Tracking natal origins of salmon using isotopes, otoliths, and landscape geology

Abstract

The inability to identify natal origins (i.e., individual rivers and hatcheries) of adult Pacific salmon in the ocean has impeded our understanding of their ocean ecology and the management of mixed‐stock fisheries. Strontium isotope (87Sr : 86Sr) ratios recorded in otoliths of fall‐run Chinook salmon (Oncorhynchus tshawytscha) from all major natural and hatchery spawning sites in the California Central Valley can be used as natural tags to identify natal origins with high accuracy (82%) and improved when additional otolith markers identified fish to hatchery (98%) or naturally spawned (94%) sources. A spatial baseline of 87Sr : 86Sr signatures was developed by targeting 87Sr : 86Sr within juvenile portions of otoliths accreted in natal streams and hatcheries using laser ablation and a multicollector inductively coupled plasma mass spectrometer. The availability and analyses of known‐origin coded wire tagged adults provides a rare test of this technique to reconstruct early life‐histories of adults (90% correct classification). By quantifying the area of watershed influenced by granitic rocks using hydrologic and geologic data layers, we explained 94% of the geographic variability in 87Sr : 86Sr in salmon otoliths. Creating a spatial map in geographic information systems relating landscape geology to Sr isotopes is a useful framework for evaluating the efficacy of Sr isotopes to track the natal origin and movement of salmonids in freshwater, estuarine, and marine environments to better understand how processes occurring in these habitats influence the growth, survival, and reproductive success of anadromous fishes.