No abstract

Quantifying the contribution of wild (naturally spawned) and hatchery Chinook salmon (Oncorhynchus tshawytscha) to the mixed-stock ocean fishery is critical to understanding their relative importance to the persistence of salmon stocks. The inability to distinguish hatchery and wild salmon has inhibited the detection of declines or recoveries for many wild populations. By using Chinook salmon of known hatchery and wild origin, we established a baseline for separating these two sources using otolith microstructure. Otoliths of wild salmon contained a distinct exogenous feeding check likely reflecting an abrupt transition in food resources from maternal yolk not experienced by fish reared in hatcheries. Daily growth increments in otoliths from hatchery salmon immediately after the onset of exogenous feeding were wider and more uniform in width than those in wild fish. The discriminant function that we used to distinguish individuals reared in hatcheries or in the wild was robust between years (1999 and 2002), life history stages (juveniles and adults), and geographic regions (California, British Columbia, and Alaska) and classified fish with ~91% accuracy. Results from our mixed-stock model estimated that the contribution of wild fish was 10% ± 6%, indicating hatchery supplementation may be playing a larger role in supporting the central California coastal fishery than previously assumed.

Advances in probe-based mass spectrometry allow for high spatial resolution of elemental and isotopic signatures in fish otoliths that can be used to address fundamental questions in fisheries ecology. Analyses of Chinook salmon (Oncorhynchus tshawytscha) otoliths from two river populations yield identical 87Sr/86Sr ratios using laser ablation multicollector inductively coupled plasma mass spectrometry (LA-MC-ICPMS) and thermal ionization mass spectrometry (TIMS). Results were obtained from freshwater otoliths with low Sr concentrations (300–800 ppm) using high spatial resolution (50 µm) corresponding to temporal histories of ~12 days fish growth. Low natural variation in 87Sr/86Sr among otoliths from the same rivers allows for conservative estimates of external precision of techniques. Thus, we demonstrate that Sr isotope ratios obtained by LA-MC-ICPMS can be accurate and precise, bypassing the time-intensive sample preparation required by microdrilling and TIMS. This technique opens the use of Sr isotopes for broader ecological questions requiring large sample sizes to characterize nursery habitats, metapopulation dynamics, and stock discrimination similar to studies that focus on elemental concentrations, thereby providing a more robust tool for some freshwater and diadromous fishes.

The inner ear of teleost fishes contains three calcareous structures (otoliths) that are part of the organs for hearing and balance. The largest of these structures, the sagitta, is usually composed of calcium carbonate crystals in the form of aragonite, but the calcium carbonate also occurs less frequently in a clear crystallized form called vaterite. We investigated the functional consequences of otolith crystal structure on hearing in juvenile Chinook salmon (Oncorhynchus tshawytscha) using the auditory brainstem response technique. A significant loss of sensitivity (2.5–6.5 dB) occurred within the primary hearing range (100–300 Hz) among salmon that had at least one vateritic sagitta. Auditory thresholds were not significantly different in fish with one vs. two vaterite sagittae. Crystallized sagittae were significantly larger and less dense than their aragonite counterparts. Saccular epithelium shape and hair bundle orientation patterns did not differ between saccules with different crystal types. There was, however, a propensity for the saccular epithelia from vateritic sagittae to have fewer sensory hair bundles. We conclude that significant hearing loss was associated with the occurrence of vateritic sagittae and suggest that hearing loss is caused by the lower density of the vaterite otoliths.

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 ( 87 Sr : 86 Sr) 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 87 Sr : 86 Sr signatures was developed by targeting 87 Sr : 86 Sr 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 87 Sr : 86 Sr 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. Endang...