Temperature, photoperiod, and water–electrolyte balance in rainbow trout, Salmo gairdneri

The influence of temperature and photoperiod upon water–electrolyte balance was considered in animals acclimated to four combinations of two temperature (2, 18 °C) and photoperiod (18 h light–6 h dark, 6 h light–18 h dark) regimes through evaluation of Na, K, Ca, Mg, and Cl levels in plasma, skeletal, and cardiac muscle and liver. Water content and distribution and approximate cellular ion levels were estimated for skeletal muscle and liver. Evidence of significant influence upon water balance was obtained in one-half of the analyses performed, with effects being particularly pronounced in cold-acclimated, 'winter'–photoperiod specimens. Temperature effects were observed in two-thirds of the comparisons made with respect to electrolyte levels. These were not biased in relation to photoperiod. Most significant photoperiod-correlated variations in electrolyte levels (over one-half of the total considered) were observed at low temperature. These observations are discussed in relation to compensation of rainbow trout for temperature perturbation of the osmoregulatory and ionoregulatory systems, and the possible basis of photoperiodic influence upon these responses.