myths: salmon and the sea

Myths: Salmon and the Sea

• Limiting factors are all in fresh water, hence marine survival does not vary

• The ocean has unlimited capacity to support salmon

• All juveniles salmonids migrate rapidly to the north

OSU Purse Seining OSU Purse Seining (1981(1981--1985)1985)OSU purse seining—1979-1985

Ocean carrying capacity

• It varies– Seasonally—Coastal Upwelling– Interannually—El Niño/La Niña– Interdecadally—Pacific Decadal Oscillation– Intercentenially

1983 El Niño

Predictions for 1983 coho returns: 1.3 million vs. 600 thousand

Timing (spring transition) and intensity are both important

Drifter tracks temperature Chlorophyll

• Strong upwelling=good survival during cool PDO

•Poor correlation with upwelling during warm PDO

•Strong stratification

Pacific Decadal Oscillation.

Warm or positive phase Cool or negative phase

OPI (hatchery) coho marine survival: PDO and stratification

0

0.02

0.04

0.06

0.08

0.1

0.12

0.1419

70

1972

1974

1976

1978

1980

1982

1984

1986

1988

1990

1992

1994

1996

1998

Return Year

Surv

ival

Why? Leading hypothesis: changes in ocean conditions impact the entire marine food-web

1976-77 PDO shift

Strong stratification in 1990s

upwelling food webs in our coastal ocean

Cool water, weak stratificationhigh nutrients, a productive “subarcticsubarctic” food-chain with abundant forage fish and few warm water predators

Warm stratified ocean, fewnutrients, low productivity “subtropicalsubtropical” food web, a lack of forage fish and abundant predators

What controls survival?Hypotheses and mechanisms

• Critical period during early ocean life is affected by ocean conditions

• Ocean productivity, prey availability and growth– Faster growth, less predation– Critical size to avoid winter starvation– Area and distribution of highly productive water

• Predation intensity—more big predators when warm waters intrude and close to shore