Note: Provided here is the Executive Summary from our 2004 Season Summary (click link below for complete report). The 2004 Season Summary has been prepared for the Bonneville Power Administration and the U.S. Army Corps of Engineers for the purpose of assessing project accomplishments. This report is not for citation in the published literature without the permission of the authors. Please contact clientservices@realtimeresearch.org for this authorization.
Last Revised : Arpil 08, 2006
[Click here for the 2004 Season Summary (pdf)]
Executive Summary
We initiated a study in 1997 to investigate the impacts of piscivorous colonial waterbirds on the survival of juvenile salmonids ( Oncorhynchus spp.) in the lower Columbia River (Roby et al. 1998; Collis et al. 2002). The study area included the Columbia River from the mouth (river km 0) to the head of the impoundment created by McNary Dam (river km 553). The species of piscivorous waterbirds investigated were Califonia gulls (Larus californicus), ring-billed gulls (L.delawarensis), glaucous-winged/western gulls (L. glaucescens X L. occidentalis), Caspian terns (Sterna caspia), double-crested cormorants (Phalacrocorax auritus), and, more recently, American white pelicans (Pelecanus erythrorhynchos) and California brown pelicans (Pelecanus occidentalis californicus). This study revealed differences in diet composition among the various bird species and colony locations (Collis et al. 2002). Terns, cormorants, and pelicans were strictly piscivorous, whereas the three gull species consumed a diverse array of food types. Gulls nesting at up-river colonies consumed primarily anthropogenic food items (e.g., cherries, potatoes, human refuse).
In general, piscivorous waterbirds nesting in the Columbia River estuary consumed more juvenile salmonids than those nesting up-river. On Rice Island (river km 34), salmonids accounted for 74% of the diet in Caspian terns, 46% in double-crested cormorants, and 11% in glaucous-winged/western gulls (Collis et al. 2002). Juvenile salmonids were especially prevalent in the diets of colonial waterbirds on Rice Island during April and May. By comparison, juvenile salmonids were significantly less prevalent in the diets of cormorants and gulls nesting lower in the estuary on East Sand Island (river km 8), presumably due to the greater availability of marine forage fishes. These results indicated that avian predation on juvenile salmonids in the lower Columbia River is more prevalent in the estuary than near the large up-river gull colonies. Furthermore, the high incidence of salmonids in the diets of Caspian terns, cormorants, and gulls nesting on Rice Island suggested that the impact of avian predation on survival of smolts would be reduced by discouraging piscivorous birds from nesting there, while encouraging nesting on East Sand Island and other sites nearer marine foraging areas.
In 1997 and 1998, Caspian terns nesting on Rice Island consumed the highest percentage of juvenile salmonids of those species of piscivorous colonial waterbirds nesting in the estuary (Collis et al. 2002). Rice Island , a dredged material disposal site, supported an expanding colony of about 8,500 breeding pairs of terns in 1998 (Collis et al. 2002). This colony was the largest known Caspian tern breeding colony in the world, and supported about two-thirds of all the Caspian terns nesting along the Pacific Coast of North America (Cuthbert and Wires 1999). Using bioenergetics modeling, it was estimated that in 1998 this tern colony consumed about 12.4 million juvenile salmonids (95% c.i. = 9.1–15.7 million), or approximately 13% (95% c.i. = 9.1%–16.9%; Roby et al. 2003) of the estimated 96.6 million out-migrating smolts that reached the estuary during the 1998 migration year. Analysis of over 36,000 smolt PIT tags recovered from the Caspian tern breeding colony on Rice Island revealed that over 13.5% of all PIT-tagged steelhead smolts (O. mykiss) that reached the estuary were consumed by terns in 1998 (Collis et al. 2001).
The magnitude of predation on juvenile salmonids by Rice Island terns led to management action in 1999 (Roby et al. 2002). A pilot study was conducted to determine whether the Rice Island tern colony could be relocated 26 km closer to the ocean on East Sand Island (river km 8), where it was hoped terns would consume fewer salmonids. Efforts to attract terns to nest on East Sand Island included creation of nesting habitat, use of social attraction techniques, and predator control, with concurrent efforts to discourage terns from nesting on Rice Island. This approach was successful, and in three years all nesting terns shifted from Rice Island to East Sand Island. Juvenile salmonids decreased and marine forage fishes (e.g., Pacific herring [Clupea pallasi], anchovies [Engraulidae], smelt [Osmeridae], and surfperch [Embiotocidae]) increased in the diet of Caspian terns nesting on East Sand Island, compared with terns nesting on Rice Island.
Our monitoring of tern management in the Columbia River estuary continued in 2004. In 2004 the estimated size of the Caspian tern colony on East Sand Island was approximately 9,500 nesting pairs. This represents about a 14% increase in the size of the colony compared to the 2003 breeding season. Nesting success at the East Sand Island colony remained high, with an average productivity of 0.92 young raised per breeding pair in 2004. During the 2004 breeding season, the diet of East Sand Island terns averaged 17% salmonids, the lowest proportion of salmonids in the diet so far recorded for this tern colony. Consumption of juvenile salmonids by the East Sand Island tern colony in 2004 was approximately 3.5 million smolts (95% c.i. = 2.9–4.0 million), ca. 9 million fewer smolts consumed compared to 1998, when all terns nested on Rice Island. The area of quality nesting habitat prepared for Caspian terns on East Sand Island (6.5 acres) and the area of habitat used by nesting terns (4.7 acres) were similar to the previous two years. Marine forage fishes were abundant in the Columbia River estuary and nesting success in 2004 was similar to 2002 and 2003, revealing no apparent incentive for Caspian terns to shift to alternative colony sites.
Although numbers of Caspian terns nesting in the Columbia River estuary have remained stable over the last 8 years, the numbers of double-crested cormorants nesting on East Sand Island have nearly tripled during the same period to ca. 12,500 breeding pairs. This colony is now the largest known breeding colony for the species in North America. Although juvenile salmonids represented only ca. 5% of the diet of cormorants nesting on East Sand Island in 2004, estimated smolt consumption by the cormorant colony (6.4 million smolts; 95% c.i. = 2.5–10.3 million) is now comparable to or greater than that of the East Sand Island tern colony. This is due largely to the greater size of the cormorant colony on East Sand Island and the greater food requirements of cormorants relative to terns. The double-crested cormorant colony on East Sand Island experienced high nesting success in 2004 (2.05 young/breeding pair), more than twice the nesting success experienced by the East Sand Island Caspian tern colony in 2004. This colony is expected to continue to expand for the foreseeable future, perhaps posing an increasing risk to survival of juvenile salmonids in the estuary.
The only other known Caspian tern breeding colony on the lower Columbia River during 2004 was on Crescent Island, just below the confluence of the Snake and Columbia rivers. The tern colony on Crescent Island consisted of about 530 breeding pairs in 2004, similar in size to the previous year. Average nesting success of Caspian terns on Crescent Island in 2004 (0.62 young raised per breeding pair) was somewhat greater than in 2003 (0.55 young per breeding pair). The diet of Caspian terns nesting on Crescent Island in 2004 consisted of ca. 70% juvenile salmonids, similar to diets of Crescent Island terns during the 2000-2003 breeding seasons. An estimated 470,000 (95% c.i. = 370,000–570,000) juvenile salmonids were consumed by Caspian terns nesting on Crescent Island in 2004. Despite the much smaller numbers of salmonid smolts consumed annually by the Crescent Island tern colony compared to the tern and cormorant colonies on East Sand Island, predation rates on particular salmonid stocks were surprisingly high, particularly in low flow years. Preliminary results from 2004 suggest the predation rate by Crescent Island terns on Snake River steelhead smolts was 23%, based on the number of PIT-tagged smolts interrogated at Lower Monumental Dam that were subsequently recovered on the Crescent Island tern colony and corrected for PIT tag collision and detection efficiencies on-colony. In-river steelhead smolts from the Snake River were more vulnerable to tern predation than in-river steelhead smolts from the Upper Columbia (4% of PIT-tagged smolts interrogated at Rock Island Dam were subsequently recovered on the Crescent Island tern colony, corrected for tag collision and detection efficiency). The high predation rate on in-river migrants from the Snake River was, however, offset by the transportation of most juvenile salmonids around the McNary Pool. Conversely, juvenile salmonids from the upper and mid-Columbia River (upstream of McNary Dam) were not transported past Crescent Island, resulting in a much larger proportion of those runs being susceptible to predation by Crescent Island terns. Predation rates on salmonids by Crescent Island terns are unlikely to increase appreciably over those observed in 2004 considering constraints on tern colony expansion, limited capacity for increased per capita smolt consumption by terns, and current high transportation rates for Snake River smolts.
In 2004, the largest colony of double-crested cormorants on the Mid-Columbia River consisted of ca. 300 pairs on Foundation Island, near Crescent Island, and the diet of Foundation Island cormorants during the chick-rearing period consisted of < 8% salmonids. The American white pelican colony on nearby Badger Island is also small (< 500 pairs) and, based on smolt PIT tag detections on the pelican colony by NOAA Fisheries, is not a source of significant smolt mortality.
A system-wide assessment of avian predation using the available data indicates that the most significant impact on survival of juvenile salmonids occurs in the estuary. Caspian terns and double-crested cormorants nesting on East Sand Island together consumed ca. 10 million smolts in 2004. Additionally, when compared to predation impacts further up river, avian predation that occurs in the estuary affects juvenile salmonids that have survived freshwater migration to the estuary and presumably have a higher probability of survival compared to those fish that have not yet completed their outmigration. Finally, juvenile salmonids from every listed stock from the Columbia River basin are susceptible to predation in the estuary because all surviving fish must migrate in-river through the estuary. For these reasons, management of terns and cormorants on East Sand Island has the greatest potential to benefit Columbia River salmonid populations across the basin, when compared to potential management of other bird populations. One possible exception is the Caspian tern colony on Crescent Island, where tern management may benefit some stocks in some years (e.g., Upper Columbia River steelhead ESU, particularly in low flow years).
Further management of Caspian terns to reduce losses of juvenile salmonids in the estuary is imminent; the Final Environmental Impact Statement for Caspian tern management in the Columbia River estuary lists the redistribution of approximately two-thirds of the East Sand Island colony to alternative colony sites in Washington, Oregon, and California as the preferred alternative (USFWS 2005). Management options to reduce or cap smolt losses to the expanding double-crested cormorant colony have yet to be considered and will require additional research and NEPA analysis. Relocation of a portion of the cormorants nesting on East Sand Island to alternative sites outside the estuary may be an option. Pilot studies designed to test the feasibility of employing habitat enhancement and social attraction (i.e., decoys, audio playback systems) to relocate nesting cormorants showed some promise; cormorants were induced to nest at two sites on East Sand Island where they had not previously nested. Restoration, enhancement, or establishment of tern and cormorant colony sites outside the Columbia River estuary would likely benefit Columbia Basin salmonids without negatively affecting protected populations of fish-eating birds.
References
Anderson, C.D., D.D. Roby, and K. Collis. 2004. Conservation implications of the large colony of double-crested cormorants on East Sand Island, Columbia River estuary, Oregon, USA. Waterbirds 27:155-160.
Collis, K., D.D. Roby, D.P. Craig, B.A. Ryan, and R.D. Ledgerwood. 2001. Colonial waterbird predation on juvenile salmonids tagged with passive integrated transponders in the Columbia River estuary: vulnerability of different salmonid species, stocks, and rearing types. Transactions of the American Fisheries Society 130:385-396.
Collis, K., D.D. Roby, D.P. Craig, S. Adamany, J. Adkins, and D.E. Lyons. 2002. Colony size and diet composition of piscivorous waterbirds on the lower Columbia River: Implications for losses of juvenile salmonids to avian predation. Transactions of the American Fisheries Society 131:537–550.
Cuthbert, F., and L. Wires. 1999. Caspian tern (Sterna caspia). in A. Poole and F. Gill, editors. The Birds of North America, no. 403. The Birds of North America, Inc., Philadelphia, Pennsylvania.
Roby, D. D., D. P. Craig, K. Collis, and S. L. Adamany. 1998. Avian predation on juvenile salmonids in the lower Columbia River. 1997 Annual Report of the Oregon Cooperative Fish and Wildlife Research Unit at Oregon State University to Bonneville Power Administration and U.S. Army Corps of Engineers, Portland, Oregon, USA.
Roby, D.D., D.E. Lyons, D.P. Craig, K. Collis, and G.H. Visser. 2003. Quantifying the effect of predators on endangered species using a bioenergetics approach: Caspian terns and juvenile salmonids in the Columbia River estuary. Canadian Journal of Zoology 81:250-265.
Ryan, B.A., B.P. Sanford, K. Collis, A.F. Evans, and D.D. Roby. In prep. Predation on juvenile salmonids by colonial waterbirds in the Columbia River basin: System-wide assessment of avian predation based on PIT tag recoveries.
USFWS ( U.S. Fish and Wildlife Service). 2005. Caspian tern management to reduce predation of juvenile salmonids in the Columbia River estuary: Final Environmental Impact Statement, January 2005. Migratory Birds and Habitat Program, Portland OR.