Otolith elemental and Sr isotopic composition as a natal tag for Biwa salmon Oncorhynchus masou subsp in Lake Biwa, Japan

We examined the elemental and Sr isotopic composition of otoliths from Biwa salmon Oncorhynchus masou subsp., an endemic subspecies of the O. masou complex in the Lake Biwa (Japan) water system, to evaluate the usefulness of otolith chemistry as a natural tag for studying homing and migration history. Fish and water samples were collected from 5 major spawning rivers and 2 hatcheries. The elemental content relative to calcium (Na/Ca, Mg/Ca, K/Ca, Mn/Ca, Sr/Ca, and Ba/Ca) and Sr isotopic ratio (Sr-87/Sr-86) of the samples were analyzed using laser ablation inductively coupled plasma mass spectrometry and thermal ionization mass spectrometry, respectively. Significant variations in otolith and water sample chemistry were detected. We also observed significant relationships between the Sr/Ca, Ba/Ca, and Sr-87/Sr-86 ratios of otoliths and those of ambient water samples from which the fish were collected. The other elemental ratios and all elemental concentrations in the otoliths showed variations independent of water chemistry. Jackknife cross-validation using quadratic discriminant function analysis showed that otolith elemental composition could classify Biwa salmon juveniles by sampling location, with a high level of accuracy (mean 79%). The mean accuracy increased to 89% when Sr isotopic ratios were combined with elemental data. This study thus demonstrated the suitability of otolith chemistry for determining fish distribution. Otolith chemistry successfully revealed the natal (hatching) locations of individual fish, facilitating further understanding of migration history and habitat use, which are essential for establishing effective fishery management strategies for this Near Threatened subspecies.