Effects of temperature and photoperiod on rainbow trout (Oncorhynchus mykiss) smoltification and haematopoiesis

•nkaα1a, nkaα1b and nkcc1a provide relevant information that the NKA activity does not reflect.•While nkaα1a and nkcc1a transcription are mainly regulated by temperature, nkaα1b is regulated mainly by photoperiod.•High water temperature could potentially compromise the adaptive and innate immune response of rainbow trout.•Increased water temperature (8 °C) during winter does not provide clear advantages for smoltification or size at harvesting.