Shifting the Salmon Freshwater-Ocean Survival Paradigm: Are Fewer Juvenile Salmon Entering the Marine Food Web Than Previously Thought?

Ocean survival models for California salmon populations often assume a stable, abundant input of juvenile salmon into the marine environment each year because of relatively continuous, large-scale California hatchery production.  However recent studies on hatchery and wild smolts from the smallest and largest California watersheds suggest instream mortality could be much greater than previously thought.  Research into avian predation on parr and smolt stage juvenile steelhead and coho salmon passing through smolt traps in small California coastal watersheds has revealed freshwater mortality rates  to ocean entry that are much higher and more variable (40-80+%) than earlier  estimates. At the large basin scale, studies in the Sacramento River have assessed survival and interannual production for hatchery and wild Chinook salmon from fry to smolt life stages using acoustic telemetry and habitat surveys. These studies suggest instream mortality may be much greater and more variable than previously suspected.  For example, out-migration survival rates from basins several hundred kilometers upstream to the Golden Gate Bridge for hatchery fish range from 5-20%, whereas survival of fish that are trucked downstream is likely higher. Further, while recent otolith microchemistry analysis estimated that naturally produced fry may contribute up to 20% to the returning adult population, monitoring surveys revealed that annual catch-per-unit-efforts of fry is inconsistent and may vary by 2 orders of magnitude. These findings may indicate that fewer salmon are contributing to marine food webs along the California coast and that the marine survival component of salmon life history may be up to an order of magnitude higher than previously thought for some stocks. Thus, we must consider the possibility that variability in the marine environment is acting on a smaller and more variable number of fish.  Given continuing anthropogenic impacts to freshwater habitats, there is potentially a need to shift the ocean component of the salmon freshwater-ocean survival paradigm from a place of risk to one of refuge.

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