Is smolting a positive or a negative developmental decision?

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Salmonid life histories are characterised by the ages at which the two primary developmental conversions, smolting and sexual maturation, are made. For Atlantic salmon, the critical seasons at which the physiological decisions are taken are thought to be soon after midsummer for the smolting conversion, and in mid-autumn for the maturity conversion. Maturity is inhibited if genetically determined performance thresholds are not met. Experiments were designed to test whether smolting was controlled in a similar sense. Sibling upper modal growth group fish (mean wet weight 10.34±0.02 g) were divided into two replicated treatment groups on 11 February. One treatment group was fed a standard ration of 3% body weight/day until 1 June; the other group was fed this same ration but only during 1 week in four. All fishes had silvered by early May. On 11 May, when 96-h challenge tests in 35‰ artificial seawater were started, the mean weights of the unrestricted and restricted groups were 16.3±1.7 and 9.4±1.2 g, respectively. By 18 May all the unrestricted and 47.5% of the restricted groups survived the challenge. By 25 May, survival of the restricted group had increased to 65%. On 1 June only one fish from the unrestricted and none from the restricted group survived the transfer. Survival after transfer was related significantly to weight at transfer: larger smolts survived better than did smaller smolts ( p<0.001). Food-deprived fish survived as well as control fish of the same size. Food restriction limited weight growth to only 12.5% at maximum (compared to 85% among the controls) over the 3-month interval, and so had an indirect effect on survival of deprived fish, but did not switch off smolting for the majority of individuals. It is concluded that smolting is not controlled in a manner analogous to maturation, but is a negative developmental decision. This is consistent with the hypothesis proposed earlier that smolting represents a developmental pathway taken by fish that have failed to mature in that developmental cycle.