Isotopic signatures of otoliths in identification of Pacific hake stocks

Pacific hake, Merluccius productus, are one of the most important groundfish along the Washington coast and the most abundant species in the Georgia Strait, British Columbia. In this study, we analyzed stable oxygen (¹⁸O/¹⁶O or d¹⁸O) and carbon (¹³C/¹²C or d¹³C) isotope ratios of hake otoliths from the two areas, and examined their stock structure and behavior. The d¹⁸O values of hake otoliths from the Washington coast ranged from +1.1 to +2.7‰, much higher than those from the Georgia Strait from -0.66 to +0.87 ‰. In contrast, d¹³C values of the same otoliths from Washington ranged from -2.0 to +0.3‰,  also higher than those from the Georgia Strait from -2.91 to -1.08 ‰. Statistical analyses on isotopic data from otolith nuclei and multi-year rings indicated that there were significant differences between the samples from the two areas in habitat conditions, food sources, and the age of sexual maturity. These results agree well with the biological observations, and are consistent with the principles of stable isotope geology. Thus we concluded that Pacific hake from the Washington coast and the Georgia Strait may, like other groundfish reported, belong to two different stocks or populations along the US-Canada Pacific west coast.