Size, growth, and origin-dependent mortality of juvenile Chinook salmon Oncorhynchus tshawytscha during early ocean residence

Abstract

Selective mortality during early life history stages can have significant population- level consequences, yet critical periods when selective mortality occurs, the strength of selection, and under what environmental conditions can be difficult to identify. Here, we used otolith microstructure and chemistry to examine the factors potentially linked to selective mortality of juvenile fall-run Chinook salmon Oncorhynchus tshawytscha from California's Central Valley during early ocean residence. Back-calculated size and growth rates of the population were com- pared across 3 sample periods: as juveniles exited the San Francisco Bay estuary (estuary-exit), after their first month at sea (summer-ocean) and 5 mo after ocean entry (fall-ocean). We com- pared mortality dynamics during years of exceptional recruitment (addition of individuals to har- vestable population; 2000 and 2001) to a year of poor recruitment (2005). Otoliths from 2005 were also analyzed for sulfur isotopes to discern hatchery from naturally spawned stock. Significant size and growth-rate selective mortality were detected during the first month at sea in the low recruit- ment year of 2005, but not in 2000 and 2001. Individuals that were larger and growing faster dur- ing freshwater and estuarine rearing were more likely to survive to summer and fall in the low recruitment year. There was a slight, but insignificant, increase in the proportion of hatchery to naturally spawned individuals from estuary-exit to fall-ocean, suggesting that fish from neither origin were overwhelmingly favored. Our results suggest that Central Valley Chinook salmon can be subject to significant size and growth-rate selective mortality resulting in low adult abundance, and this mortality appears independent of origin.