Latitudinal Variation in Life-History Characteristics of Sea-Run Migrant Brown Trout Salmo trutta

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Abstract
(1) The chief objective was to compare life-history characteristics of sea-run migrating brown trout, Salmo trutta L., in a latitudinal gradient from 58 to 70.degree.N in Norway in order to reveal biogeographic trends in life-history characters. (2) The annual length increment of brown trout during the second year in fresh water was signficantly, negatively correlated with latitude (r2=0.520), and significantly, positively correlated with river temperature (.degree.days) (r2 = 0.700), and length of the growth season (number of days t > 4.degree.C, r2 = 0.549). The annual length increment of brown trout during the first year after smolting was significantly, negatively correlated with latitude (r2 = 0.339), and significantly, positively correlated with sea temperature (r2 = 0.309). (3) Mean smolt age increased significantly with latitude (r2 = 0.686), but decreased significantly with increasing river (r2 = 0.447) and sea (r2= 0.606) temperature (.degree.days), and length of the growth season in the river (r2 = 0.387). Means smolt size increased significantly with latitude (r2 = 0.394), but decreased significantly with increasing sea temperature (r2 = 0.328). (4) Sea age at maturity increased with latitude (r2 = 0.244) as did the coefficient of variation of sea age at maturity (r2 = 0.353). Sea age at maturity depends on growth rate at sea. A younger age at maturity was observed in the populations where the length increment of immatures at sea was reduced in the second year relative to the first year after smolting (r2 = 0.526).