Northwest Fisheries Science Center

Physical Oceanographic Considerations

The marine and anadromous faunae over which NOAA Fisheries exercises stewardship occupy diverse habitats in the coastal ocean off Washington, Oregon, and California.  This biogeographic region has been collectively termed the Coastal Upwelling Domain (Ware and McFarlane 1989).  Dominant fisheries species within this domain include market squid, northern anchovy, Pacific sardine, Pacific hake, Pacific mackerel, jack mackerel, Pacific herring, rockfish, flatfish, sablefish, and coho and Chinook salmon.

Within this domain, several smaller–scale physical zones are recognized, including: 

  1. A near shore zone where juvenile fall Chinook salmon, sand lance, and smelts reside

  2. The upper 10–20 m of the water column across the continental shelf and slope, where many pelagic fishes reside, including juvenile coho and Chinook

  3. The benthic and demersal habitats on the continental shelf (English sole), at the shelf break (whiting, rockfish), and beyond the shelf break to depths of 1500 m (sablefish, Dover sole, and thornyheads)

Each of these physical zones has unique circulation patterns that affect spawning and larval transport, and each is subject to different physical conditions.  These differing conditions lead to species–specific variations in growth, survival, and recruitment.  Moreover, since many species have pelagic larvae/juvenile stages, recruitment is affected by broad–scale variation in both ocean productivity, which affects the feeding environment of larval and juvenile fish, and in ocean circulation, which affects the transport of eggs and larvae. 

The Coastal Upwelling Domain is part of the California Current system, a broad, slow, meandering current that flows south from the northern tip of Vancouver Island (50°N) to Punta Eugenia near the middle of Baja, California (27°N).  The California Current extends laterally from the shore to several hundred miles from land.  In deep oceanic waters off the continental shelf, flows are usually southward all year round.  However, over the continental shelf, flows are southward only in spring, summer, and fall:  during winter, flow over the shelf reverses, and water moves northward as the Davidson Current.

These biannual transitions between northward and southward flow over the shelf occur in during March April and October November and are respectively termed the "spring transition" and "fall transition."  Another important feature of circulation within the Coastal Upwelling Domain is the deep, poleward flowing undercurrent found year round at depths of 100–300 m over the outer shelf and slope.  This current seems to be continuous from Southern California (33°N) to the British Columbia coast (50°N). 

Coastal upwelling is the dominant physical element affecting production in the Coastal Upwelling Domain.  In the continental shelf waters off Washington and Oregon, upwelling occurs primarily from April to September, whereas upwelling can occur year round off the coasts of northern and central California.  Upwelling in offshore waters also occurs through Ekman pumping and surface divergence in the centers of cyclonic eddies, but these processes will not be discussed further here. 

Coastal upwelling works as follows:  winds that blow from the north (towards the equator) result in the offshore transport of waters within the upper 15 m of the water column.  This offshore transport of surface waters is balanced by onshore movement of cold, nutrient rich waters from a depth of about 100–125 m at the shelf break region.  When winds are strong, this cold (8°C), nutrient rich water surfaces within 5 miles of the coast.  The result is high production of phytoplankton from April through September fueled by a nearly continuous supply of nutrients and concomitant high biomass of copepods, euphausiids, and other zooplankton during summer. 

Coastal upwelling is not a continuous process.  Rather, it is episodic, with favorable (equatorward) winds blowing for 1–2 week periods, interspersed by periods of either calm or reversals in wind direction.  These pulses in the winds produce what are called "upwelling events."  Interannual variations in the length and number of upwelling events result in striking variations in the level of primary and secondary production.  Thus, the overall level of production during any given year is highly variable, and is dependent on local winds. 

We do not yet know if there is an optimal frequency in upwelling event cycles, but one can easily imagine scenarios in which prolonged periods of continuous upwelling would favor production in offshore waters because nutrient rich waters would be transported far to sea.  The other extreme is one in which winds are weak and produce upwelling only in the very nearshore zone, within a mile or two of the coast.  In this case, animals living in waters off the shelf would be disadvantaged.  Any process that leads to reduction in the frequency and duration of northerly winds will result in decreased productivity and vice versa.  The most extreme of these processes is El Niño, which disrupts coastal ecosystems every 5–10 years.

Despite the existence of high plankton biomass and productivity, coastal upwelling environments present unique problems to fish and invertebrate populations who must complete their life cycles there.  This is because the upwelling process transports surface waters and the associated pelagic larvae and juvenile life stages away from the coast and towards the south, away from productive habitats.  Typical transport rates of surface waters are 1 km per day in an offshore direction and 20–30 km per day southward.

Zooplankton and larval and juvenile fishes, which live in the food–rich surface layers (i.e., the upper 15 m of the water column ), can be transported rapidly offshore, out of the upwelling zone, and into relatively oligotrophic waters.  Bakun (1996) argues that for any animal to be successful in such environments, the adults must locate habitats that are characterized by enrichment, with some mechanism for concentrating food (for larvae), and that offer a way for larvae to be retained within the system.

Perhaps because of its problems related to transport (and loss), many species do not spawn during the upwelling season.  Species such as Dover sole, sablefish, Dungeness crab, and pink shrimp each spawn during the winter months, before the onset of upwelling.  Other species perform an extended migration to spawn in regions where there is no upwelling. 

Hake, for example, undertakes an extended spawning migration, during which adults swim south to spawn in the South California Bight in autumn and winter, outside of the upwelling region and season.  This migration extends from Vancouver Island (ca. 49°N) to southern California (35°N), a distance of several thousand kilometers.  The return migration of adults and the northward drift of larvae and juveniles take place at depth, where fish take advantage of the poleward undercurrent.

Still other species, such as English sole, spawn in restricted parts of an upwelling system where advective losses are minimized, such as in bays or estuaries.  Salmonids and eulachon smelt spawn in rivers, completely outside the upwelling system.  Finally, species such as rockfish simply bypass the egg and larval stages and give birth to live precocious "juvenile" individuals.