Oxygen isotope fractionation in otoliths: experimental results from four North Pacific and Arctic gadid species

Abstract

High-latitude climate warming is expected to have wide-ranging effects on habitats, ecosystems, and the fish species that occupy them. Not all fish species will be able to adapt to increasing temperatures. We investigated oxygen isotope fractionation in fish otoliths and its relationship to environmental temperature and thermal histories of individual fish. Fish from 4 gadid species, Gadus macrocephalus, Boreogadus saida, Eleginus gracilis, and G. chalcogrammus, representing North Pacific and Arctic regions, were reared in a range of controlled temperatures (0-20°C). We estimated 4 new species-specific otolith oxygen isotope fractionation equations, a relationship between otolith δ18O and temperature (T) in the form δ18Oo - δ18Ow = m × T°C + b and also in a second form using the fractionation factor α: 1000 ln α = a × (1000 TK-1) + c, where o is otolith, w is water, and m, b, a and c are regression coefficients. In using the first form, B. saida was the most unique among the 4 species, with the steepest slope (-0.23) and the highest intercept (32.99‰ Vienna PeeDee Belemnite [VPDB]). G. macrocephalus had the lowest slope (-0.17) and the lowest intercept (31.76‰ [VPDB]). Results of an ANCOVA test indicated that the 4 fractionation equations were not statistically different (F = 2.25, p > 0.087). However, when we applied the 4 new fractionation equations to δ18Oo measured in wild-caught B. saida otoliths, the species-�specific fractionation equation resulted in the closest match between measured and predicted water temperatures. These new fractionation equations represent new tools for investigating temperature effects on fish biota and will also improve paleotemperature reconstruction, especially for high-latitude species.