Ontogenetic shifts in energetic content and diet of juvenile Yukon River Chinook Salmon

Over the last decade the production of Yukon River Chinook Salmon has declined to levels eliciting the closure of commercial and reduction of subsistence fisheries, respectively. The decline in production is not yet understood. Recently, low numbers of juvenile-per-spawner, and subsequently, returns-per-spawner, seem to confirm the hypothesis of low production due to high early life history mortality. Whether this mortality is more pronounced in the freshwater or marine phase of Chinook salmon early life history is not well understood. The failure of multiple stocks throughout western Alaska seems to suggest a large scale and more likely marine explanation. A better understanding of the transition between freshwater and marine feeding and energetic strategy, and thus early marine survival, will increase our ability to forecast and understand adult returns. Freshwater, estuarine, and marine phase juveniles were collected throughout three different survey efforts from May-September during both 2014 and 2015. Samples were preserved and returned to Auke Bay Labs (NOAA) in Juneau, Alaska for dietary and energetic processing. Both dietary composition and energetic content shifted over time, with the dietary composition generally increasing in oceanic influence over time and with increased body length. Additionally, prey items ingested shifted with increasing time from surface oriented prey field to pelagic prey fields. Energetic content continued to increase over time and fish body length, especially with the introduction of highly energetic marine prey content. Additionally, a shift in long standing dietary patterns of September marine juveniles was observed for the 2014 and 2015 survey years. Diets shifted from a predominantly piscivorous nature in previous years to a diet dominated by juvenile crab.