Developmental stage of Atlantic salmon parr regulates pituitary GH secretion and parr–smolt transformation

Abstract

Atlantic salmon respond to increased photoperiod by elevation of plasma growth hormone (GH) levels during the period leading up to parr–smolt transformation, both under natural and farming conditions. The aim of the present study was to elucidate the ontogeny of this response, by exposing juvenile Atlantic salmon to increased daylength at different stages of development. Salmon fry were reared on 12 h light:12 h dark from first feeding and thereafter exposed to continuous light from a mean fork length of 5 cm (LL5) or 8 cm (LL8). Plasma GH increased in smolting individuals in the LL8 group, whereas in the LL5 group as well as in parr of the LL8 group, plasma GH levels remained low. Growth rate in length increased significantly in both groups in response to increase in daylength. Plasma IGF-I varied to a lesser degree, although in late July, the LL5 IGF-I levels exceeded those of the LL8 IGF-I levels. In the LL8 group, 33% of the fish smoltified 6 weeks after onset of continuous light, whereas all the fish in the LL5 group failed to smolt. At termination of the experiment, the incidence of precocious maturing males in the LL5 and LL8 group were 38% and 62%, respectively. In conclusion, based on the lack of a GH response in fish smaller than 8 cm, it is hypothesized that the development of a functional light-pituitary axis leading to an increase in plasma GH levels, sets the threshold developmental stage for successful parr–smolt transformation of Atlantic salmon.