Evolutionary ecology of arctic char (Salvelinus alpinus (L.)) : Intra- and interspecific interactions in circumpolar populations

Sammanfattning: This thesis examines a suite of ecological and genetic mechanisms influencing the fundamental and realized niche borders of European and North American populations of the Arctic char (Salvelinus alpinus (L.)) species complex in allopatry as well as in sympatry with brown trout (Salmo trutta) or brook trout (Salvelinus fontinalis).The Arctic char has a circumpolar distribution and is the only freshwater fish taxon in lakes at highlatitudes and at high altitudes in subarctic and alpine regions. Whereas abiotic factors such as extremetemperature constrain the reproduction in the north, biotic factors such as interspecific competition cause the niche of the Arctic char to be compressed in the south.ln allopatric populations of Arctic char, phenotypic flexibility, size bimodality and cannibalism were found to be characteristic features and to increase in magnitude with latitude and altitude. Several food niches were found to be exploited by Arctic char. An ontogenetic switch to cannibalism was found to enhance survival, and also reproductive success as older females have larger eggs and are more fecund. Cannibalism may commence at a body sire of about 200 mm. The switch is very fast and is associated with a pause in the reproductive investment, while allocation to somatic growth is maximized. Individual char with a relatively large gape on average had higher condition, were more likely to become cannibalistic and probably achieved a higher reproductive output, since their gonads were on average larger. As a cost of cannibalism, larger char accumulated cestodan parasites, which reduced their survival and were responsible for the termination of the large-sized modal group in the population.In sympatry with brown trout in Scandinavia competitive and predatory interactions were found to beimportant. However, a temperature dependent dietary shift during winter allowed an ecological release on the part of the char, the realized niche of which could therefore approach the fundamental niche as determined from allopatric populations. Analogous processes were found to occur in Newfoundland between Arctic char and brook trout, where sympatry in high and low altitude lakes also caused a differential switch in the direction of piscivory. The evolutionary divergence between Arctic char and brook trout in sympatry is probably retarded as a result of hybridization and introgression, which are more frequent at the borders of the distribution of the respective species.