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The aim of this study was to determine the effects of variable and constant thermal conditions on embryonic development of three endangered fish species from the genus Leuciscus. Wild living spawners of dace Leuciscus leuciscus (L.), ide L. idus (L.) and chub L. cephalus (L.) were obtained from rivers belong - ing to the Pasleka River basin (northeastern Poland) and next transported to a hatchery for the purpose of conducting artificial reproduction. The obtained eggs were fertilized and next incubated under a variety of thermal protocols: slow gradual heating of water (M1), rapid increase in temperature at the end of incuba tion (M2), fluctuating temperature (M3) and two constant temperatures (optimum and sublethal). Variable thermal changes of water were also continued after hatching, up to the yolk sac resorption by larvae. During the study it was shown that relative to incubation at optimal temperatures (12.3°C for dace, 15.7°C for ide and 19.0°C for chub), thermal modifications had no clear influence on a decrease in survival rates (differences among regimes did not exceed 5%) and increase in developmental deformations (differences below 1.0%) observed among the hatched embryos. The duration of egg incubation and developmental rate increased with increasing temperature . In the systems with modified temperature the embryonic development of dace (from fertilisation to commencement of exogenous food intake) took from 9.5 to 22.5 days, for ide from 6.1 to 12 days and for chub from 5.0 to 10.5 days. The yolk sac resorption stage in the particular species occurred from 11.7 to 23 days for dace, 7.5 to 13.2 days for ide and 5.5 to 12.8 days for chub. Different time of hatch - ing was also reflected in the level of ontogeny of hatched embryos. Among the fish hatched at modified temperatures the largest sizes, similar to those characteristic of fish incubated at optimum temperatures, were observed in individuals originating from variants where the temperature fluctuated. The developmental disproportions among the embryos of studied species originating from different thermal regimes observed at the time of leaving the egg shells were definitely larger than during the following development stages. This study also confirmed that the embryos of studied species can adapt to increasing water temperature due to global warming up to 19.0 (dace), 23.0°C (ide) and 27.5°C (chub). The obtained results are very important not only for practical purposes but also from physiological and ecological aspects.

Effect of variable and constant thermal conditions on embryonic and early larval development of fish from the genus Leuciscus (Cyprinidae, Teleostei)