Effects of waterborne iron overload and simulated winter conditions on acute physiological stress response of whitefish, Coregonus lavaretus

Two-year-old whitefish (Coregonus lavaretus) were exposed for 30 days to episodic iron overload in iron-rich humic water (5%) supplemented with inorganic iron (5 mg Fe L−1). Two parallel laboratory exposures were performed, one under conditions simulating winter and the other under conditions simulating spring. After exposure, some of the fish were subjected to acute handling stress in the form of a short air challenge to reveal possible modification of the primary and secondary stress responses. In whitefish sampled without additional handling, iron accumulated in the liver (under spring conditions) and gills (under winter and spring conditions); plasma catecholamine and β-estradiol (both winter and spring groups) as well as blood hematocrit (winter group only) levels were depressed; blood glucose (winter group only) and red blood cell (RBC) Na+ levels (spring group only) were increased. In handled whitefish, liver glycogen phosphorylase (GPase), RBC, and blood glucose stress responses were impaired by the applied exposure conditions, which reflected natural iron-rich humic water. Exposure also removed some physiological effects of the applied ambient conditions: plasma catecholamines and β-estradiol, gill Na+/K+-ATPase, and RBC K+ concentration were not different in two iron-exposed fish groups, whereas there was a difference in reference fish. Thus, the physiological effects of this type of subchronic exposure, together with alterations in the acute stress response, can lead to incorrect conclusions being drawn from the results, if the effects of time-dependent stress response are ignored. In conclusion, waterborne iron overload may impair the optimal capacity of whitefish to carry out their normal physiological functions such as responding to external threats.