| Summary: | A study was made of three populations of Arctic charr, Salvelinus alpinus (L. ). from different habitats. Differences were found In morphology. feeding habits. growth, energetics and reproductive tactics. A pelagic. planktivorous. rather than benthic. life history was adopted by populations in competition with other salmonids. Measurements showed morphological coincidence with these life styles. Growth rate in body length was similar among the populations but growth in weight was related to the productivity of the habitat. Final body size was determined by the onset of sexual maturity. Large, deep, lowland fjord-like lochs produced bigger, heavier charr than a smaller. less temperate hill loch and a meso-eutrophic Hebridean loch. Body resources (lean. lipid, energy). expressed as values for fish of a standard size, were greater in the lowland loch population. These charr invested more resources in somatic sexual dimorphism while the hill charr invested more in gonad tissue. The age at maturity and fecundity were greater in the lowland charr but egg size was smaller. The size of offspring at the onset of feeding was directly related to egg size. It Is hypothesized that the requirement for yolk to ensure alevin survival may initiate adaptation of the life history. The lowland charr (hypermorphic. large, old, fecund with small but variable egg size) resemble the root stock of post-glacial colonization. Large egg size, early maturity (with its demographic advantages) and 'small body size with reduced fecundity represent a chain of responses to more stringent environments.
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