Abstract
The serendipitous discovery of a body‐size dimorphism amongst the sexually mature Arctic charr Salvelinus alpinus of Loch Tay is described. Sexually mature Arctic charr, collected by gill netting on spawning areas, showed a clear and distinct bimodal size distribution with no overlap in fork length distributions. The upper (19–29 cm L F ) and lower modes (8–16 cm) were not solely the result of sex or age differences. Analysis of stable isotope ratios of C and N in muscle showed highly significant differences in mean δ 13 C and δ 15 N between populations, demonstrating a difference in trophic ecology between the two body‐size morphs. Overlap in the range of δ 13 C and δ 15 N values for the two morphs, however, suggested that they occasionally shared a common diet. Data from other studies strongly indicated that the proximate and ultimate mechanisms that control body‐size dimorphisms in Arctic charr differed between sites. Clear differences in trophic ecology in the Loch Tay Arctic charr suggested that the available feeding opportunity may differ for the two morphs. The most likely proximate mechanism resulting in this dimorphism is growth rate differences resulting from differences in food availability for the two subgroups occupying alternative foraging niches in Loch Tay.