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 populationlevel 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 compared 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 compared mortality dynamics during years of exceptional recruitment (addition of individuals to harvestable 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 recruitment year of 2005, but not in 2000 and 2001. Individuals that were larger and growing faster during 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.

Science Advancements Key to Increasing Management Value of Life Stage Monitoring Networks for Endangered Sacramento River Winter-Run Chinook Salmon in California

“…• Chemical analyses used to reconstruct migration patterns (Sturrock et al, 2015), population of origin (Barnett-Johnson et al 2008;, and diet (Weber et al 2002) • Alaska hatcheries "thermally mark" fish by varying temperatures in unique patterns visibly recorded in otoliths (Scott et al 2001) • Freshwater growth rates linked to higher ocean survival (Woodson et al 2013) by battery life, tags can possibly be recovered later in downstream monitoring surveys, or upon return in adult carcass surveys, or at the LSNFH. Handheld PIT-tag readers can be used at any monitoring location where fish are handled to identify the unique code of each PIT tag.…”

“…Winter-run sized juveniles migrate past Knights Landing with increases in streamflow (>400 m 3 s -1 ) or turbidity generally coincident with the first fall or winter storm events, and are thought to rear in the Delta for approximately 1 to 4 months before they enter the ocean (Martin et al 2001;del Rosario et al 2013;Poytress et al 2014). The ocean is a critical environment for SRWRC, and that is where most of their growth occurs (MacFarlane 2010; Woodson et al 2013;Wells et al 2016). Finally, adults return in the winter to the upper Sacramento River and spawn predominantly as 3-year-olds in the following summer.…”

Environmental conditions impacting juvenile Chinook salmon growth off central California: An ecosystem model analysis

“…While oceanic dispersal of California Central Valley Chinook salmon presumably evolved in response to improved growth potential in persistently productive marine waters, a reasonable short-term objective for salmon during dispersal is to make movement decisions that maximize their growth rate (i.e., the movement toward higher prey concentrations used in the present simulations) as mortality rates are thought to be largely size dependent [Cross et al, 2009;Woodson et al, 2013]. An improved understanding of the tension between local growth and larger-scale dispersal may be obtained via statistical models that quantify relationships between juvenile salmon survival and ocean conditions at diverse spatiotemporal scales.…”

“…During years of reduced forage biomass, Chinook salmon exhibit poorer condition which is linked to increased mortality [Holtby et al, 1990;Woodson et al, 2013] presumably associated with relatively greater predation on smaller fish [Tucker et al, 2013]. Measuring growth of salmon at sea, especially during their first year, is difficult because the size distribution of populations changes in response to both growth and size-dependent mortality, and populations are moving and mixing with other populations.…”

“…Early marine mortality varies as a function of growth and size [Holtby et al, 1990;Woodson et al, 2013], such that slower growing fish have a lower probability of survival when first entering the ocean during less productive conditions. During years of reduced forage biomass, Chinook salmon exhibit poorer condition which is linked to increased mortality [Holtby et al, 1990;Woodson et al, 2013] presumably associated with relatively greater predation on smaller fish [Tucker et al, 2013].…”