Tern Management on the Columbia Plateau, 2016
In 2016, the U.S. Army Corps of Engineers - Walla Walla District and the U.S. Bureau of Reclamation (BOR) continued implementation of the Inland Avian Predation Management Plan (IAPMP) to reduce predation by Caspian terns (Hydroprogne caspia) on U.S. Endangered Species Act (ESA)-listed populations of salmonids (Oncorhynchus spp.) from the Columbia River basin (USACE 2014). The primary objective of management in the third year of implementation was to reduce the numbers of Caspian terns breeding at colonies on Goose Island in Potholes Reservoir and on Crescent Island in McNary Reservoir to less than 40 breeding pairs each. To accomplish this task, the availability of suitable Caspian tern nesting habitat was nearly eliminated on both islands by installing a variety of “passive nest dissuasion” materials prior to the 2016 nesting season, materials that were designed to preclude tern nesting on both islands. In addition, on Crescent Island willows had been planted over extensive areas to preclude tern nesting over the long-term. Ultimately, 4.3 acres, or more than 85% of the upland area of Goose Island and nearby islets, were covered with passive nest dissuasion materials consisting of stakes, rope, and flagging. On Crescent Island, about 2.4 acres of potential Caspian tern nesting habitat were covered with passive nest dissuasion materials consisting of fences rows of privacy fabric, as well as stakes, rope, flagging, and woody debris; any remaining open areas on Crescent Island had been planted with willows prior to the 2016 nesting season. On both islands, passive dissuasion was placed over all the area where Caspian terns have previously nested, as well as all areas of open, sparsely-vegetated habitat that might be used by ground-nesting Caspian terns or gulls (Larus spp.). An effort was also made to prevent nesting by the two species of gulls that nest abundantly on both islands (California gulls [L. californicus] and ring-billed gulls [L. delawarensis]), on the theory that nesting gulls would attract prospecting Caspian terns and could limit the efficacy of efforts to dissuade Caspian terns from nesting on the two islands. Once Caspian terns and gulls arrived on Goose and Crescent islands to initiate nesting, active nest dissuasion (i.e. human hazing) was used to try to dissuade both Caspian terns and gulls from nesting anywhere on either island.
Both California and ring-billed gulls quickly acclimated to both the passive and active dissuasion employed at Goose Island and, as in 2014 and 2015, gulls initiated nesting and laid eggs despite dissuasion efforts. Once gulls laid eggs, hazing gulls that were attending eggs was precluded due to the risk of causing gull nests to failure. As the area on Goose Island with active gull nests expanded, the opportunities to actively haze Caspian terns that were prospecting for nest sites on Goose Island declined. Nevertheless, between the passive dissuasion deployed on potential Caspian tern nesting habitat and active dissuasion (hazing, including use of a green laser) to deter prospecting terns, nesting by Caspian terns on Goose Island and nearby islets was prevented in 2016. This is the first year since nest dissuasion activities were initiated at Goose Island that efforts were successful in preventing Caspian terns from successfully nesting on Goose Island; in 2014, 159 breeding pairs nested on Northwest Rocks near Goose Island, and in 2015 two breeding pairs of Caspian terns nested on the main island under the passive dissuasion near the site of the former colony. Prior to management (2004-2013), an average of 343 breeding pairs of Caspian terns nested on Goose Island each year.
Despite the use of a combination of passive and active dissuasion on suitable Caspian tern nesting habitat on Goose Island during the 2014-2016 breeding seasons, some Caspian terns have continued to display high fidelity to Potholes Reservoir as a nesting area. This fidelity is likely due to Caspian terns nesting on Goose Island since 2004 and the persistence of a large gull colony on the island, which attracts prospecting Caspian terns to the site. Another factor that might explain the strong fidelity of Caspian terns to the Potholes Reservoir area is the paucity of alternative Caspian tern colony sites in the vicinity. As was the case in 2015, Caspian tern use of Goose Island for roosting and nesting in 2016 was largely limited to areas near the island’s shoreline, which gradually were exposed during the nesting season as reservoir levels receded. Active nest dissuasion (hazing), collection of any Caspian tern eggs discovered, and high rates of gull predation on newly-laid Caspian tern eggs were factors in preventing the formation of a Caspian tern colony on Goose Island in 2016. Only six Caspian tern eggs were discovered in five different nests on Goose Island during the 2016 nesting season. Of those six tern eggs, four were collected under permit and two were depredated by gulls soon after they were laid. By comparison, a total of 43 Caspian tern eggs were found on Goose Island and nearby islets in 2015.
Passive and active nest dissuasion techniques were successful in preventing all nesting and roosting by both Caspian terns and gulls on Crescent Island during the 2016 nesting season, as was the case in 2015. The results during the 2015 and 2016 nesting seasons were somewhat unexpected because Caspian terns and gulls had nested consistently on Crescent Island for nearly three decades prior to 2015. One factor that likely contributed to the absence of nesting Caspian terns on Crescent Island was the use of closely-spaced fence rows of privacy fabric as passive dissuasion in much of the suitable Caspian tern nesting habitat on Crescent Island, including the former colony area; similar fencing was not deployed at Goose Island due to shallow rocky soils. Another factor was the successful dissuasion of all gulls from nesting on Crescent Island in both 2015 and 2016; gulls are breeding associates of Caspian terns and attract prospecting Caspian terns to nest near their colonies. At Goose Island, gull nesting could not be prevented using the passive and active dissuasion techniques at our disposal, whereas at Crescent Island gulls never habituated to the passive and active dissuasion techniques. Instead, gulls abandoned Crescent Island as a nesting site and established a new colony on Badger Island (located on the Columbia River just one kilometer upriver from Crescent Island) in both 2015 and 2016. Similarly, Caspian terns displaced from Crescent Island relocated to an alternative colony site on the Columbia River, the Blalock Islands in John Day Reservoir (70 river kilometers downriver from Crescent Island), where Caspian terns have nested in small numbers over the last decade. Resightings of Caspian terns that were previously color-banded on Crescent Island confirmed that there was a large influx of terns to the Blalock Islands colony from the colony on Crescent Island in 2015, and many of these same color-banded terns renested at the Blalock Islands colony in 2016.
System-wide action effectiveness monitoring determined that Caspian terns attempted to nest at five different colonies in the Columbia Plateau region in 2016. Four of these occupied colony sites had been used in previous years, and one site was new in 2016. The formerly occupied sites included the Blalock Islands on the Columbia River (483 breeding pairs in 2016, down from 677 breeding pairs in 2015), Twinning Island in Banks Lake (6 breeding pairs in 2016, down from 64 breeding pairs in 2015), Harper Island in Sprague Lake (3 breeding pairs in 2016, down from 10 breeding pairs in 2015), and an unnamed island in Lenore Lake (39 breeding pairs in 2016, up from 16 breeding pairs in 2015). In 2016, an incipient Caspian tern colony became established on a small, low-lying island in northeastern Potholes Reservoir, where 144 breeding pairs of Caspian terns attempted to nest before the colony was abandoned in early June. As was the case in 2015, the largest Caspian tern colony in the Columbia Plateau region was on the Blalock Islands, where 72% of all the Caspian terns in the region nested during 2016. Compared to the average historical size of the Caspian tern colony on the Blalock Islands during 2005-2014 (58 breeding pairs), the colony was 11 times larger in 2015 and eight times larger in 2016.
The total estimated breeding population of Caspian terns in the Columbia Plateau region during 2016 was 675 breeding pairs at five separate colonies. This represents a 23% decline in the total number of Caspian terns breeding in the Columbia Plateau region compared to the pre-management average during 2005-2013 (873 breeding pairs). Although nest dissuasion actions implemented on Goose and Crescent islands in 2016 were effective in preventing all Caspian terns from nesting at these two colonies, formerly the two largest tern colonies in the region, it did not result in a commensurate reduction in the total number of Caspian terns breeding in the region. This was primarily due to the more than 8-fold increase in the number of Caspian terns nesting in the Blalock Islands in 2016 compared to the pre-management average for that colony. The Blalock Islands colony during 2015-2016 was similar in size to the largest Caspian tern colony recorded anywhere in the Columbia Plateau region since intensive monitoring began in 2000.
Resightings of Caspian terns that were previously color-banded indicated strong site fidelity to the Potholes Reservoir area, despite the third year of efforts to dissuade Caspian terns from nesting at Goose Island and nearby islets. The Blalock Islands experienced a large influx of nesting Caspian terns in 2015, both from the colony on Crescent Island and the colony on Goose Island, but most immigrants came from Crescent Island. Many of the terns that immigrated to the Blalock Islands in 2015 returned to that colony in 2016. Although most Caspian terns dissuaded from Goose and Crescent islands remained in the Columbia Plateau region during the 2015 and 2016 nesting seasons, some terns dispersed to breeding or non‐breeding sites along the coasts of Washington and Oregon, as well as to colonies on Corps-constructed islands in interior Oregon and northeastern California. Results from a more robust analysis to estimate inter-annual movement rates and the number of terns that moved among colonies are also included this report.
The goal of the IAPMP is to reduce Caspian tern predation rates (percentage of available fish consumed by terns) on ESA-listed salmonid populations to less than 2% per tern colony per year (USACE 2014). Based on an analysis of former predation rates by Caspian terns nesting in the Columbia Plateau region during 2007-2015 and data on the size of breeding colonies (number of breeding pairs) in 2016, we predicted that predation rate goals would be achieved for many, but not all, tern colonies and salmonid populations in 2016. Predicted predation rates on ESA-listed salmonid populations were close to zero (≤ 0.2% per ESA-listed population) for Caspian terns nesting at Goose Island due to the near complete abandonment of the colony in 2016. Predicted predation rates on ESA-listed salmonid populations by Caspian terns nesting on Crescent Island were even closer to zero (< 0.1% per ESA-listed population), due to the complete abandonment of the colony in 2016. Because of the large size of the Caspian tern colony at the Blalock Islands in 2016 (483 breeding pairs), however, predicted predation rates were above the 2% threshold for Upper Columbia River steelhead (4.9%; 95% prediction interval [PI] = 3.8 - 6.4%), Snake River steelhead (4.6%; 95% PI = 4.0 - 5.6%), Snake River sockeye salmon (2.6%; 95% PI = 1.6 - 5.0%), and Snake River spring/summer Chinook salmon (2.0%; 95% PI = 1.8 - 2.3%).
There is a lack of sufficient data to estimate former predation rates, or predict current predation rates, on ESA-listed salmonid populations by Caspian terns nesting on (1) Twinning Island in Banks Lake, (2) the small unnamed island in Lenore Lake, and (3) the incipient colony in northeastern Potholes Reservoir. Salmonid PIT tags were recovered, however, from these three nesting sites following the 2016 breeding season and actual (as opposed to predicted) estimates of predation rates are pending further analysis; results will be presented as part of a report to the Grant County Public Utility District and the Priest Rapids Coordinating Committee in January 2017. Actual predation rate estimates based on smolt PIT tag recoveries will also be available for terns that nested on the Blalock Islands in 2016. Due to the small numbers of Caspian terns that were present on Goose Island and the complete abandonment of the Crescent Island tern colony, PIT tag recovery was not conducted at either of these sites and, consequently, predicted predation rates represent the best estimate of predation rates in 2016.
In summary, management to eliminate breeding colonies of Caspian terns on Goose Island in Potholes Reservoir and on Crescent Island in McNary Reservoir, formerly the largest breeding colonies for the species in the Columbia Plateau region, was fully successful in 2016, the third year of implementation of the IAPMP. Consequently, predation on juvenile salmonids by Caspian terns nesting at these two colony sites was effectively eliminated. Numbers of breeding Caspian terns in the Columbia Plateau region have declined from pre-management levels due to the management of colonies on Goose and Crescent islands, with the regional population size declining by 23%. Based on resightings of banded Caspian terns, most terns that were displaced from colonies on Goose and Crescent islands have remained in the region, and many have attempted to nest at alternative colony sites in the region. Most notable was the post-management increase in the size of the formerly small breeding colony in the Blalock Islands. Caspian terns nesting in the Blalock Islands during 2015-2016 have consumed sufficient numbers of juvenile salmonids to at least partially compensate for reductions in smolt consumption due to tern management at Goose and Crescent islands. Nesting habitat for Caspian terns in the Blalock Islands is dependent on reservoir level; quality tern nesting habitat is only available when reservoir levels are below full pool. A new tern colony also appeared on a small island in northeastern Potholes Reservoir, but this colony failed before any chicks were fledged due to predators reaching the island as reservoir levels dropped. Based on results during the first three years of implementation of the IAPMP, the goal of the Plan to reduce predation rates on ESA-listed salmonid populations below 2% per tern colony per year throughout the Columbia Plateau region will not be achieved until alternative tern nesting habitat is reduced from current levels, especially in the Blalock Islands and perhaps at the new colony in Potholes Reservoir.
-Bird Research Northwest
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