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

Abstract

The feasibility of stable oxygen and carbon isotope ratio (δ 18 O and δ 13 C) analyses in sagittal otoliths of sockeye salmon (Oncorhynchus nerka) was tested by analyzing the seasonal and annual otolith zones of 44 samples collected from different localities in the northeast Pacific coast. The δ 18 O and δ 13 C values of these otoliths ranged from -14.23 to +1.62‰ and from -15.18 to -3.17‰, respectively. The δ 18 O variations can be divided into two stages from freshwater (-14.2 to -2.5‰) to marine (-2.5 to +1.6‰) that were consistent with the life history of sockeye salmon from juvenile to adult stages. The transition occurred after age 1, during which the timing of seaward migration of smolts was different. The marine component of the isotope variation in sockeye salmon otoliths (ages &gt; 2) was uniform but showed a consistent and strong shift towards oceanic changes around 1996. Thus, δ 18 O and δ 13 C values of otoliths can be potentially used as a chemical tracer in population identification, and their marine portions can be used in the study of ocean environmental changes.