Sulfur isotopes in otoliths allow discrimination of anadromous and non-anadromous ecotypes of sockeye salmon (Oncorhynchus nerka)

Oncorhynchus nerka occur both as anadromous sockeye salmon that spend most of their life in the ocean, and as non-anadromous kokanee salmon that remain in fresh water their entire lives. We assessed whether stable isotopes of sulfur (delta(34)S) in otoliths could be used to distinguish sockeye salmon and kokanee ecotypes that are otherwise difficult to identify when they share a common freshwater rearing environment. We also investigated the chemical link between salmon and their diet by measuring delta(34)S in various fish tissues (eggs, muscle, scales) and zoo-plankton. delta(34)S (mean +/- SE) in sockeye salmon eggs (1 8.7 +/- 0.4%o) and marine zooplanlcton (20.5 +/- 0.1 parts per thousand) were enriched by 10-14 parts per thousand compared with kokanee eggs and freshwater zooplankton. delta(34)S in the otolith cores of sockeye salmon (19.2 +/- 0.7 parts per thousand) and kokanee salmon (5.3 +/- 1.1 parts per thousand) were similar to delta(34)S in marine and freshwater zooplankton, respectively, indicating that the core is derived from maternal yolk tissue and reflects the maternal diet. delta(34)S in the freshwater growth zone of otoliths did not differ significantly between sockeye (5.9 +/- 1.1 parts per thousand) and kokanee salmon (4.4 +/- 1.2 parts per thousand), and was similar to freshwater zooplankton. The mean difference between delta(34)S in the otolith core and first year of growth was 13.3 +/- 1.4 parts per thousand for sockeye and 0.65 +/- 1.3 parts per thousand for kokanee salmon. A quadratic discriminant function developed from measurements of delta(34)S in otoliths of known maternal origin provided perfect classification rates in cross-validation tests. Thus, sulfur isotope ratios in otoliths are effective in discriminating between anadromous and non-anadromous ecotypes of O. nerka.