We initiated a study in 1997 to assess the impacts of piscivorous waterbirds (i.e., Caspian terns, double-crested cormorants, and several gull species) on the survival of juvenile salmonids in the lower Columbia River. These investigations indicated that Caspian terns nesting on Rice Island, a dredged material disposal island at river mile 21 in the Columbia River estuary, were the most significant avian predator of juvenile salmonids on the lower Columbia River. Rice Island supported an expanding population of about 16,000 nesting Caspian terns until 2000. This was the largest Caspian tern breeding colony in the world, and consisted of about 67% of all the Caspian terns nesting along the Pacific Coast of North America. Diet analysis indicated that Caspian terns nesting on Rice Island consumed more juvenile salmonids than any other prey type (73% of prey items in 1997-1998). Using bioenergetics modeling, we estimated that in 1998 Caspian terns nesting on Rice Island consumed about 12.4 million juvenile salmonids (95% c.i. = 9.1 – 15.7 million), or approximately 12.8% (95% c.i. = 9.4% - 16.3%) 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 steelhead smolts were more vulnerable to tern predation than other species of salmonids. In 1998, more than 13.3% of all PIT-tagged steelhead smolts that reached the estuary were consumed by Caspian terns nesting on Rice Island.
The magnitude of predation on juvenile salmonids by Caspian terns nesting on Rice Island led to management action in 1999. A pilot study was conducted to determine whether the Rice Island tern colony could be relocated 26 km (16 miles) closer to the ocean on East Sand Island (river mile 5), where it was hoped terns would consume fewer salmonids. Efforts to attract Caspian 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. About 1,400 pairs of Caspian terns nested at the new colony site on East Sand Island in 1999. In 2000, about 8,500 pairs of Caspian terns nested on East Sand Island, or 94% of all Caspian terns nesting in the estuary. During 2001–2010 all Caspian terns nesting in the Columbia River estuary used East Sand Island; colony size during this period averaged 9,300 breeding pairs.
As predicted, Caspian terns nesting on East Sand Island foraged more in marine and brackish water habitats than did Caspian terns nesting on Rice Island. The diet of East Sand Island terns averaged from 17% to 47% salmonids during the years 1999-2010; in comparison, the diet of Rice Island terns averaged 77% and 90% salmonids in 1999 and 2000, respectively. The relocation of the Caspian tern colony from Rice Island to East Sand Island resulted in a sharp drop in consumption of juvenile salmonids by terns nesting in the Columbia River estuary. Total consumption of juvenile salmonids in 2000, when most terns nested on East Sand Island, was estimated at 8.2 million (95% c.i. = 6.7–9.7 million), a reduction of about 4.2 million (34%) compared to 1998. Annual smolt consumption by Caspian terns nesting on East Sand Island during 2001–2010, when all terns nesting in the estuary used East Sand Island, ranged from 3.9 to 6.7 million. This represents a 46% to 69% reduction in estimated annual smolt mortality due to Caspian tern predation compared to 1998.
Caspian terns nesting on East Sand Island in 2007 nevertheless consumed an estimated 4.8 – 6.2 million smolts, with some stocks listed under the U.S. Endangered Species Act still suffering significant losses to Caspian tern predation. To achieve further reductions in consumption of juvenile salmonids by Caspian terns in the estuary, however, it will be necessary to reduce the size of the East Sand Island tern colony by relocating a portion of the colony to alternative sites outside the estuary.
In 2008, the U.S. Army Corps of Engineers began implementing the plan “Caspian Tern Management to Reduce Predation of Juvenile Salmonids in the Columbia River Estuary,” outlined in the Final Environmental Impact Statement (FEIS) and Records of Decision (RODs) signed in November 2006. This management plan seeks to redistribute a portion of the East Sand Island tern colony to alternative colony sites in Oregon and California by 2015. As alternative Caspian tern nesting habitat is created or enhanced, the available tern nesting habitat on East Sand Island will be reduced from its historic size (6 acres) to 1 – 1.5 acres. Once fully implemented, the plan would reduce the East Sand Island Caspian tern colony from its current size (approximately 9,850 nesting pairs) to approximately 3,100 – 4,400 nesting pairs. This reduction in the size of the East Sand Island Caspian tern colony is intended to reduce tern predation on smolts in the Columbia River estuary by an estimated 2.4 – 3.1 million smolts annually.
Monitoring and evaluation will be necessary to determine if the Caspian Tern Management Plan is successful in reducing smolt consumption by Caspian terns and other avian predators that nest on the lower Columbia River. As part of this study, we will continue to investigate the food habits and smolt consumption of Caspian terns nesting in the Columbia River estuary, as well as Caspian terns nesting at prospective alternative colony sites in Oregon and California. Our work at alternative colony sites will assess the potential impacts of larger, permanent Caspian tern colonies on local stocks of forage fishes, particularly fish species of special concern (e.g., juvenile salmonids, Warner suckers, Delta smelt). This study will also investigate whether low food availability or locally abundant nest predators may render some former or prospective Caspian tern colony sites as population sinks. Together these studies will be important in evaluating the efficacy of the Caspian Tern Management Plan in reducing predation on Columbia Basin salmonids while at the same time minimizing impacts to other listed fish outside the basin and maintaining the viability of the West Coast Caspian tern population.