Isotopic composition of otoliths as a chemical tracer in population identification of sockeye salmon (Oncorhynchus nerka)

The feasibility of stable oxygen and carbon isotope ratio (delta(18)O and d elta(13)C) analyses in sagittal otoliths of sockeye salmon (Oncorhynchus ne rka) was tested by analyzing the seasonal and annual otolith zones of 44 sa mples collected from different localities in the northeast Pacific coast. T he delta(18)O and delta(13)C values of these otoliths ranged from -14.23 to +1.62 parts per thousand and from -15.18 to -3.17 parts per thousand, resp ectively. The delta(18)O variations can be divided into two stages from fre shwater (-14.2 to -2.5 parts per thousand) to marine (-2.5 to +1.6 parts pe r thousand) that were consistent with the life history of sockeye salmon fr om juvenile to adult stages. The transition occurred after age 1, during wh ich the timing of seaward migration of smolts was different. The marine com ponent of the isotope variation in sockeye salmon otoliths (ages > 2) was u niform but showed a consistent and strong shift towards oceanic changes aro und 1996. Thus, delta(18)O and delta(13)C values of otoliths can be potenti ally used as a chemical tracer in population identification, and their mari ne portions can be used in the study of ocean environmental changes.