Sex-based divergence in isotopic compositions of north temperate freshwater fishes

Differences in reproductive allocation of male and female fishes could lead to sexual divergence in nutritional requirements, trophic ecologies, and metabolisms that may be reflected in isotopic compositions. We compared muscle C:N ratio, δ 13 C, and δ 15 N between mature females and males for eight fish species from 21 North American populations. We also compared isotopic compositions between muscle and ova in a subset of females to determine if observed sex differences may be associated with fractionation during oogenesis. Muscle δ 13 C and δ 15 N did not vary greatly between males and females for most species but significant differences were observed in walleye ( Sander vitreus ) and burbot ( Lota lota ). Among-individual variation in isotopic composition, estimated as dispersion in δ 13 C–δ 15 N space, did not differ significantly between the sexes. Ova had significantly higher C:N ratio and lower δ 13 C than female muscle in all species with the greatest contrast between tissues seen in burbot. Ova had significantly higher δ 15 N than female muscle in most species with the greatest contrast between tissues in walleye. Sex-based divergence in isotopic composition of northern fishes is most evident in lean-fleshed piscivores, and may be related to selective mobilization of nutrients from soma to ovaries.