Otoliths speak out: why the Pacific halibut in Puget sound are different

Pacific halibut, Hippoglossus stenolepis, is one of the most important commercial groundfish and is managed as a single coast-wide population from Alaska to northern California. Nevertheless, genetic investigations did not show success in detecting the population structure of the species. Here I report stable oxygen and carbon isotope analyses (δ18O and δ13C) in otoliths to discriminate the stock differences from two sample locations between the Washington coast (WC) and the northern Puget Sound (PS), and two sample years in 2007 and 2008. In general the δ18O values of halibut otoliths from WC ranged from −0.2 to 1.8‰, higher than the PS samples from −0.5 to 1.4‰. In contrast, the δ13C values from WC ranged from −3.6 to −1.0‰, lower than the PS samples from −3.2 to −1.2‰. Results from the otolith nuclei (age-0 halibut) and the 8th (the earliest maturity age for male halibut) and edge otolith rings (the latest location where the fish lived) showed significant differences between halibut samples from PS and WC. In particular, the sample location difference (between PS and WC) in both δ13C and δ18O data was significant and markedly larger than the sample year difference (between 2007 and 2008). These isotopic signatures provide evidence that the PS halibut may belong to a distinct stock that is significantly different from WC halibut.