Salmon origin in California's Sacramento-San Joaquin river system as determined by otolith strontium isotopic composition

Geochemical methods for distinguishing salmon of different runs would impro ve management practices designed to mitigate for declines in salmon populat ions in California's Sacramento-San Joaquin river system, Strontium isotopi c measurements show a strong relationship between the Sr-87/Sr-86 ratio in hatchery water and the Sr-87/Sr-86 Patio in the otoliths (aragonitic ear bo nes) of juvenile chinook salmon (Oncorhynchus tshawytscha) raised in those waters. As a result of differences in basin geology from north to south alo ng the western slope of the Sierra Nevada, important salmon spawning rivers within the Sacramento-San Joaquin river system have distinct Sr-87/Sr-86 r atios. Of the 10 rivers in this study, those in the Sacramento River draina ge have lower Sr-87/Sr-86 ratios (0.7039-0.7063) than those in the San Joaq uin River basin (0.7068-0.7092), with the exception of the American River, which has the highest Sr-87/Sr-86 ratios in this study (average 0.7100), Th e combination of distinct river Sr-87/Sr-86 ratios and the relationship bet ween water and otolith Sr isotope ratios indicates that this geochemical me thod can be used to identify the origin (and potentially the migration hist ory) of juvenile, out-migrating salmon in the Sacramento-San Joaquin system .