Juvenile migration in brown trout: a consequence of energetic state

1. We explored the mechanisms determining age and size at juvenile migratio n in brown trout Salmo trutta L. A Cs-133 tracer methodology was used to es timate food consumption of juvenile brown trout in a Norwegian stream, and the energy budgets of early migrants and stream residents were compared. 2. Fast-growing brown trout migrated to the lake earlier and at a smaller b ody size than slower-growing individuals. The 2+ migrants were significantl y larger than those that remained 1 or more years longer in the stream. The 3+ migrants were significantly larger than the 2+ migrants. Some fast-grow ing males matured in the stream, whereas all females left the stream before maturing sexually. 3. The food consumption and the energy budgets for 2+ migrants were more th an four times higher than those of the resident 2+ fish. Total energy alloc ated to growth was also higher among migrants, and the total metabolic cost s were five times higher among migrants than among resident fish. 4. The proportional energy allocation to growth among the 2+ migrants was m uch lower (about half) than that of those remaining longer in the stream. T he reduction in the proportion of energy available for growth from age 1+ t o 2+ was larger among migrants (88%) than among resident fish (68%). Reduct ion in the proportion of energy available for growth is a probable explanat ion for why migrations are initiated at age 2. 5. Our study supports the hypothesis that fast-growing individuals shift th eir niche earlier and at a smaller body size than slower-growing individual s because they maintain higher metabolic rates and are energetically constr ained at a younger age by limited food resources than slow growers.