Is smolting a positive or a negative developmental decision?

Salmonid life histories are characterised by the ages at which the two prim ary developmental conversions, smelting and sexual maturation, are made. Fo r Atlantic salmon, the critical seasons at which the physiological decision s are taken are thought to be soon after midsummer for the smelting convers ion, and in mid-autumn for the maturity conversion. Maturity is inhibited i f genetically determined performance thresholds are not met. Experiments we re designed to test whether smelting was controlled in a similar sense. Sib ling upper medial 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 ot her group was fed this same ration but only during 1 week in four. All fish es had silvered by early May. On 11 May, when 96-h challenge tests in 35 pa rts per thousand artificial seawater were started, the mean weights of the unrestricted and restricted groups were 16.3 +/- 1.7 and 9.4 +/- 1.2 g, res pectively. By 18 May all the unrestricted and 47.5% of the restricted group s survived the challenge. By 25 May, survival of the restricted group had i ncreased to 65%. On 1 June only one fish from the unrestricted and none fro m the restricted group survived the transfer. Survival after transfer was r elated significantly to weight at transfer: larger smelts survived better t han did smaller smelts (p < 0.001). Food-deprived fish survived as well as control fish of the same size. Food restriction limited weight growth to on ly 12.5% at maximum (compared to 85% among the controls) over the 3-month i nterval, and so had an indirect effect on survival of deprived fish, but di d not switch off smelting for the majority of individuals. It is concluded that smelting is not controlled in a manner analogous to maturation, but is a negative developmental decision. This is consistent with the hypothesis proposed earlier that smelting represents a developmental pathway taken by fish that have failed to mature in that developmental cycle. (C) 1998 Elsev ier Science B.V. All rights reserved.