Stable Isotopic Composition of Otoliths from Hatchery and Wild Chinook Salmon in Makah Bay, Washington
Yongwen Gao, Russell Svec, Steve Joner, Joe Hinton, and Dave Zajac
Abstract.—Chinook salmon Oncorhynchus tshawytscha is an important species to the Makah Indian Tribe of Washington. To examine the stock structure and identify their natal sources, 38 Chinook salmon otoliths from Makah Bay and 10 otoliths each from the brood returns of the Hoko Hatchery and the Makah National Hatchery were collected and analyzed for stable isotope ratios (δ18O and δ13C). The δ18O values of the Makah Bay Chinook salmon otoliths ranged from –8.2‰ to +1.2‰ Vienna Peedee belemnite (VPDB), while the δ13C values from the same otoliths ranged from –11.8‰ to –3.0‰ VPDB. In contrast, the isotopic compositions of otoliths from the two hatcheries were not significantly different from each other (t-test, p = 0.33 for δ18O and p = 0.46 for δ13C) and were within the range of otoliths of the Makah Bay mixed catch. Because the nucleus of otoliths corresponded to the starting time of fish life history, isotopic signatures from otolith nuclei indicate the different natal sources of hatchery and wild Chinook salmon. We estimated that 85% of Makah Bay Chinook salmon were hatchery-released and about 15% were of a wild origin. Furthermore, stable isotopic variations extracted from otolith annuli of seven representative fish showed a similar pattern on Chinook salmon’s life history and were consistent with the previous isotopic studies on otoliths of sockeye salmon O. nerka. Thus, we concluded that stable isotopic composition of Chinook salmon otoliths might be a powerful tool for stock discrimination and a useful means for identifying the relative proportions of hatchery-released and wild salmonids.