Atlantic salmon, Salmo salar L., and Arctic char, Salvelinus alpinus (L.):comparative correlation between iodine-iodide supplementation, thyroid hormone levels, plasma cortisol levels, and infection intensity with the sea louse Caligus elongatus

The interrelationship between plasma cortisol levels, iodine-iodide nutriti onal supplementation, plasma thyroid hormone levels (tri- and tetra-iodothy roaine, T-3 and T-4 respectively), and infection intensity with the sea lou se Caligus elangatus were investigated in Atlantic salmon, Salmo salar L., and Arctic char, Salvelinus alpinus (L.). Cortisol-implanted Atlantic salmo n had significantly higher mean intensities of sea lice than salmon not imp lanted with cortisol. Untreated Arctic char had significantly higher plasma cortisol levels and sea lice infections than untreated salmon. Cortisol-im planted salmon had lower plasma levels of T-3 and T-4. Salmon and char trea ted with iodized feed and iodinated water had higher plasma T-3 and T-4 lev els, higher plasma T-3/T-4 ratios, and lower plasma cortisol levels than co ntrols. Sea lice infections were significantly reduced on salmon treated wi th either iodized feed (P < 0.05) or iodinated water (P < 0.05). Char treat ed with iodized reed also had significantly reduced infections (P < 0.05). In general, iodized feed had a greater effect than iodinated water in reduc ing sea lice numbers. This study indicates that elevated plasma cortisol le vels in salmon and char result in lower plasma levels of thyroid hormone an d an increased susceptibility to sea lice infections. It is concluded that providing the fish with sufficient iodine-iodide increases thyroid hormone levels, reduces plasma cortisol levels, and reduces susceptibility to sea l ice infections.