2012 Final Annual Report
We conducted field studies in 2012 to (1) assess the impact of avian predation on survival of juvenile salmonids (Oncorhynchus spp.) in the Columbia River estuary, (2) monitor the efficacy of on-going management actions designed to reduce the impact of Caspian terns (Hydroprogne caspia) on salmonid smolt survival in the estuary, (3) test management strategies for limiting the availability of nesting habit for double-crested cormorants (Phalacrocorax auritus) at East Sand Island in the Columbia River estuary, and (4) evaluate the impacts on smolt survival of piscivorous colonial waterbirds (i.e., Caspian terns, double-crested cormorants, American white pelicans Pelecanus erythrorhynchos, California gulls Larus californicus, and ring-billed gulls L. delawarensis) that nest in the Columbia Plateau region.
The Caspian tern breeding colony on East Sand Island, the largest of its kind in the world, consisted of about 6,400 breeding pairs in 2012, continuing the downward trend in colony size from the peak of about 10,000 pairs in 2008. The Caspian tern colony on East Sand Island produced a total of about 410 fledglings in 2012, compared to complete breeding failure at this colony in 2011. The proximal factor responsible for colony failure in 2011 and very poor nesting success in 2012 (an average of only 0.06 young raised per breeding pair) was intense disturbance by bald eagles (Haliaeetus leucocephalus) and associated predation on tern eggs and chicks by glaucous-winged/western gulls (L. glaucescens/occidentalis). The average proportion of juvenile salmonids in the diet of Caspian tern diets during the 2012 nesting season was 34%, similar to 2009-2011. The estimated total smolt consumption by Caspian terns nesting at East Sand Island in 2012 was 4.9 million (95% c.i. = 3.9 - 5.8 million), similar to 2011. Recoveries of smolt passive integrated transponder (PIT) tags on the Caspian tern colony at East Sand Island indicated that tern predation rates in 2012 were highest on steelhead populations (7.4 – 10.0%, depending on the population), followed by salmon populations (0.7 – 2.2%, depending on the population), based on ESA-listed PIT-tagged smolts last interrogated passing Bonneville Dam on the Columbia River or Sullivan Dam on the Willamette River; there were indications that predation rates on some ESA-listed salmonid populations were trending lower in 2012 compared to 2010 and 2011. To further reduce the impacts of predation by Caspian terns nesting at East Sand Island on salmonid stocks from the Columbia River basin, more terns will need to be relocated to colonies outside the basin; the management objective is to reduce the size of the East Sand Island tern colony to 4,000 breeding pairs or less, < 40% its pre-management size (ca. 10,000 breeding pairs), while attracting the displaced Caspian terns to alternative colony sites built for terns elsewhere.
Caspian tern management actions continued in 2012, with the U.S. Army Corps of Engineers, Portland District (Corps) further reducing the area of suitable tern nesting habitat on East Sand Island to 1.58 acres, 32% of its former area. This habitat restriction caused Caspian terns to nest at higher densities (average of 1.06 nests/m2) than previously seen in the Columbia River estuary. The Corps has built a total of nine new islands as alternative Caspian tern nesting habitat since early 2008, six in interior Oregon and three in the Upper Klamath Basin region of northeastern California. Six of these nine new islands supported nesting Caspian terns in 2012, including the 1-acre rock-core island built early in 2012 on Malheur Lake in Malheur National Wildlife Refuge, where 232 pairs nested. Nest predators, both mammalian and avian, and apparent low forage fish availability at some sites (Crump Lake and Summer Lake Wildlife Area), limited Caspian tern colony size and nesting success at five of the six tern colonies that formed on the Corps’ new islands in 2012. Substantial numbers of Caspian terns from the colony on East Sand Island in the Columbia River estuary, however, are visiting these new islands; 64 terns originally banded in the Columbia River estuary were re-sighted on the Corps’ new Malheur Lake tern island and 83 were re-sighted on the Corps’ Upper Klamath Basin tern islands in 2012.
Data on diet composition of Caspian terns nesting on Corps-constructed tern islands in interior Oregon and northeastern California indicated that in 2012 terns from these colonies primarily consumed cyprinids (i.e., chub Gila spp., fathead minnows Pimephales promelas, and common carp Cyprinus carpio), centrarchids (i.e., crappie Pomoxis spp.), and ictalurids (i.e., bullhead Ameiurus spp.). Catostomids (suckers), several species of which are listed under the Endangered Species Act, were not identified in the diet of Caspian terns nesting on Corps-constructed tern islands at Sheepy Lake, Tule Lake Sump 1B, and Summer Lake during 2012. One juvenile sucker (species unknown) was observed at the Caspian tern colony on the Corps-constructed tern island in Crump Lake during 2012; suckers represented a very small percentage (< 0.1%) of identifiable prey items at this Caspian tern colony. No sucker PIT tags were recovered from Caspian tern colonies on the Corps-constructed tern islands in either interior Oregon or northeastern California during 2012.
The double-crested cormorant colony on East Sand Island in the Columbia River estuary consisted of about 12,300 breeding pairs in 2012, the largest colony of its kind in western North America and similar in size to 2011 (ca. 13,000 breeding pairs). Juvenile salmonids represented about 20% (by biomass) of the double-crested cormorant diet in 2012, compared to about 19% in 2011. Our estimate of total smolt consumption by double-crested cormorants nesting on East Sand Island in 2012 was 18.9 million smolts (95% c.i. = 14.0 – 23.8 million), not significantly different from the number of smolts consumed by cormorants from this colony in 2011. Annual smolt consumption by double-crested cormorants nesting on East Sand Island has been trending upward since 2003, until 2012 when estimated consumption leveled off. As in other recent years, estimates of total smolt consumption by East Sand Island cormorants were significantly higher than that of Caspian terns nesting on East Sand Island in 2012. Recoveries of smolt PIT tags on the East Sand Island cormorant colony in 2012 indicated that population-specific predation rates ranged from 0.6% to 7.2% for populations originating upstream of Bonneville Dam on the Columbia River or upstream of Sullivan Dam on the Willamette River. Compared to predation rates on salmon populations by Caspian terns nesting on East Sand Island, predation rates by double-crested cormorants nesting on East Sand Island (2% – 4%) were generally higher; however, the highest estimate of population-specific predation rates for East Sand Island cormorants was on steelhead from the upper Columbia River (7.2%), less than the maximum population-specific predation rate by East Sand Island terns (10.0%).
Using both bioenergetics-based estimates of smolt consumption at the level of the salmonid species and PIT tag-based estimates of population-specific predation rates on salmonids, it is possible to develop a more comprehensive understanding of the impacts of avian predators on survival of juvenile salmonids from across the basin. Population-specific predation rates based on PIT tag recoveries indicated that the impacts of Caspian terns and double-crested cormorants nesting on East Sand Island are substantial for several salmonid populations originating upstream of Bonneville Dam. Bioenergetics-based estimates of smolts consumed indicated that other salmonid populations from across the basin are also negatively affected, and some significantly so. Genetic identification of smolts in the diet of Caspian terns and double-crested cormorants indicated that salmonid populations from the basin that are infrequently PIT-tagged, but ESA-listed (e.g., Upper Willamette River steelhead, Lower Columbia River Chinook salmon), are also consumed in significant numbers by terns and cormorants nesting at East Sand Island. Overall smolt consumption by East Sand Island cormorants has been higher during 2010-2012 compared to the previous decade, while predation rates on ESA-listed populations originating upstream of Bonneville Dam have not shown this same trend. Impacts of double-crested cormorant predation on sub-yearling Chinook smolts originating downstream of Bonneville Dam, however, have been substantial during 2010-2012.
In 2012, the Corps expanded a pilot study initiated in 2011 to test possible strategies for limiting the size of the double-crested cormorant colony on East Sand Island. An eight-foot-high privacy fence was built to bisect the colony and visually separate 62% of the nesting area used by the colony in 2010 from the remainder of the colony. Using human disturbance to haze cormorants during the nest initiation period, cormorants were successfully dissuaded from using this 62% of their former nesting area. Some hazed cormorants were satellite-tagged or radio-tagged to follow their movements to prospective new nesting sites. About 55% of these tagged cormorants dispersed from the East Sand Island colony after tagging, but nearly all eventually returned to the Columbia River estuary and attempted to nest on East Sand Island. Tagged cormorants dispersing from East Sand Island were detected at colonies and roost sites (1) on the lower Columbia River below Bonneville Dam, (2) the outer Washington coast (Willapa Bay and Grays Harbor), (3) Puget Sound, and (4) northern Salish Sea (San Juan Islands; Strait of Georgia; Vancouver, BC area). Only one tagged cormorant was detected on the north coast of Oregon (Cannon Beach).
Caspian terns, double-crested cormorants, American white pelicans, California gulls, and ring-billed gulls are native piscivorous colonial waterbirds that nest in the Columbia Plateau region. The total number of Caspian terns nesting in the Columbia Plateau region was about 1,000 breeding pairs at six colonies in 2012, as high as or higher than any other year during 2005-2011. The two largest Caspian tern breeding colonies were at Goose Island (463 pairs) in Potholes Reservoir, WA and at Crescent Island (422 pairs) on the mid-Columbia River. The third largest breeding colony was recently formed at Badger Island (60 pairs) on the mid-Columbia River. A small number of Caspian terns (n = 8) that were originally banded as adults on East Sand Island in the Columbia River estuary – where management actions to reduce the size of the colony are being implemented – were re-sighted at colonies in the Columbia Plateau region during 2011 and 2012; some of these banded terns (n = 4) were confirmed to be nesting at colonies in the Columbia Plateau region. The movement of banded Caspian terns that had previously nested on East Sand Island to colonies in the Columbia Plateau region was not seen during 2006-2010, before tern management intensified at East Sand Island. Natal dispersal of terns banded as chicks at East Sand Island to the colonies in the Columbia Plateau region has also been confirmed. Caspian tern movements from East Sand Island to colonies in the Columbia Plateau region, if substantial, could off-set benefits to salmonids of tern management in the estuary because per bird impacts on smolt survival are higher for terns nesting in the Columbia Plateau region compared to those nesting in the estuary, where marine forage fishes (anchovy, smelt, surfperch, etc.) tend to dominate the diet.
Total numbers of double-crested cormorants nesting in the Columbia Plateau region increased slightly in 2012, from an average of about 1,350 breeding pairs during 2005-2011 to about 1,550 breeding pairs at four colonies in 2012; the largest colonies were in the North Potholes Reserve (992 nesting pairs) and on Foundation Island in the mid-Columbia River (390 nesting pairs). Numbers of American white pelicans nesting on Badger Island in the mid-Columbia River, a colony that experienced rapid growth during 2004-2010, appear to have stabilized at about 2,100 adults. The numbers of gulls nesting on Miller Rocks, a colony located just downstream of John Day Dam on the Columbia River, were similar to those observed in recent years (ca. 4,500 adults). Following the abandonment of the large California gull colony on Three Mile Canyon Island (ca. 6,200 adults were counted on-colony in 2009), there was a commensurate increase in the number of California gulls nesting on islands in the Blalock Islands complex in 2012; in 2012 ca. 7,300 nesting California gulls were counted on one island in the Blalock Islands, whereas in 2009 no gulls nested there.
Salmonid smolts represented 83% of Caspian tern prey items at the Crescent Island colony and 30% of prey items at the Goose Island colony, resulting in an estimated 730,000 juvenile salmonids consumed by Caspian terns nesting at these two colonies combined in 2012. Estimates of predation rates based on PIT tag recoveries on Caspian tern colonies indicate that impacts were highest on survival of Upper Columbia River steelhead (estimated predation rate of 17.3% by Goose Island terns), Snake River steelhead (estimated predation rate of 2.8% by Crescent Island terns), and Upper Columbia River spring Chinook (estimated predation rate of 2.5% by Goose Island terns). Smolt PIT tag recoveries on Caspian tern colonies located at Twining Island on Banks Lake (> 45 km from the Columbia River) and at Harper Island on Sprague Lake (> 65 km from the Snake River) indicated that Caspian terns from those two colonies commuted long distances to the mainstem rivers to consume ESA-listed juvenile salmonids.
Studies to refine estimates of avian predation rates based on smolt PIT tags recovered on colonies of double-crested cormorants, California gulls, and ring-billed gulls were conducted in 2012. These studies resulted in correction factors for PIT tag deposition rates, the proportion of PIT tags consumed by birds that were subsequently deposited on-colony, as opposed to off-colony. These correction factors are needed to more accurately measure predation rates on juvenile salmonids by these three species of piscivorous colonial waterbirds. Previously, estimates of PIT tag deposition rates were only available for Caspian terns. Deposition corrected results from 2012 indicated that predation rates on steelhead populations by gulls nesting at certain colonies in the Columbia Plateau region were as great or greater than those of nearby Caspian tern and double-crested cormorant colonies. For example, predation rate estimates indicate that ca. 4% of the available Snake River steelhead and Upper Columbia River steelhead were consumed by gulls nesting on Crescent Island in 2012; ca. 4% of available Snake River steelhead and ca. 6% of available Upper Columbia River steelhead were consumed by gulls nesting on Miller Rocks in 2012. Predation rates on most populations of salmon by gulls nesting at the Crescent Island and Miller Rocks colonies were, however, generally less than 1.0%, with the exception of the predation rate on Snake River sockeye salmon by gulls nesting on Miller Rocks (ca. 5%). PIT tag-derived predation rates by double-crested cormorants nesting on Foundation Island, corrected for PIT tag deposition rate, indicated that predation rates were highest on Snake River sockeye salmon (2.5%) and Snake River steelhead (2.4%). Data on PIT tag deposition rates for American white pelicans nesting at the colony on Badger Island are not currently available. Minimum predation rate estimates (not corrected for PIT tag deposition rates) indicate that American white pelicans consumed less than 0.3% of the available smolts in 2012, regardless of salmonid population.
Resource management agencies are currently developing a management plan aimed at reducing avian predation rates on ESA-listed salmonids in the Columbia Plateau region, especially on steelhead smolts from the Upper Columbia River and Lower Snake River populations. Previous and future research efforts will help inform this process so that the resultant management initiatives are science-based, defensible, cost-effective, and have a high probability of success.
-Bird Research Northwest
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