Tern smolt consumption increases; cormorant numbers up near Ilwaco

Cormorants are eating more and more salmon.

EAST SAND ISLAND - Caspian terns' consumption of juvenile salmonids grew last year, as did the sheer numbers of another avian predator - doubled-crested cormorants - in the Columbia River estuary and elsewhere, according to a draft research report released this week.

Tern consumption of young salmon and steelhead jumped during the spring and summer of 2006 to 32 percent of the birds' diet, up from an estimated 23 percent in 2005 and 18 percent in 2004, according to the 2006 season summary of ongoing research that began in 1997, "Research, Monitoring, and Evaluation of Avian Predation on Salmonid Smolts in the Lower and Mid-Columbia River."

The draft report is at www.columbiabirdresearch.org.

While the size of the East Sand Island tern colony near Ilwaco has not changed much since 2000, its consumption of salmon and, particularly, steelhead took an uptick last year. Researchers estimate the migratory birds ate 5.3 million smolts last year as compared to 3.6 million in 2005.

That consumption still pales in comparison to 1998, when more than 12 million smolts were consumed by terns, which fly north in spring to nest on the island. The colony there is considered to be the largest in the world, numbering 9,200 pairs in 2006, according to the study.

The birds' potential impact on the salmonids is heightened in the estuary, because all basin stocks, including 12 listed under the Endangered Species Act, swim through on their way to the ocean.

The increase in the terns' salmon consumption is puzzling, since ocean conditions seemed to bode well for abundance of the birds' main food sources - marine forage fish. Salmon consumption only increased slightly from 2004 to 2005, when ocean conditions seemed poor for marine fishes.

"It's not as simple situation as we'd like," said Don Lyons, an OSU Ph.D. candidate involved in the study.

While overall abundance of anchovies - the terns' staple - may have been higher in 2006, that doesn't mean it was higher within the birds prey range. The marine fish might have concentrated elsewhere along the coast where their quality of life was higher. The timing of marine fishes' presence in the estuary also might have not coincided with the terns' time of need, in which case the birds might have focused more on salmon.

Lyons called the terns opportunistic feeders, taking "what's around and what's near the surface." That's why, perhaps, shallow swimming steelhead are a favorite prey, the researchers theorize.

Of the juvenile salmonids consumed in 2006, the researchers estimated that 39 percent were coho salmon, 26 percent were yearling Chinook salmon, 18 percent were steelhead, 16 percent were subyearling Chinook salmon and 1 percent were sockeye salmon.

Steelhead again were hardest hit at East Sand, and at the mid-Columbia's Crescent and Rock islands. Predation rates were as high as 20 percent at East Sand on particular groups of PIT-tagged steelhead in 2006 that had been either identified passing Bonneville Dam or released from barges below the dam. The dam is the last hydro project the fish pass on their way to the ocean.

"Steelhead seem particularly vulnerable" to tern predation, according to consultant Ken Collis, co-principal investigator for the study along with Dan Roby of Oregon State University. Preliminary data indicates that cormorants at East Sand Island are less selective toward steelhead.

At Crescent Island, salmonids represented 63 percent of the diet of 448 nesting tern pairs last year. Data compiled from PIT tags collected on the island indicate that 12.3 percent of the Snake River hatchery-reared, in-river steelhead migrants, and 7.5 percent of the wild steelhead smolts, were consumed by terns. Those numbers were lower than estimates for 2005's migration, 18.6 and 14.5 percent respectively, but more fish were migrating in-river last year. Total steelhead consumption at Crescent Island in 2006 was up an estimated 22 percent from 2005. The Snake River steelhead are listed under the ESA as threatened.

"Because fewer Snake River steelhead were transported around McNary Pool in 2006 compared to 2005, however, a larger proportion of the Snake River steelhead population was susceptible to predation from Crescent Island terns in 2006, which corresponds with the higher total consumption of steelhead by Crescent Island terns in 2006 compared to 2005," the draft summary says. Increased spill last year - imposed under court order -resulted in more migrants passing downstream in-river via spillways and fewer fish being collected and transported.

A colony on nearby Rock Island grew from six tern pairs in 2005 to 110 in 2006. No consumption data was available for that colony.

This year the researchers will outfit 9,000 run-of-the-river Snake River steelhead smolts encountered at the juvenile fish facility at Lower Monumental Dam in an attempt to address some of those avian predation questions.

The intentional sampling and tagging of steelhead smolts at Lower Monumental would allow them to test hypotheses about how differences in smolt morphology, condition, abundance, origin, river conditions, and dam operations are associated with differences in smolt vulnerability to avian predation. Lower Monumental Dam represents the edge of the foraging range of Caspian terns nesting at Crescent Island.

The larger cormorants had a salmonid-light diet, 2 percent of their fish consumption in 2005 compared to the terns' 23 percent at East Sand. But they consume more overall poundage to meet their energy needs.

And their presence continues to grow, up 10 percent over 2005 and 275 percent since 1997 in the estuary, according to the draft report. The colony, also the world's largest, included 13,750 breeding pairs last year.

"Cormorant numbers there have really exploded," Collis said.

Cormorant smolt consumption data analysis is not yet complete for 2006, the combined smolt loss to terns and cormorants is likely in the 7 to 10 million range as it was the previous two years, "if not higher."

The birds nest elsewhere in the Columbia River basin but in much smaller numbers. Estimated smolt losses to terns, cormorants and other birds in the estuary is more than an order of magnitude higher than in the Mid-Columbia region.

"Additionally, when compared to the impact of avian predation on the mid-Columbia, avian predation in the estuary affects juvenile salmonids that have survived freshwater migration to the ocean and presumably have a higher probability of survival to return as adults compared to those fish that have yet to complete out-migration," the draft report says.

Attention focused on the Caspian terns in the later 1990s after their presence swelled from a handful the previous decade. The birds were attracted to the estuary as historic nesting sites along the coast and inland were lost because of development and other factors. They initially chose Rice Island as a primary nesting site. Both Rice and East Sand are manmade, created with dredged materials from the lower river's navigation channel.

In 1999 and 2000 the Corps of Engineers led an effort to lure the tern colony from Rice to East Sand, which is 15 miles nearer the Pacific Ocean. It was believed the move would help reduce salmon consumption by putting them in an environment with a more diversified menu. More marine forage fish are available nearer the Pacific.

The plan worked, with overall salmonid consumption falling from that 12 million-smolt peak.

Meanwhile, the Corps developed a plan to further reduce tern predation in the estuary by dispersing half of the birds to other sites in Oregon and California. Implementation of the January 2005 final environmental impact statement's strategies have been stalled awaiting funding and permission from Congress.

"I still have no authority to implement," the Corps' avian predation project manager, Jeff Dorsey, said this week. Acquiring land or conservation easements, preparing suitable habitat, establishing needed research, monitoring and evaluation schemes and doing other work specified in the EIS require money, and specific authority under the Water Resources Development Act.

Such legislation is being sought but its fate at this point remains unknown, Dorsey said.

Both the researchers and Dorsey say that a similar path may be necessary for cormorants - a painstaking evaluation of their status and needs, and of their impacts on Columbia basin salmon and steelhead. The collection of data to underpin a potential National Environmental Policy Act EIS for cormorant management has begun with an increased emphasis in recent years within the framework of the OSU study.

"People see that there are a lot of questions on cormorants that need to be answered" before any management decisions can be made, Collis said.

The 2006 report, he said, "is a little bit more extensive than it has been in the past" with more cormorant data, as well as more PIT tag information.

A dispersal would "seem more appropriate in the context of the cormorant colony on East Sand Island, which constitutes nearly 50 percent of the entire breeding population ...," the draft report says. Collis said the researchers believe the colony expansion is not due to overall growth in the West Coast population, but again the result of the lost of suitable habitat elsewhere.

Pilot studies conducted within the OSU research to lure cormorants to nearby islands have "shown some promise," the report says.

The estuary research is being funded primarily by the Bonneville Power Administration through the Northwest Power and Conservation Council's fish and wildlife program. The funding level established by Bonneville last week is $505,000 per year for the 2007-09 period. The Corps is funding the upriver portion of the study.

Recommended for you

(0) comments

Welcome to the discussion.

Keep it Clean. Please avoid obscene, vulgar, lewd, racist or sexually-oriented language.
Don't Threaten. Threats of harming another person will not be tolerated.
Be Truthful. Don't knowingly lie about anyone or anything.
Be Nice. No racism, sexism or any sort of -ism that is degrading to another person.
Be Proactive. Use the 'Report' link on each comment to let us know of abusive posts.
Share with Us. We'd love to hear eyewitness accounts, the history behind an article.