Life history plasticity in chinook salmon: relation of size and growth rate to autumnal smolting

An experiment was conducted to assess the relative influence of body size and growth rate on autumnal smolting of under-yearling spring chmook salmon, Fish were sorted into large (>85 mm) or small (<75 mm) size categories in July. Subsequently, some fish from each size group were reared at reduced ration, resulting in four treatments: Large-HiFeed, Large-LoFeed, Small-HiFeed, and Small-LoFeed. Starting in July, and continuing through November, length and weight, and samples for gill Na+-K+-ATPase, plasma T-4 and IGF-I were obtained at 2-week intervals. On seven occasions throughout October and November, 10 fish from each treatment group were PIT tagged and movement patterns were assessed. Autumnal smolting clearly occurred in all treatment groups, indicated by elevated gill Na+-K+-ATPase activities, attaim-nent of a silvery skin coloration, and an active swimming behavior pattern. However, fish from the Large-HiFeed treatment clearly differed in a number of respects from fish from the Small-LoFeed treatment from August through September: condition factor was higher, skin was more silvery, plasma T-4 and IGF-I levels were higher, and gill Na+-K+-ATPase activities were higher. In addition, fish from the Large-HiFeed group showed greater levels of activity than fish from the Small-LoFeed group on most occasions, in October and November. These data extend knowledge of the phenomenon of autumnal smolting in chinook salmon. Further, they demonstrate that differences in growth through the summer-autumn period has a significant effect on both physiological and behavioral expression of smolting. (C) 2003 Published by Elsevier Science B.V.