Columbia River Plume Fronts and Their Influence on Juvenile Salmonids Orton, P.M., D.A. Jay, R. Emmett, R. Brodeur, and C. Morgan Funded by the Bonneville Power Administration

Horizontal density gradients, Salmon Survival Alaska: warm PDO –salmon survival –riverflow –mixing, gyre upwelling Oregon: cold PDO –salmon survival –coastal upwelling –riverflow HIGH  LOW  HIGH  Positive Anomalies Base figure provided courtesy of GLOBEC NEP

Why horizontal density gradients? Circulation patterns –reduced export from coastal zone –increased onshore transport opportunities Concentration processes (Bakun, 1996) –eddies –fronts: river plume, tidal mixing, upwelling, … are relatively productive concentrate surface-seeking organisms attract predators

 What is the physical character of frontal regions of the Columbia River plume?  Do currents concentrate juvenile salmon and prey at these fronts?  Are these regions favorable habitats for juvenile salmonids? Columbia River Plume, May ‘01

Image: Satellite-borne Scanning Aperture Radar (SAR) backscatter, Oct 1, 2001 (similar flow / wind conditions) May 23-28, 2001 Two vessels, helicopter Tides: spring or near-spring Riverflow: 33% of normal freshet flow Winds: NW knots, except last day Field Study, general summary

Fisheries Vessel Measurements PLUME OCEAN FRONT CTD: profiles and continuous surface, Meter Net, Neuston Net, Fish Trawl

Velocities: Acoustic profiler, Home Depot drifters Sal, Temp: Tow-Yo CTD Surface Flow-thru system - CTD: Sal, Temp - LISST: particle size, transmissivity - AC-3: Chlorophyll Physics Vessel Measurements

Front Structure, North Side, 9h past HHW Total depth: 39m At 1m:  S /  z= m -1  S /  x = m -1 F F At frontline Instrument trackline

Horizontal Velocities Total depth: 39m From above: velocity at 4m depth Cross-section PlumeOcean F F F

Temporal/Spatial Development of Fronts

Sea surface Plume Pycnocline Base Results Case A: Salmon in top 2m -- find front 6.5 hours later Case B: Salmon in pycnocline -- do not find front Are Salmon Encountering This Front? 5 cm s AssumptionsObservations Mean horizontal salmon swim velocity is zero Salmon “ejected” from river 4 hrs after frontogenesis

 What is the physical character of frontal regions of the Columbia River plume?  Very strong vertical, horizontal gradients  Convergence toward front, downwelling  Intensity decreases rapidly with time  Do currents concentrate juvenile salmon and prey at these fronts (from De Robertis et al)?  Some evidence for higher abundance of juvenile chum and coho salmon  Biomass of prey ingested by the salmon was elevated at fronts -- primarily Cancer magister  Are these regions favorable habitats for juvenile salmonids? Qualified Yes Summary

Acknowledgements: Greg Krutzikowsky, stellar captain of the R/V Sea Otter SAR image courtesy of NOAA/NESDIS Office of Research and Applications What’s Next ? Plume front study, May 2002 Evaluate salmon relationship to tidal mixing fronts How are these features affected by climate? Sal = AVHRR SST