Journal of Fish Biology (1998) 52, 1259–1271 Trophic polymorphism amongst Arctic charr from Loch Rannoch, Scotland C. E. A*, D. F *, F. A. H †, R. B. G §, C. M. A* A. F. W‡ *Fish Biology Group, University Field Station, University of Glasgow, Rowardennan, Glasgow, G63 0A W , †Fish Biology Group, Division of Environmental and Evolutionary Biology, Institute of Biomedical and Life Sciences, University of Glasgow, Glasgow, G12 8QQ and ‡SOAEFD Freshwater Fisheries Laboratory, Faskally, Pitlochry, Perthshire, PH16 5LB, Scotland, U.K. (R eceived 24 M ay 1997, A ccepted 22 February 1998) This paper describes the results of a multivariate and univariate morphometric analysis of three groups of Arctic charr Salvelinus alpinus from Loch Rannoch, Perthshire, Scotland, distin- guished a priori on the basis of coloration (bright v. cryptic) and site of capture (from two locations 12 km apart). The analysis produced a clear and highly significant distinction, not only between brightly and cryptically coloured charr, but also between the two cryptically coloured groups, one of which was more robust in terms of several size-independent head measurements. Stomach analysis showed that the brightly coloured charr fed entirely on zooplankton, the less robust, cryptically coloured fish, fed on benthic macroinvertebrates, whereas a significant proportion of the diet of the more robust cryptically-coloured form consisted of other fish. Differences in length at age also distinguished the three forms, with the robust, piscivorous charr, which live longer and potentially reach a larger final size, attaining smaller sizes at a given age. These data clearly confirm the previous identification of a distinct planktivorous morph of Arctic charr in Loch Rannoch and extend the morph analysis by distinguishing two additional morphs, a benthivorous morph and a piscivorous morph that are morphologically and ecologically distinct. The results are discussed in the context of other systems in which sympatric morphs of Arctic charr have been described. 1998 The Fisheries Society of the British Isles Key words: sympatric polymorphism; head structure; trophic morphology. INTRODUCTION Phenotypic variation between populations of the same species is common in fishes, particularly freshwater forms. Such variation has been interpreted typically as being the result of local evolutionary pressures acting upon a common ancestral form in different ways and is seen often as one step towards full speciation (Skulason & Smith, 1995). The occurrence of such poly- morphisms is particularly common in certain taxonomic groups, the salmonids, for example (Taylor, 1991), resulting in part from differences in the degree to which species have retained phenotypic flexibility (Day et al., 1994; Skulason & Smith, 1995). Among the salmonids, the Arctic charr Salvelinus alpinus (L.) exhibits the greatest extremes of phenotypic variability, in the form of between- population variability in body shape, in colour and in behaviour Behnke, 1984). Additionally in this species, it is not uncommon to find disjunction of phenotypic §Present address: Natural Resources Scotland, Berbice, Blair Atholl, Perthshire, PH18 5SZ, Scotland, U.K. 1259 0022–1112/98/061259+ 13 $25.00/0/jb980676 1998 The Fisheries Society of the British Isles