CORTISOL-INDUCED CHANGES IN OXYGEN-CONSUMPTION AND IONIC REGULATION IN COASTAL CUTTHROAT TROUT (ONCORHYNCHUS-CLARKI-CLARKI) PARR

The influence of cortisol on oxygen consumption and osmoregulatory var iables was examined in coastal cutthroat trout (Oncorhynchus clarki cl arki) parr kept in fresh water (FW) and transferred to seawater (SW). Intraperitoneal implants containing cortisol (50 mu g g(-1)) in vegeta ble oil resulted in elevated plasma cortisol titres similar to those o bserved in fish following a 24h SW exposure. Cortisol treatment signif icantly increased the oxygen consumption and plasma glucose levels of trout in FW, consistent with the glucocorticoid role of cortisol. Cort isol treatment did not cause any changes in plasma ion concentrations or gill Na+,K+-ATPase activity in FW after 10 days. Cortisol-implanted fish exposed to SW for 24h showed slightly improved ion regulatory ab ility compare to non-implanted controls. The results of this study sug gest that during SW transfer in juvenile salmonids, increases in corti sol may act as both a mineralocorticoid and a glucocorticoid, dependin g on the developmental state of the fish (e.g., smelt versus parr). Fu rthermore, the relative energetic costs of osmoregulation and that of the stress associated with SW transfer cannot be discerned using whole -animal oxygen consumption rates.