| Abstrakt: |
The Central Valley fall-run Chinook salmon (Oncorhynchus tshawytscha) is the dominant population complex supporting the California and Southern Oregon commercial salmon fishery. The stock is largely dominated by hatchery production and has shown high variability in adult returns, suggesting that hatchery practices are critical to the long-term sustainability of the fishery. We compiled information from numerous sources to synthesize trends in the number, location, size, and timing of fall-run Chinook salmon released from the five Central Valley hatcheries between 1946 and 2012. Approximately 2 billion fish were released during this period, nearly half of which were released from the single federally operated hatchery. Juveniles have been planted off-site in the estuary with increasing frequency since the early 1980s, particularly by state-operated hatcheries. Approximately 78% of all releases occurred between January and June, including ~25% in April and ~20% in May. Release timing and size trends differed among hatcheries, and were correlated. For example, the Coleman and Nimbus hatcheries tended to release small fish (< 5 g, on average) early in the year, while the Feather, Mokelumne, and Merced hatcheries tended to release larger fish (> 10 g, on average) later in the year. Moreover, sizes-at-release (by month) have increased since the 1980s, leading to the emergence of a new life-history type that now comprises nearly all of the estuary releases: springtime releases of large ocean-ready "advanced smolts." We collapsed release timing and size data into a single index of lifehistory diversity and our results indicate a reduction in juvenile life-history diversity through time, with decreased variability in release number, timing, and size in recent years. Together, these results indicate a reduction in the diversity of life-history types represented in the fall-run Chinook salmon hatchery releases, which may be a factor that contributes to the decreased stability of the Central Valley fall-run Chinook salmon stock complex. [ABSTRACT FROM AUTHOR] |
