The size of the double crested cormorant colony on the lower Columbia River’s East Sand Island grew little from 2009 to 2010, but the avian predators’ consumption of juvenile salmon and steelhead skyrocketed, according to preliminary estimates from researchers that monitor the birds.
The researchers’ “best estimate” is that the 13,600 breeding pairs that nested on the island, and their chicks, this year gobbled up about 19 million young salmon and steelhead, Oregon State University’s Dan Roby told participants in this week’s Anadromous Fish Evaluation Program annual review in Portland.
“It’s clearly more than we’ve ever seen,” Roby said of the cormorants’ consumption. The estimated consumption last year, 11.1 million salmonids, had been the high count on a record dating back to 2003. The colony totaled 12,100 breeding pairs in 2009.
The overall research project has been ongoing since 1997. It aims to assess avian predation impacts on the 13 Columbia River basin salmon and steelhead stocks that are listed under the Endangered Species Act.
The cormorants’ increased consumption this year was because salmonids made up 17 percent of the big birds’ diet as compared to 9 percent in 2009. Roby said he had heard reports the abundance of alternative prey, such as marine species like anchovies and sardines, was low this year, particularly early in the season.
Accurate estimates of the total number of salmon and steelhead that arrived in the estuary are hard to come by. But preliminary estimates compiled by the Fish Passage Center indicate that more than 141 million young fish were released from hatcheries across the basin. That data will be finalized sometime early next year. The number of wild fish that launched toward the Pacific this year is uncertain.
The study is a collaborative project between Oregon State University, Real Time Research, and the USGS-Oregon Cooperative Fish and Wildlife Research Unit conducting research, monitoring, and evaluation regarding the issue of avian predation on juvenile salmonids in the Columbia River estuary. Roby is co-principle investigator for the study along with Real Time’s Ken Collis.
The study is funded through the U.S. Army Corps of Engineers’ AFEP program, which is run on annual congressional appropriations that are reimbursed to the U.S. Treasury by the Bonneville Power Administration. The Corps, along with the Bureau of Reclamation, operates the hydro projects that make up the Federal Columbia River Power System. Bonneville markets the power generated in the FCRPS.
Because the Corps and its cooperating federal and state partners are implementing a management plan to reduce predation on smolts by Caspian terns in the estuary, much of the Columbia River estuary avian predation research effort is directed at monitoring and evaluating the efficacy of bird management. According to a study abstract those same managers are now amidst a process to consider the efficacy of a management plan aimed at reducing cormorant predation in the lower river.
East Sand Island is home to the largest known double breasted cormorant colony. It represents about 41 percent of the estimated West Coast population of 30,000. The colony grew steadily from about 5,000 breeding pairs in 1997 to a peak of nearly 14,000 in 2007, then dropped off to only 11,000 in 2008 before resuming an upward trend the past two years.
“This past year, 2010, was definitely an anomaly,” Roby said. In 2006 and 2007 the cormorants’ consumption was about 9 million annually when a similar number of birds (just under 14,000 nesting pairs) settled on the island.
Beginning in 2003 the researchers made a concerted effort to collect the necessary data to generate estimates of annual smolt consumption by cormorants at this colony using bioenergetics modeling. The estimates indicate that total smolt consumption varies widely from year to year. The best estimate in 2005 was only about 2 million smolts.
The preliminary data summarized by Roby indicates that well over half of the double crested cormorants consumption, 12.4 million smolts, were subyearling fall chinook. That includes listed wild Snake River fish as well as unlisted wild fish from the mid-Columbia. The cormorants next favorite target was coho (3.2 million).
The researchers continued their evaluation of East Sand’s Caspian tern colony, which is also considered to be the largest in the world. A total of 8,300 pairs nested at the island this year, which is down slightly from the 2000-2010 average. The colony was relocated from Rice Island, which is farther upstream, to its current site back in 2000 with the goal of bringing the birds closer to alternative prey such as marine species.
The relocation achieved its goal of reducing the tern predation on salmon. At Rice Island the terns were estimated to eat as many as 12 million smolts annually. At East Sand they have an average (2000-2009) consumption of 5.3 million, including an estimated 5.3 million this year. The proportion of juvenile salmonids in tern diets during the 2010 nesting season was 33 percent, which was similar to recent years.
The East Sand colony’s population has been relatively constant through the years, averaging about 9,300.
The management plan being implemented involves reducing the area of suitable habitat at East Sand Island and encouraging the terns to relocate to alternative habitat created outside the Columbia Basin. But so far, with the East Sand area reduced by 38 percent, the population remains relatively constant because the original area was bigger than the colony needed. With the habitat shrinkage, the nesting density has greater.
In 2010, 8,300 breeding pairs used slightly less than the 3.1 acres of habitat that was prepared for them. In 2011, the area of tern nesting habitat will be reduced to 2 acres, probably causing more of the terns to nest elsewhere, Roby said. The ultimate goal is to reduce the area of habitat for Caspian terns down to 1 acre, but reaching that goal depends on providing at least 8 acres of suitable alternative nesting habitat for terns outside the basin.
So far the Corps has built eight new tern nesting islands with a total acreage of 7.3 acres, but due to the severe drought in the Upper Klamath Basin, only five islands were surrounded by water this year, Roby said. Caspian tern colony size and nesting success was very low at four of the five islands.