[5] Birds Rule Lower River
Despite a program to move some salmon-munching terns away from the mouth of the Columbia, researchers say tern and cormorant populations are still growing there, and they've got the evidence to prove it.
The researchers detected more than 80,000 PIT tags from bird colonies in the Basin this year--most in the lower estuary near the river mouth--but about 20 percent were found on islands in the reservoir behind McNary Dam.
The latest results were announced earlier this month at the Corps of Engineers' annual research review in Walla Walla, where scientists said the 80,000 PIT tags represent a minimum of 3.6 percent of all PIT-tagged juvenile fish released into the basin in 2009.
That adds up to 7 million to 16 million smolt sandwiches a year, on average.
With the western population of double-crested cormorants growing at a 3-percent clip as a whole, the lower estuary number of about 12,000 breeding pairs was up 10 percent from 2008. That means more than 40 percent of the entire western population of the cormorants is located on East Sand Island, close to the mouth of the Columbia.
In 2008, nearly 10,000 nesting pairs of terns were counted on East Sand Island, where they had been relocated from an upriver site at Rice Island a few years earlier.
The scientists estimated about 9 percent of the tern diet was made up of salmonids this year, down from 12 percent in 2008. In 2008, about 30 percent of the diet was salmonids.
However, a closer look at the data shows that nearly 10 percent of all the juvenile steelhead detected at Bonneville Dam were consumed by birds downriver, along with nearly 3 percent of the spring/summer chinook smolts that had been detected passing the dam.
Another important finding is the high mortality level of young fall chinook released in the lower Columbia. Researchers estimated about 30 percent of the PIT-tagged subyearlings released below Bonneville Dam ended up on the tern and cormorant colonies near the mouth of the river.
Other research with steelhead found that nearly 10 percent of PIT-tagged steelhead were consumed by waterbirds in McNary Pool and about 2 percent in John Day/The Dalles Pool.
The scientists also reported that in 2008, more than 20 percent of PIT-tagged steelhead released at Lower Monumental Dam on the lower Snake were later consumed by avian predators in the estuary.
For steelhead released into the mid-Columbia, the researchers found that about 5 percent were eaten in McNary Pool and 2 percent in JD/The Dalles pools.
They also estimated that the colony (486 pairs) of Caspian terns in the Potholes area of the upper Columbia was responsible for knocking off about 14 percent of the upper Columbia steelhead run.
"A comparison of predation between different avian species and colonies indicated that Caspian terns on East Sand Island, Crescent Island, and Potholes Reservoir consumed the largest proportion of available PIT-tagged steelhead, followed by double-crested cormorant colonies on East Sand and Foundation islands," read a study abstract. "Predation by gulls and American white pelicans was relatively minor in comparison to that of terns and cormorants."
White pelicans may not be a problem, but California brown pelicans, listed for protection under the ESA, are showing up in large numbers. In April, the first brown pelican was sighted at East Sand Island. By the middle of June, nearly 13,000 of them were roosting on the island. That's nearly twice the previous high count for June, according to other researchers' weekly updates. East Sand Island has turned out to be the pelicans' largest known post-breeding nighttime roost site.
Gulls in the region are adding more salmonids to their diets as well, according to the report, which said "recent increases in numbers of smolt PIT tags recovered on Miller Rocks in The Dalles Pool, where about 4,600 pairs of gulls now nest, have raised concerns about the impact of gull predation on survival of salmonid smolts."
Researchers said this year, 5,500 smolt PIT tags were deposited on the Miller Rocks gull colony, compared to 4,211 tags deposited in 2008, and the increase in consumption likely reflects both an increase in size of the gull colony, as well as an increase in foraging intensity at the nearby John Day Dam and The Dalles Dams. "The magnitude of predation on salmonid smolts by Miller Rocks gulls appears to be unique to this gull colony," researchers concluded.
Scientists estimated that gulls alone ate 80,000 or so salmonids at John Day--between 1.5 and 3 percent of the nearly 4 million smolts that swam by during the three-month study period.
Other work has been done to monitor the effectiveness of efforts to reduce tern predation and begin action to move cormorants.
Using satellite tags, researchers tracked 36 double-crested cormorants nesting at East Sand Island. Cormorants were tracked this fall to sites in the Puget Sound region (n=15); lower Columbia River (n=5); northern California (n=3); interior British Columbia (n=1); John Day Dam (n=1); and Salton Sea, Calif. (n=1).
The Corps of Engineers established two new colony sites for terns in interior Oregon, both at Summer Lake Wildlife Area. Caspian terns have colonized both islands, for a total of 15 pairs. Five terns that had been banded in the Columbia River estuary were again seen at the Summer Lake tern islands.
Oregon State University scientist Dan Roby said two other sites built in 2008 were more successful. The Crump Lake tern island in Warner Valley, Ore., attracted about 700 pairs of terns. Eighteen terns that had been banded in the Columbia River estuary were re-sighted on Crump Lake Island.
He said more than 80 percent of the diet of the Crump Lake and Summer Lake terns is made up of tui chub.
But another site, ready last year at Fern Ridge, had the same success rate as last year--zero.
Roby said his group has also been studying the nesting ecology of Caspian terns in San Francisco Bay, as part of a Corps proposal to build another site for terns in 2010.
The largest Caspian tern colony in the Bay Area is on Brooks Island, where about 10 percent of their diet is already made up of salmonids. Roby said fish managers are concerned that adding more terns from the Columbia River estuary might reduce survival of ESA-listed salmonids from the Sacramento River.
Recoveries of over 2,000 smolt-coded wire tags from the Brooks Island tern colony found that 98 percent of the smolts consumed by terns were non-ESA-listed fall-run chinook. Nearly all of the coded wire tags recovered from the tern colony (99.7 percent) were from smolts released from net pens in San Pablo Bay. -B. R.
The following links were mentioned in this story:
Anadromous Fish Evaluation Program
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