Evidence for the role of growth hormone as a part of a 'light–pituitary axis' in growth and smoltification of Atlantic salmon (Salmo salar)

Abstract

Atlantic salmon (Salmo salar) parr were subjected to one of two photoperiod regimens: a simulated natural photoperiod, or a reciprocal photoperiod (with a light-to-dark ratio opposite to that of the natural photoperiod). The animals were held at 10 °C from mid-December onward and were sampled at 3-week intervals.By February, salmon held under the reciprocal photoperiod were significantly longer, had lower condition factors, and looked and behaved more like smolts than did those under the natural photoperiod. A separate experiment showed that salinity tolerance increased at this time. In June the situation was reversed; the salmon kept under the natural photoperiod showed greatest growth and high salinity tolerance.Longer days in either regimen coincided with an increase in number and apparent activity of pituitary somatotrops of the animals. Adrenocorticotropin (ACTH), pars intermedia, and prolactin cells were also judged to be active under these conditions. A role for growth hormone and other peptides as components of a 'light–pituitary axis' is discussed in relation to the parr–smolt transformation of Atlantic salmon.

The influence of dietary lipid inclusion and daily ration on growth and smoltification in 1+ Atlantic salmon (Salmo salar) parr

“…However, similar numbers of fish within the lipid treatments entered the UMG and underwent successful smoltification, suggesting that high fat levels may not be necessary for smoltification (Saunders et al, 1982). In support, the previously documented winter reduction in lipid content (Komourdjian et al, 1976;Saunders and Henderson, 1978;Higgins and Talbot, 1985) occurred at similar rate in all of the treatments. Although Higgins and Talbot (1985) recorded this reduction to occur at similar rates in upper and lower mode fish it has typically been linked to the physiological demands imposed by smoltification (Woo et al, 1978;Birt and Green, 1986;Helland and Grisdale-Helland, 1998).…”

Comparison of Gene Expression in the Gill of Salmon (Salmo salar) Smolts from Anadromous and Landlocked Populations

“…Migration of young salmon to sea is associated with major changes in morphology, physiology and behaviour of the fish, collectively called the parr-smolt transformation (smoltification), which remodels the young freshwater-adapted fish (parr) to the seawateradapted fish (smolt) (Hoar 1976, Folmar & Dickhoff 1980, McCormick & Saunders 1987. The parr-smolt transformation occurs in spring under the control of hormonal system entrained by increasing day-length and rising water temperature (Komourdian et al 1976, Ban et al 2007. Although smoltification happens only once in the lifetime of the wild fish, it is a reversible process that can occur several times under hatchery conditions.…”

Effects of size, age and photoperiod on hypoosmoregulation in brook trout, Salvelinus fontinalis

“…It is clear, however, that smoltification occurs without seawater induction through the influence of environmental cues (e.g. photoperiod and lunar rhythms), resulting in hormonal activation of the parr-smolt transformation (Komourdjian et al, 1976;Folmar and Dickhoff, 1980;.…”

“…Among the biochemical changes involved in smoltification is an increase in moi sture content whi ch coi nci des wi th decreased 1 i pi d levels (Komourdjian et al, 1976;Saunders and Henderson, 1978). Al though moi sture content was si gni fi cantly di fferent in brook trout from Rivi ère à 1 a Trui te and the Matamek r.…”