Temperature, the abiotic master environmental factor, has a profound effect on the abundance and distribution of animals on the earth, in particular ecotherms. Ectothermic animals (fishes, frogs, reptiles, and invertebrate animals), cannot physiologically regulate their body temperature, and are therefore especially vulnerable to temperature changes to which they are not adapted but to which they might be exposed on the warming earth. Indeed, understanding how climate change will affect animal life is one of the greatest challenges of the current biological research. Climate warming is considered to be the most critical environmental threat, in particular for aquatic ecosystems and their biodiversity. According to the current scenarios, the annual mean temperature in Finland is projected to rise by 2-5°C and 2-7°C by the 2050s and 2080s, respectively. The duration of ice cover in lakes will become shorter, winters will be milder and extreme temperature peaks are likely to be higher and occur more often which inturn will affect fish populations in Finland. Predicted increases in ambient temperature will acts as a leading factor which control the boundary of habitats, locomotion, reproduction, development, immune defense and general performance level of fishes and other ectotherms, and thereby will impact the distribution and abundance of animal species and possibly distort the precise balance of the ecosystems to which they belong.