SynopsisSibling cannibalism in pike, Esox lucks, larvae and juveniles living in outdoor rearing ponds was studied using stomach contents analysis. For the two initial densities tested (6 and 18 larvae rnm2, equivalent to 12 and 36 larvae mm3), cannibalism was non-existent during the larval period (13 to 35 mm total length) and was observed only during the juvenile stages. Initial density of larvae influenced both the date of first detection of cannibalistic individuals and the rate of development of cannibalism in the population. At initial stocking densities of 18 larvae mM2 (36 larvae m-"), cannibalism was observed from 21 days after the start of exogenous feeding (mean total length: 60mm) onwards. At a mean total length of 100mm and for initial stocking densities of 6 and 18 larvae mm2 (12 and 36 larvae me3), the average proportions of cannibals in the populations of juveniles were 7.8% and 41.3% and the cannibals accounted for 15.5% and 65.9% of the total pike biomass, respectively. In stomachs of cannibals, young pike were the dominant prey in terms of weight. Dry weights of invertebrate-prey were lower in cannibals than in non-cannibals of similar size. Cannibalism among pike juveniles was characterized by the prey being swallowed whole and head first in the vast majority of cases. There was a strong positive correlation between predator and prey size and the mouth size of a cannibal was found to be an important constraint determining maximum victim size. The overall mean ratio of pike prey length to pike cannibal length was 66.2% and the average ratio of prey head depth to predator mouth width amounted to 87.6%. Prey size selection could be demonstrated for several length-groups of cannibals. These results are compared with the characteristics of early cannibalism in other fish species.

Survival, biomass and diet of pike, Esnx lucius, larvae and juveniles were studied over 3 years by stocking free embryos at a density of six fish m ~' in 12 drainable outdoor ponds. The ponds were sequentially drained at six larval/juvenile developmental stages, up to a total length (T.L.) of 139 mm. The mean rate of survival at harvest decreased irregularly over time and the highest mortality rates were recorded during the early larval period (13 to 27 mm T.L.) and two intervals of the juvenile period (46 to 99 mm T.L. and 121 to 139 m m T.L.). Mean biomass increased dramatically between 46mm T.L. (19.8 kg ha-.') and 121 mm T.L. (181.8 kg h a -' ) and stabilized between 121 and 139mm T.L. Sharp increases in the mean weight coefficient of variation were recorded during the early larval period (1 3 to 27 mm T.L.) and between 74 and 121 mm T.L. (development of cannibalism). Diet breadths were relatively narrow in pike larvae and reached maximum levels in 99 mm T.L. juveniles. Average-sized pike exhibited a sequence of six-dependent shifts from a diet composed primarily (in terms of weight) of microcrustaceans (at 13 mm T.L.), to chironomid larvae (at 74 to 99mm T.L.), and finally macrocrustaceans (at 121 to 139 mm T.L.). Cannibalism was detected first among the largest fish at the 74 mm T.L. stage.Between-year diet similarity at various developmental stages was consistently high. In 70mm T.L. fish harvested from different ponds, we found significant among-pond differences in diet composition; however, similar trends of diet changes in relation to fish size were observed from pond to pond. Our results are discussed in light of existing knowledge of young pike trophic ecology and current aquaculture practices.