| Summary: | Niche diversification of polymorphic Arctic charr can be altered by multiple anthropogenic stressors. The opossum-shrimp (Mysis relicta) was introduced to compensate for reduced food Resources for fish following hydropower operations in Lake Limingen, central Norway. Based on habitat use, stomach contents, stable isotopes (d13C, d15N) andtrophically transmitted parasites, the zooplanktivorous upper water-column dwelling ‘normal’ morph was clearly trophically separated from two sympatric deep-water morphs (the ‘dwarf’ and the ‘grey’) that became more abundant with depth ([30 m). Mysis dominated (50–60%) charr diets in deeper waters ([30 m), irrespective of morph. Mysis and/or zooplankton prey groups caused high dietary overlap ([54%) between the ‘dwarf’ morph and the two other ‘normal’ and ‘grey’ morphs. After excluding Mysis, the dietary overlap dropped to 34% between the two profundal morphs, as the ‘dwarf’ fed largely on deepwater zoobenthos (39%), while the ‘grey’ morph fed on fish (59%). The time-integrated trophic niche tracers (trophically transmitted parasites and stable isotopes) demonstrated only partial dietary segregation between the two deep-water morphs. The high importance of Mysis in Arctic charr diets may have reduced the ancestral niche segregation between the deep-water morphs and thereby increased their resource competition and potential risk of hybridization.
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