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

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. One of the most challenging aspects of understanding population structure and connectivity for migratory species is identifying the natal origins of individuals across broad geographic areas where populations potentially mix. The use of isotopes and the development of spatial maps of isotopic variation (isoscapes) to track migrations have advanced our knowledge of population structure and feeding ecology in terrestrial taxa (reviewed in Hobson 1999; Hobson and Wassenaar 2008). Fewer empirical examples or isoscapes exist in aquatic systems, despite the fundamental role that connectivity plays in understanding the demography of populations. Such information and tools would extend our understanding of spatial mechanisms of population persistence for marine and anadromous fishes and aid in determining critical aquatic habitats (e.g., nurseries, tributaries) for reproduction, survival, and growth of endangered species and those targeted by fisheries. Chinook salmon (Oncorhynchus tshawytscha) from the California Central Valley (CCV) make significant contributions to fisheries along the west coast of North America largely because of hatchery supplementation of the fall-run (Barnett-Johnson et al. 2007). Like many formerly abundant Chinook salmon stocks (e.g., Columbia River, Klamath-Trinity River), freshwater populations of CCV Chinook salmon vary in their extinction risks under the U.S. Endangered Species Act. All four runs of wild (naturally spawned) Chinook salmon in the CCV are listed as endangered (winter run), threatened (spring run), or are candidates (fall and late-fall run; Fisher 1994). Fall-run CCV Chinook salmon alone constitute 85–95% of the ocean salmon landings in California, resulting in US$56 1 Corresponding author ([email protected]). Present address: Institute of Marine Sciences, University of California, Santa Cruz, 100 Shaffer Road, Santa Cruz, California 95060. 2 Present address: Minnesota Pollution Control Agency, 520 Lafayette Road, Saint Paul, Minnesota 55155.