Isotopic Signatures of Otoliths as Management Tools in Groundfish Stocks

Pacific halibut (Hippoglossus stenolepis), Pacific cod (Gadus macrocephalus), and Pacific hake (Merluccius productus) are commercially important groundfish along the United States Pacific coast. However, little is known about the stock structure of these species because genetic studies did not provide sufficient data in stock identification. Here I present an alternative method, using stable oxygen and carbon isotope ratios (δ¹⁸O and δ¹³C) of otoliths to identify the stock structure of multi-fish species in both Puget Sound (PS) and the Washington west coast (WC). In general, the δ¹⁸O values of Pacific halibut otoliths from WC ranged from -0.22 to 2.11‰, much higher than those from PS from -0.50 to 1.35‰. In contrast, δ¹³C values of the same halibut otoliths from WC ranged from -3.61 to -1.31‰,  a little lower than those from PS from -3.18 to -1.15‰. Similar variations were found in otoliths of Pacific cod and Pacific hake. 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. For Pacific halibut otoliths, the site differences (between PS and WC) are much larger than the year differences (between 2007 and 2008) in isotopic compositions. These isotopic signatures provide evidence that the PS groundfish may belong to a different stock from WC.