The role of light and endocrine factors in the development of bimodality of growth in the juvenile Atlantic salmon (Salmo salar L.)

Abstract

The timing of the development of bimodality in the frequency distribution of length and weight within sibling populations of juvenile Atlantic salmon was independent of change in day-length or photoperiod. However, photoperiod affected the proportion of fishes in the upper modal group, from none under short photoperiod to higher proportions under constant light. The photoperiod exerted its effects on the development of bimodality in two ways, through its proportional effects (length of daylight per se) and by its differential effects (rate of change in day-length). It is postulated that the rate of change in day-length exerted its effect during a period of sensitiveness to light, called the "critical photosensitive period" (CPP). Temperature had an unimportant role in development of bimodality relative to light since fish grew well in constant light when the temperature was declining, but poorly under short constant photoperiod at a time when the water temperature was increasing. The biochemical and morphometric assessment of cellular growth (nucleic acid content and muscle fibre diameter measurements) showed that fish growth was due to both cell enlargement (hypertrophy) and proliferation (hyperplasia). The preponderance of one process over the other depended on developmental stage and season. The lowest muscular DNA content and highest RNA-DNA ratio was reached at the end of July, independent of the photoperiod or family, indicating therefore that cellular processes of growth (e.g. cell enlargement and protein precursor accumulation) were photoperiod independent and under endogenous seasonal regulation. The lowest water content and highest dry weight values were reached by late summer for all photoperiodic treatments indicating their endogenous character. Specific growth rate was correlated significantly with RNA-DNA ratio before midsummer and with the natural logarithm of RNA concentration after that. RNA concentration then, as an index of growth, was more affected by cell size during spring than during summer. It is concluded that RNA concentration alone was an appropriate index of growth when it was not affected by cell size. Fishes of the upper modal group synthesised more protein and lipids than those of the lower group, since the former showed consistent and significantly higher dry and wet weights, total RNA and RNA concentrations, total DNA, DW-DNA ratios and RNA-DNA ratios. The female salmon allocated more resources to gonadal growth than the males did as assessed by histological measurements. Among males upper modal group fishes showed more advanced testicular development than did the lower modal ones. These results were at variance with the hypothesis that the development of bimodality was due to differential allocation of resources to somatic and gonadal growth. Gonadal growth was highly independent of the gonadotropes, since they did not show any parallel cytological changes with the gonads. The Somatotropes (GH cells) and thyrotropes (TSH cells) were involved in fish growth regulation, because they showed consistent and in some cases significant increases in cell and nucleus diameters and cell numbers during the somatic growth season. The prolactin cells (LTH cells) were larger in female and in small fish. No interaction was found between the two factors (sex and size). This result suggested that the t wo fish groups were under osmotic stress. It was concluded that photoperiod exerted its effects on fish body growth through the hypothalamo-hypophysial axis, since good correlations were found between morphometric estimates of pituitary cell activity and somatic growth and development.