Photoperiod and temperature affect plasma growth hormone levels, growth, condition factor and hypoosmoregulatory ability of juvenile Atlantic salmon ( Salmo salar) during parr-smolt transformation

Abstract

Nine groups of juvenile Atlantic salmon ( Salmo salar) were subjected to one of three different daylengths (2L:22D, 8L:16D and 14L:10D) for the duration of 1, 2 or 5 months, and subsequently to 24L:0D. Four groups on natural photoperiod were exposed to a combination of two different temperature regimes (6 and 11°C) with or without night illumination (5 lux), and one group was kept on 24L:0D. Plasma growth hormone (GH) levels, growth rate and hypoosmoregulatory ability increased in response to increased daylength, independent of season. A smoltification-associated decrease in condition factor was absent in fish on 24L:0D and in fish exposed to only 1 month of reduced daylength. Is is concluded that GH has direct roles in stimulating growth and increasing hypoosmoregulatory ability during parr-smolt transformation. Increasing daylength is of overriding importance as a “zeitgeber” for increases in GH levels, growth and hypoosmoregulatory ability during smoltification. Different physiological changes associated with smoltification may be put out of phase by the manipulation of environmental cues, and thus possibly cause an incomplete smoltification.