (230 ind. m− 3) concentration of pike larvae in cages, which on the 12th day of rearing made 3%; and on the 19th day, 42%. Pike larvae could be reared without additional feeding in illuminated cages up to 2 weeks. Over this period, their mean standard length increased from 13.1 to 20.0 mm. Further rearing of pike larvae does not make any point because their output started to decrease considerably as a result of cannibalism.