PHOTOPERIOD REGULATION OF PLASMA GROWTH-HORMONE LEVELS DURING PARR-SMOLT TRANSFORMATION OF ATLANTIC SALMON - IMPLICATIONS FOR HYPOOSMOREGULATORY ABILITY AND GROWTH

The influence of photoperiod on plasma growth hormone (GH) levels during parr-smelt transformation was studied in Atlantic salmon, using a specific salmon GH radioimmunoassay; in addition, the impact of endogenous changes in GH levels on hypoosmoregulatory ability and growth was assessed using seawater challenge tests and measurements of growth rates. Groups of fish were kept on simulated natural photoperiod or continuous light, and subgroups were subjected to reciprocal transfers between these two light regimes at different times. For Atlantic salmon on simulated natural photoperiod, GH levels increased during the parr-smelt transformation, from 0.6 ng ml(-1) in January-March to 6.6 ng ml(-1) in June, while the corresponding change in fish on continuous light was from 0.4 to 1.0 ng ml(-1). The study demonstrates that photoperiod is a major zeitgeber for the increased GH levels during the parr-smelt transformation of Atlantic salmon. The data further support the view that exposure to continuous light into fall and winter causes a ''free running'' of an endogenous rhythm governing smelting and a subsequent phase-delay of the parr-smelt transformation-related increase in plasma GH levels. A strong positive nonlinear correlation was found between specific growth rate and GH levels, with growth rate increasing rapidly with increasing GH levels up to 2-3 ng ml(-1) at which point near-maximal growth rate is reached, revealing that relatively small increases in GH levels may be of great importance for an increased specific growth rate. A positive nonlinear correlation was also found between GH and hypoosmoregulatory ability, leading to the conclusion that a regime of continuous light has adverse effects on seawater tolerance and that a period of short daylength is necessary for the normal smelt-related physiological improvement in seawater adaptability. (C) 1995 Academic Press, Inc.