Estuary Birds Double Smolt Consumption
by Bill Rudolph
At the Corps of Engineers' recent research review in Walla Walla, bird researchers stunned the audience when they told fellow scientists that cormorants and terns in the lower Columbia have doubled their consumption of salmon and steelhead smolts over the past two years.
Together, the birds on East Sand Island, near the mouth of the Columbia, ate more than 27 million smolts, with the cormorants estimated to have consumed more than 22 million smolts. Researchers estimated that about 18 million of those young fish were subyearling fall chinook, 3 million were young coho and the rest nearly equal parts yearling chinook and steelhead.
Consumption was up, but the cormorant population actually declined a bit from the 2010 estimate, when more than 13,000 breeding pairs were estimated to inhabit the colony. The birds' productivity was down in 2011 as well, when compared to 2010, according to preliminary findings. The long-term average productivity of more than two chicks per pair declined to about 1.5.
The report also said the size and nesting success of the cormorant colony may have peaked--and "seems limited by predation."
The researchers reported on a pilot study for testing the feasibility of using privacy fencing and human disturbance to reduce bird numbers. Those actions kept cormorants from nesting on 15 percent of the colony.
As for the Caspian terns, which were the original focus of the research on bird predation in the lower Columbia more than 10 years ago, a concerted effort to reduce their colony size at East Sand Island has helped to reduce their numbers. The tern colony has declined from nearly 11,000 breeding pairs in 2008 to around 7,000 breeding pairs in 2011.
Smolt consumption has dropped in recent years as well--from nearly 7 million in 2006 to less than 5 million in 2011.
Terns didn't play favorites when it came to their diet, either. They were estimated to have consumed about a million subyearling chinook, about half a million yearling chinook, and a million-plus each of young coho and steelhead.
The terns now use only about two acres on the island for nesting, down from more than four acres in the early part of the last decade.
Nesting success for the terns was essentially zero this year, after several eagles began harassing the colony. But that didn't mean they weren't busy preying on young salmon and steelhead, instead of heading for new grounds built by the Corps to reduce tern numbers in the lower Columbia.
But several tern nesting sites in eastern Oregon showed little attraction to terns from the Columbia estuary. A Tule Lake site of two acres was home to only 34 tern breeding pairs, with only one chick fledged.
Researchers are now working with the Corps of Engineers on a draft EIS for managing the East Sand Island cormorant colony, which has nearly tripled in size since 1998, beginning a process already completed for the terns, whose colony has actually shrunk since then--by around a thousand breeding pairs.
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