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RESEARCH PRODUCT

Some like it deep : Intraspecific niche segregation in ruffe (Gymnocephalus cernua)

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Generalist fishes commonly show intraspecific niche segregation along the littoral–pelagic resource axis in lakes. Recent studies have shown that the deep, cold and seemingly unproductive profundal zone can also offer underutilised resources and facilitate specialised individuals, and can contribute to lake food webs via methane-derived carbon pathways. Despite numerous examples from salmonid fish species, such intraspecific niche segregation along a littoral–profundal resource axis has not been reported in percids or other predominantly littoral benthivorous fishes. Here, we describe a case of ruffe (Gymnocephalus cernua: Percidae) populations consisting of shallow- and deep-water dwelling individuals in two post-mining lakes in the northern Czech Republic. Results from stable isotope (δ15N and δ13C) mixing models indicated that littoral and profundal food resources dominated the long-term diets of ruffe individuals caught from shallow- and deep-water habitats, respectively. No similar niche segregation along the littoral–profundal gradient was observed in coexisting perch (Perca fluviatilis: Percidae) which used more pelagic food than the benthivorous ruffe. The observed littoral–profundal niche segregation in ruffe was more pronounced in the macrophyte-rich and poorly oxygenated Lake Milada, where high habitat complexity and strong interspecific interactions in the littoral zone as well as the absence of competing fish species in the profundal zone likely promote narrow trophic niches of shallow- and deep-water dwelling ruffe Our study provides novel evidence of littoral–profundal niche segregation in a predominantly benthivorous fish species. The results also show that intraspecific niche variation can be affected by lake ecosystem characteristics, in particular by the availability of and competition for littoral and profundal resources. Although more research is needed to confirm potential early divergence of shallow- and deep-water dwelling ruffe as well as cascading effects on lake ecosystem processes (e.g. nutrient cycling and competitive and predatory interactions), our study indicates that individual ruffe may show limited habitat and food-web coupling due to their restricted use of shallow or deep resources.