Natural Resilience in Arctic Char: Life-History, Spatial and Dietary Alterations Along Gradients of Interspecific Interactions

The Arctic char (Salvelinus alpinus (L.) sp. complex) is the northernmost group of freshwater fish, and is also the most vulnerable, especially to rapid environmental changes. Arctic char has thus frequently been studied as an indicator of human impacts such as gillnetting, water-level regulation, introductions of alien species, and acidification, as well as associated efforts to compensate for these environmental changes, in northern Scandinavia. Still, Arctic char inhabit environmental extremes from landlocked lakes in the High Arctic to high altitudes lakes with low diversity of fish and oligotrophic deepwater lakes far outside the polar region. The long-term responses of Arctic char to environmental variations and interspecific interactions reveal a high degree of ecological resilience and phenotypic plasticity. The shifting trophic roles of Arctic char as cannibal, predator or prey, its niche use and life-history changes in lakes with variable fish species richness are illustrated from comparative studies in Sweden and Newfoundland. When allopatric in northern low productive lakes, landlocked populations are structured by cannibalism, where Arctic char serve as both predator and prey. In sympatry with other fish species, Arctic char serve as prey, with their diet successively shifting from benthos to zooplankton as interspecific competition for food increases. Interactions with more efficient zooplankton feeders such as coregonids (Coregonus spp.) or predators such as pike (Esox spp.) can be detrimental. Thus, while the distribution of Arctic char in the north is restricted by physical factors, the southern range is thus restricted by biological factors. Consequently, the niche of Arctic char is increasingly compressed in lakes along latitudinal and altitudinal gradients associated with increased diversity of fish, although certain natural key conditions offer Arctic char temporarily asylum in the inescapable process towards extinction. Low temperature in winter allows Arctic char to temporally extend its dietary and spatial niche use in low diversity lakes, while in southern lakes with more complex fish communities, including key species such as profundal smelt (Osmerus spp.), Arctic char is allowed to regain its original characteristics as a predator. In addition, Arctic char demonstrate associated alterations of life-history characteristics, such as size and life span, and thus reproductive traits demonstrating similarities in northern cannibals and southern predators. Although including a high degree of resilience, changes seen in southern marginal Arctic char may have been adaptive and thus the product of an evolutionary fast response caused by natural selection due to ultimate interspecific competition and predation.