Growing large in a low grade environment: size dependent foraging gain and niche shifts to cannibalism in Arctic char

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Abstract
Arctic char (Salvelinus alpinus) living in allopatry in lakes often show a distinct size dimorphism where the large char prey upon small char. We studied the relationship between size scaling of energy intake, total metabolic costs and levels of storage energy related to the niche shift from invertebrate feeding to cannibalism in an alpine Arctic char population. Gill‐net samples consisted of two distinct size modes, small invertebrate feeding char (92–172 mm total length) and large cannibalistic char (265–606 mm total length). Food consumption was estimated for a one year period using a radioactive tracer (137Cs). The energy intake followed a hump‐shaped relationship with fish size for invertebrate feeding char and increased monotonically with size for cannibalistic char. Total metabolic costs increased exponentially with size for both groups. Char feeding in the invertebrate niche had a maximum net energy gain at 140 mm total length with an upper size limit for positive net energy gain at 180 mm. For cannibals the relationship between size and net energy gain formed a hump shaped function with maximum net energy gain at 400 mm and a lower size limit for positive net energy gain at 230 mm. The size at maturation for both small and large char was close to the size for maximum energy gain in their respective niches. Immature individuals close to the upper size limit for positive net energy gain in the invertebrate niche had low levels of storage lipids but tended to have high mass specific growth rates. This may imply a reallocation from reversible (stored) energy to structural (length) growth as a strategy to transform into the size‐range where the cannibalistic niche successfully could be exploited.