Development of seawater tolerance and concurrent hormonal changes in fed and fasted Arctic charr at two temperature regimes

Anadromous Arctic charr cease feeding and rely on endogenous energy stores during overwintering in fresh water. During spring, they undergo seasonal development of seawater (SW) tolerance (hypoosmoregulatory capacity) and seaward migratory behaviour. As hormones known to be involved in smoltification are also prime regulators of growth and metabolism, the present study set out to investigate whether the seasonal development of hypoosmoregulatory capacity and accompanying changes in cortisol, thyroid hormones and insulin-like growth factor-I (IGF-I) levels are influenced by metabolic and nutritional status. In addition, the present study also investigated the effect of early spring water temperatures (1.5-month advance) on the above parameters. Hatchery-reared offspring of anadromous Arctic charr were either fed or fasted from October 1998 to July 1999. In fish held at ambient water temperature, hypoosmoregulatory capacity improved from March, and reached a level indicating prime SW tolerance in late June. Hypoosmoregulatory development was accompanied by significantly higher gill Na+,K+-ATPase activity, and low levels of thyroid hormones and total IGF-I in the fed and fasted groups. Of the hormones investigated, only plasma cortisol levels in fasted fish resembled the pattern typically seen in smolting salmonids, with peak levels coinciding with development of SW tolerance and increased gill Na+,K+-ATPase activity. Elevation of water temperature in early spring seemed to impair the development of hypoosmoregulatory capacity in this high-Arctic species. It is concluded that long-term fasting associated with overwintering does not impair seasonal development of SW tolerance in Arctic charr, but elevated temperatures in early spring may compromise this developmental process. The lack of hormonal changes regarded as typical for parr-smolt-transforming salmonids may reflect regulatory adjustments at other levels (receptor concentration/affinity, hormone turnover and binding proteins). Furthermore, the hormonal changes seen in the present study may resemble a seasonal resmoltification typical of repeat migrants of this species, rather than parr¡smolt-transforming first-time migrants.