Has
the PDO shifted to favor Columbia Basin salmon?
by ED
HUNT |
posted 02.09.04 |
It's complicated.
That's the word from researchers at the meeting of the American Geophysical
Union, held last week in Portland.
Of course we already knew it was complicated. Over the past 100 years -- based
on catch records and other data -- Pacific salmon have gone through a boom and
bust cycle. Decades of good years followed by decades of bad years. Salmon
return in great numbers in Alaska when the nets are empty down on the Columbia
and vice versa.
The reason for this shifting abundance is called the PDO -- Pacific Decadal
Oscillation. Shifting currents in the Pacific ocean bring changes in winds and
water temperatures which in turn bring different food sources for salmon and
different precipitation patterns inland -- which can mean more or less water at
the right time for salmon spawning streams.
Sometimes the weather and ocean favored Columbia Basin salmon -- set them up
with lots of nice fatty good stuff to eat and cool water to enjoy -- other times
the water was warmer and the food more scarce, or less nutritious. The bottom
line is when things are good in the ocean -- more salmon survive and return to
spawn.
Things change every 20 to 30 years -- from "positive to negative" or
from cooler to wetter. While a cool or warm pattern lasts over decades, it
seemed to flip from one to the other rather quickly in relative terms. When it
does, the marine critters readily respond.
"We see fundamental changes in ecosystem structure occurring almost
instantaneously," NOAA researcher Franklin Schwing told the Oregonian.
About a year or two after researchers at the University of Washington and Oregon
State University identified the PDO as a major player in salmon abundance,
however, something was happened out in the ocean itself. Something changed. Was
it a shift in the PDO?
Certainly salmon abundance started improving in about 1998 and now -- subsidized
with millions of hatchery fish flushed down the rivers every year-- salmon
returns are the best they've been since the Bonneville Dam was built in 1938. To
the uninitiated, it certainly appeared that the climate regime had shifted in
favor of the Columbia basin again.
So we can expect at least another 15 years of great salmon fishing right?
Well, it turns out it's more complicated than that. This PDO shift -- if that is
what it is -- isn't following the pattern of the past 100 years.
That means the good times for salmon may not last. That also means whatever
pattern we're in now is less predictable than we thought.
For salmon that means an uncertain future.
To be sure something is going on out there and it has been good for salmon.
As the Oregonian's
Joe Rojas-Burke reported, researchers found major changes in copepod
populations. Copepod are little crustaceans that are food for the fish that
salmon like to eat. Warm water copepods dominated since the last PDO shift in
1977, yet after the 1998 event, cool-water copepods took over -- their
populations increasing by 70 to 140 percent in some cases the following year.
"For predator fish such as salmon, the shift to cold-water prey species
could explain gains in survival," Rian Hooff said. "Cold-water copepod
varieties are larger and better-quality food for salmon because, unlike their
southern cousins, they store energy in a waxy fat layer," Rojas-Burkel
wrote.
Actually, salmon don't eat a lot of copepods, Hooff told Tidepool, but new
research has identified what appears to be a link between copepod populations
and fish abundance and survival at the higher trophic levels -- including
salmon. Hooff found that the type of copepods prevelant in the ocean (fat or
thin) could be positively correlated with coho salmon survivial.
"The results suggest that changes in copepod community structure may be a
helpful indicator, albeit indirect, for forecasting salmonid survival/returns in
subsequent years," Hooff said.
Yet other indicators don't fit the profile of a PDO shift. For the most part,
fishing in Alaska hasn't crashed the way one would expect. Then there are the
warm water copepods who showed up -- along with warmer waters -- in 2003. The
strong westerly flow and warm water currents don't match previous PDO
conditions. Indeed, what's going on out there no longer seems to resemble either
a positive or a negative PDO.
It may be that there is more than two alternating regimes of the PDO. Perhaps
global warming or some other new factor is shaking things up. Maybe something
unprecedented is happening.
Or maybe we're just reading the tea leaves wrong.
"Today we don't have the tools or the understanding to accurately monitor
or predict the states of Pacific decadal variability," Nathan Mantua , a
research scientist at the University of Washington and NOAA's Joint Institute
for the Study of the Atmosphere and Oceans. "We look at sea surface
temperature because it is relatively easy and it has a long historical record.
But without knowing the physics of what causes this phenomenon to happen, we
don't really know what the key parameters are."
So what does all this mean for salmon?
While the good news is that whatever is happening out there in the ocean has
been good for salmon -- fat copepods, cool water or whatever -- the bad news is
that it may not last. The message we need to take away is that we must redouble
our habitat restoration efforts, doing all we can to improve wild salmon
returns while ocean conditions are favorable. The abundance we see now in our
streams is really a message to us that the clock is ticking, and that we have to
be aggressive in our salmon restoration efforts while the ocean, weather and
wind our in our favor.
Indeed, salmon abundance itself is more complicated than it looks.
While the most recent report from the federal government says that all the
Columbia Basin stocks (except the Snake River sockeye) are "clearly in less
jeopardy of extinction" -- that doesn't mean they are recovering.
Hatcheries release200
million salmon a year. Shut off the salmon-hatchery spigot and salmon
abundance would drop by 80 percent -- and many wild salmon populations are still
not reproducing enough to increase their populations -- some are still
declining.
That still leaves them especially vulnerable to a change in the weather.
Source: http://www.tidepool.org/original_content.cfm?articleid=106655
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