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Arctic charr ( Salvelinus alpinus) is generally considered an inferior piscivore compared to brown trout ( Salmo trutta). However, we demonstrate that a recently described profundal spawning piscivore PP-morph of Arctic charr in a subarctic lake has evolved typical piscivore traits (i.e. large jaws, robust skulls), similar to the co-occurring brown trout but different from the sympatric littoral spawning omnivore LO-morph of Arctic charr. A few large-sized LO-morph, most large-sized PP-morph and trout constitute the piscivore guild, but they differed in prey fish selection. Of the fish-eating deep-water PP-morph, 52% had consumed charr, with a peak during the polar night period (68-80% in November-January). In contrast, the LO-morph and trout were mainly (~90%) inter-specific piscivores eating stickleback. Consequently, the PP-morph was the main charr predator and started to feed on fish at a smaller size and took larger prey relative to their body length than the trout. Stable isotope (δC, δN) ratios reflect the dietary specialisations in the upper-water (LO-morph, trout) and the deep-water (PP-morph) environments, besides the piscivorous behaviour of the PP-morph and trout. The existence of a shallow-water piscivore (trout) may explain the origin of this PP-morph, taking benefit of underutilised resources of small-sized Arctic charr in the deep-water habitat. [ABSTRACT FROM AUTHOR]

A population of small-sized Arctic charr living their entire life cycle in deep water was recently found in Lake Skogsfjordvatn, Northern Norway. Their phenotype and life history closely resembles the profundal charr morph in another subarctic Norwegian lake, Fjellfrøsvatn. Fjellfrøsvatn has one littoral omnivorous and one small profundal benthivorous (PB) charr morph coexisting with brown trout, whereas Skogsfjordvatn has migratory salmon, trout, charr and eel populations in addition to threespined sticklebacks, resident trout and three resident charr morphs of which one is a large piscivorous profundal morph. We predicted that the small profundal charr morph in Skogsfjordvatn would have a similar trophic niche to the PB-morph in Fjellfrøsvatn and tested this by using stomach content and stable isotope (δC and δN) analyses. Profundal benthic invertebrates, mainly chironomid larvae, Pisidium sp. mussels and small crustaceans, dominated the diets, and the δC and δN values indicated almost identical long-term trophic niches of the PB-morphs in both lakes. The prediction was therefore supported. This seems to indicate that the small profundal Arctic charr morphs in these lakes have evolved in parallel, in nearly identical and stable habitat and dietary niches in spite of likely different ecological interactions within the contrasting fish communities. [ABSTRACT FROM AUTHOR]