Title: A Snapshot of Salmon in Oregon
Category: Active
Blog Entry: Predators
Many researchers see increased pedation by creatures such
as sea lions, triggered by human-related changes in the ecosystem, as one
of numerous factors contributing to the decline of salmon.
By Theresa Novak
Salmon once were so abundant, they were a main food source for animals
of the land, air and water.
But as the number of salmon have decreased, concerns have increased that
not enough is being done to control the number of natural predators that
feed on them.
Seals and sea lions are two examples of salmon predators that now seem
out of balance with their prey.
The population of these fur-bearing, ocean-going mammals dropped very
low in the early part of the century due to demand for their pelts. Fishermen
sometimes shot them on sight because they competed for fish and raided fishing
nets.
While seal and sea lion numbers rebounded after the Marine Mammals Protection
Act in 1978 prohibited harming them, the numbers of salmon have fallen to
historic lows.
Some people thought the two trends were related, a view that gained popular
attention by the well-publicized salmon-gobbling antics of Hershel, the
determined sea lion.
In the 1980s, the big sea lion had a big appetite for mature steelhead
salmon. His habit of gobbling them as they swam into the locks in the Ballard
district of Seattle earned him the reputation for almost single-handedly
wiping out a steelhead run.
Hershel also proved to be determined. Despite being moved many miles
from the locks at Seattle, he found his way back.
Nowadays, Hershel is an attraction at Sea World.
But nuisance animals such as Hershel, who go into a feeding frenzy over
migrating salmon, prompted an amendment to the Marine Mammals Protection
Act allowing federal marine fisheries officials to remove--even kill--particular
individuals that are devastating a salmon run.
"When you have 600-700 adult sea lions (in a given area) and you
only have an expected return of 2,500 wild coho, you have to ask yourself
whether that is an acceptable loss," said Robin Brown, a marine mammals
specialist with the Oregon Department of Fish and Wildlife.
And as wildlife officials struggle to control one predator whose numbers
are on the rise, the impacts of other predators are being better understood.
A three-year research project by wildlife biologist Daniel Roby of Oregon
State University and OSU researchers Larry Davies and Carl Schreck indicates
that sea birds known as Caspian terns are gobbling millions of salmon smolts
as they swim down the Columbia River estuary and head toward the Pacific.
The terns have established what may be the largest breeding colony in
the world. More than 8,000 nesting pairs live on a man-made sandbar called
Rice Island, produced during dredging of the Columbia River. The seven-mile
finger of sand has formed the perfect place for the terns, but its effect
on salmon appears devastating.
According to research by the Roby-Schreck team, between 5 million and
20 million salmon smolts headed for the estuary in 1997 fell victim to terns
nesting on Rice Island. That amounted to up to 30 percent of the smolts
that were headed for the estuary.
The numbers vary so greatly because it isn't known how much of the loss
of the smolts was due to terns and how much of it was due to gulls and cormorants,
which also feed on smolts.
Yet as sea birds, the terns are federally protected, once again posing
the question: How do you prevent one protected species from pushing another
closer to extinction?
However, neither Brown nor the history of salmon predator control supports
a full-scale attack on salmon predators such as the terns, mostly because
such efforts in the past have produced unwanted side effects.
For example, after a 1991 study showed that northern pike, once known
as squawfish, were eating up to 61 percent of the salmon smolts entering
the pool of the John Day Dam, an aggressive program was started to reduce
the number of these native fish.
The plan had some unexpected consequences. Predators that normally ate
the northern pikeminnow began eating salmon instead. The same thing happened after
populations of northern lampreys, once an abundant eel-like fish that was
the preferred food of seals and sea lions, began declining because of changes
in the lamprey habitat.
Non-native predators such as walleye, shad and small-mouthed and large-mouthed
bass also prey upon salmon smolts. Largely unknown is what sorts of ocean-going
predators may be seeking out salmon as food because their usual supply of
prey is not available.
Those concerned with salmon restoration are taking action and gathering
more information and authority so that natural predators don't wipe out
salmon while humans continue efforts to restore salmon habitat.
Now the Oregon Department of Fish and Wildlife is seeking authority to
remove or occasionally destroy nuisance seals and sea lions without having
to go through federal channels.
The ODFW also is seeking ways to reduce the number of exotic fish that
out-compete or eat salmon.
The way that salmon are released from hatcheries and barged around dams
also is being changed to make it tougher for predators to catch the salmon
when they are most vulnerable.
More will need to be done in the way of research and compromise so that
the struggle for survival between salmon and their predators will again
be a fair fight. <a xhref="http://eesc.orst.edu/salmon/human/predators.html">"A Snapshot of Salmon in Oregon," EM 8722, published September
1998.
Updated: 11/10/1998; 04:38 PM</a>
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