Do you know a scientific definition for Coldwater fishes ?

Answer 1

There is no single definition - here's one of the most commonly used definitions based on temperature range at which species are typically found:
Coldwater: 11.0°-14.9°C,
Cool water: 21.0°-24.9°C, and
Warm water: 27.0°-30.9°C (Magnuson et al. 1979).

JOHN J. MAGNUSON, LARRY B. CROWDER and PATRICIA A. MEDVICK (1979) Temperature as an Ecological Resource. American Zoologist 1979 19(1):331-343

Here's another approach based on the highest reported average mean weekly temperatures: for cold-water: 22°C; cool-water29°C; and warm-water: 30°. Armour(1991).

Armour, C. L. 1991. Guidance for evaluating and recommending temperature regimes to protect fish. Instream Flow Information Paper 28, Biological Report 90(22), U.S. Fish and Wildlife Service, Fort Collins, CO.

Lyons et al. (1996) designated cold-water, cool-water, and warm-water stream communities as those with maximum daily average temperatures of <22°C, 22-24°C, and >24°C, respectively.

Lyons, J., L. Wang, and T. D. Simonson. 1996. Development and validation of an index of biotic integrity for coldwater streams in Wisconsin. North American Journal of Fisheries Management 16:241-256.