THE ARCTIC CHARR SALVELINUS-ALPINUS IN THINGVALLAVATN

The coexistence of four morphs of arctic charr in Thingvallavatn, Iceland. was demonstrated by investigations of morphology, habitat use, diet, endoparasitic fauna, life history, time and place of spawning, early ontogeny, and population genetics. Head morphology in embryos, juveniles and adults revealed the presence of two charr morphotypes, each including two morphs: (1) a benthic morphotype including large benthivorous (LB-) and small benthivorous (SB-) charr, featuring overshot mouth and relatively large pectoral fins, and (2) a pelagic morphotype including planktivorous (PL-) and piscivorous (PI-) charr, featuring terminal mouth and relatively small pectoral fins. SB-charr mature sexually (50%) at 2 (males) and 4 yr (females); asymptotic length is 133 mm. LB-charr mature sexually at 8 yr; asymptotic length is 553 mm. PL-charr mature sexually at 4 (males) and 5 yr (females); asymptotic length is 205 mm. PI-charr mature sexually at 6 yr; asymptotic length is 302 mm. SB- and LB-charr both live in the littoral zone, but they segregate in microhabitat: LB-charr live epibenthically, whereas SB-charr live mainly in the interstitial spaces of the stony substrate. Their diet, however, is similar (Schoener's D = 0.34 - 0.89). Both feed mainly on the snail Lymnaea peregra. PI-charr live mainly epibenthically, feeding on sticklebacks and juvenile charr. PL-charr is the most numerous morph in the lake. It lives both in the pelagic and in the epibenthic zone, feeding on crustacean zooplankton and chironomid pupae. The young of the year of all morphs live in the littoral zone where they feed on chironomid larvae. All morphs spawn in the littoral zone, LB-charr spawn in July-August, PL- and PI-charr spawn in September-November, whereas SB-charr spawn during August to November. Genetically, the morphs are very similar; Nei's genetic distances were between 0.00004 to 0.00126. The only significant difference found was between SB-charr and the other three morphs combined. The four morphs are conspecifics, although they may be reproductively segregated to some extent.