The role of growth and seasonal fat dynamics in the maturation of Atlantic salmon (Salmo salar) parr

Growth studies of individually tagged 1+ Atlantic salmon parr revealed no difference in specific growth rates between maturing and non-maturing male parr. However, maturing parr had lower mean condition factors than non- maturing males during March, and were characterized by greater increases in condition factor during April. Changes in condition factor during April were related to feeding opportunity during that month, and a relationship between April increases in condition factor and maturation rates of males was confirmed in 0 sibling populations of salmon parr. A relationship between condition factor increases during April and maturation in autumn was also confirmed for grilse and reconditioned kelts. Maturing male parr replenished non-mesenteric fat stores during April, and the mesenteric store in May. In comparison lipid replenishment and deposition in non-maturing fish was delayed until May and June respectively. The April increases in condition factor of maturing males are therefore symptomatic of the earlier replenishment of lipids depleted during winter. The mesenteric fat store is an important reserve utilized during maturation in male parr and contains up to 40% of the total lipid content of salmon parr. Its size decreases significantly during the later stages of gonadal development, while the relative size of females and non-maturing males's increases. Mesenteric fat levels are highest in maturing males in July, just before gonadal growth accelerates, and there is a strong correlation between GSI and mesenteric fat levels at this time. Because feeding in maturing male parr is depressed between August and October, the size of the mesenteric store is likely to be important in sustaining gonad differentiation, as well as in the elaboration of secondary sexual characters. Seasonal manipulations of growth rate resulted in variations in the maturation rate of male parr. Increased feeding and growth during April and May increased maturation rates whereas decreased feeding resulted in delayed replenishment of fat reserves and lower maturation rates compared with controls. Changes in growth during other months had little effect on maturation rates. The results indicate that maturation is initiated in a proportion of male parr as early as in winter, but is suppressed if fat deposition into the mesenteric store is below a genetically determined level by the end of May. However, the timing of fat deposition into the mesenteric store is dependendant on the prior replenishment of other body stores, and so is particularly sensitive to fat dynamics in April. The manipulation of maturation rates by altering growth opportunity in April and May occurs despite the fact that physiological changes leading to maturation are already in train. Thus maturation is switched off in many male parr by reduced feeding and growth during spring months. This maturation suppression switch, related to growth in fat reserves during spring months, provides the means by which growth exerts some control over maturation, and is likely to be responsible for much of the correlation between fast growth and early age of maturation in salmonids. The switch is time specific and is believed to be adaptive. It is likely to prevent maturation in the autumn if the winter is long and spring is late. A late spring shortens the growing season, and the probability of acquiring sufficient fat reserves for successful spawning and overwintering would be low in such summers. The physiological mechanisms by which growth in fat reserves during spring could affect maturation are discussed, and a hypothetical model for the role of fat stores in the hormonal control of maturation, is presented