Isotopic niche overlap between co-occurring capelin (Mallotus villosus) and polar cod (Boreogadus saida) and the effect of lipid extraction on stable isotope ratios

Climate change is expected to drive shifts in abundance and distribution of marine forage fishes and possibly result in dietary overlap among sub-Arctic and Arctic species. Stable isotopes of carbon and nitrogen (delta C-13, delta N-15) were used as a proxy of dietary niche breadth and overlap between co-occurring, immature capelin (Mallotus villosus) and polar cod (Boreogadus saida) collected in the western Canadian Arctic, Darnley Bay, NT, during August 2013. Stable isotope ratios were determined from muscle tissue to quantify the range of delta C-13 and delta N-15, along with dietary niche breadth metrics (standard area ellipses and total area) and niche overlap between lipid-extracted and nonextracted muscle tissues of capelin and polar cod. Lipid extraction influenced the values of delta C-13, delta N-15, and C:N ratio in polar cod, but only delta C-13 in capelin tissue. Lipid extraction influenced the interpretation of dietary niche breadth and extent of overlap between co-occurring species, such that overlap of capelin within the niche of polar cod increased (from 53.0 to 89.7%) when lipids were extracted. We recommend the use of lipid extraction to standardize delta C-13 values when assessing dietary niches and extent of overlap between co-occurring fishes that differ in lipid content. Species-specific lipid normalization equations for delta C-13 ratios provided in this study can be used to correct delta C-13 ratios from nonextracted tissues of polar cod in future research. Overall, the high degree of dietary niche overlap between immature capelin and polar cod in the western Arctic suggests there is a high potential for competition between these fishes while immature.