Polymorphism and speciation in Arctic charr

The Arctic charr Salvelinus alpinus exhibits 1-4 sympatric morphs in postgl acial lakes, of which one or two are epibenthic zoobenthos feeders, one is a limnetic planktivore and one is a piscivorous form. In addition, northern rivers support partly migratory populations with anadromous and freshwater resident fish. The morphs vary in their coloration, morphology, life histo ry, behaviour and genetic characteristics. The morphs usually differentiate according to their ontogenetic stage at maturity, which parallels paedomor phism in amphibians. The young usually start as epibenthic zoobenthivores, but may become pelagic at a certain size according to the predation risk ex perienced at that time. From a length of >20-25 cm, charr start to become p iscivorous. The proportion of piscivorous fish increases with increasing bo dy size. In partly anadromous populations. fish that mature before smelling become freshwater resident, the others anadromous. In some rivers, the mor phs occupy separate niches (epibenthic and limnetic), from emergence onward s. The morphs exhibit different degrees of reproductive isolation that vary from a high degree of interbreeding to complete isolation. Usually, they s pawn within morph (assortative: mating). but alternative male mating behavi our (sneaking, fighting) may occur in stream-spawning populations and at gr eat depths in lakes. Morphologically specialized morphs appear to Feed more effectively than intermediate forms. and selection according to feeding mo de, site fidelity and associated assortative mating are prerequisites for t he evolution of the different morphs. Charr morphs develop into stable feed ing niches under conditions of intense intraspecific competition when there is little competition with other species. Sympatric morphs exhibit differe nt degrees of speciation but similar morphs in different systems are not in dividual species because of (1) their polyphyletic origin, (2) the supporti ng systems are often young, transient environments making the future situat ion for the populations uncertain. and (3) the genetic differentiation amon g morphs is low. Sympatric morphs may interbreed and produce fertile hybrid s. Nevertheless, sympatric charr morphs should be managed as separate speci es. Changes in the natural conditions or human impacts to which the morphs are adapted will have a strong influence on the persistence and survival of each different morph. (C) 2001 The Fisheries Society of the British Isles.