Tern Management on the Columbia Plateau, 2017
In 2017, 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 fourth year of implementation was to limit the numbers of Caspian terns breeding at colonies 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 at these sites by installing a variety of “passive nest dissuasion” materials prior to the 2017 nesting season, materials that were designed to preclude tern nesting at both locations. In addition, on Crescent Island, willows had been planted over extensive areas of the island to preclude tern nesting over the long-term. On both Goose and Crescent 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.). Ultimately, 4.3 acres, or more than 85% of the upland area of Goose Island 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 in 2016; additionally, all open areas on Crescent Island had been planted with willows and other native vegetation prior to the 2016 nesting season. Finally, the island in northeastern Potholes Reservoir that was used by Caspian terns for nesting in 2016 (0.15 acre) and four nearby islands where terns were observed prospecting in 2017 (0.3 acre) were covered in passive dissuasion to prevent terns from nesting at those sites. An effort was also made to prevent nesting by the two species of gulls (California gulls [L. californicus] and ring-billed gulls [L. delawarensis]) that have nested at Goose and Crescent islands, on the theory that nesting gulls would attract prospecting Caspian terns and could limit the efficacy of efforts to dissuade Caspian terns from nesting at these managed sites. Once Caspian terns and gulls arrived to initiate nesting, active nest dissuasion (i.e. human hazing) was used to try to dissuade both Caspian terns and gulls from nesting on Goose and Crescent islands, as well as on other islands in the northern portion of Potholes Reservoir.
As was the case in 2015-2016, passive and active nest dissuasion techniques were successful in preventing all nesting and roosting by both Caspian terns and gulls in upland areas on Crescent Island during the 2017 nesting season, the third year of management at this site. Prior to management (2005-2013), an average of 403 breeding pairs of Caspian terns nested on Crescent Island. The complete abandonment of Crescent Island by nesting terns beginning in the first year of management was somewhat unexpected because Caspian terns and gulls had nested consistently on Crescent Island for nearly three decades prior to management. 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 and willow plantings as passive nest dissuasion measures in all the suitable Caspian tern nesting habitat on Crescent Island, including the former colony area; fencing and willow plantings were not deployed at Goose Island due to shallow rocky soils. Another factor was the successful dissuasion of gulls from nesting on Crescent Island in 2015-2017; 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 nest dissuasion techniques at our disposal, whereas at Crescent Island gulls never habituated to the nest dissuasion techniques implemented there. 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 2015-2017. Similarly, many Caspian terns displaced from Crescent Island relocated to unmanaged colony sites on the Columbia River, including the Blalock Islands in John Day Reservoir (70 river kilometers downriver from Crescent Island) in 2015-2017 and Badger Island in 2017, where Caspian terns have nested in small numbers intermittently over the last decade.
Despite the use of passive and active nest dissuasion techniques on Goose Island and elsewhere in Potholes Reservoir, some Caspian terns continued to display high fidelity to Potholes Reservoir as a nesting area in 2017, the fourth year of management at this site. 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, both before and after management, which continues to attract 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 2015-2016, Caspian tern use of Goose Island for roosting and nesting in 2017 was largely limited to areas near the island’s shoreline, areas that became exposed during the nesting season as reservoir levels receded. Despite high fidelity of terns to the area, active nest dissuasion (hazing), collection (under permit) of any Caspian tern eggs discovered, and high rates of gull depredation on newly-laid Caspian tern eggs were factors in preventing the formation of a Caspian tern colony on Goose Island and elsewhere in Potholes Reservoir in 2017. This is the second consecutive year that nest dissuasion activities initiated at Goose Island were successful in preventing Caspian terns from successfully nesting there; in 2014, 159 breeding pairs nested on Northwest Rocks near Goose Island, and in 2015 two breeding pairs of Caspian terns nested under the passive dissuasion near the former colony location on Goose Island. Prior to management (2005-2013), an average of 367 breeding pairs of Caspian terns nested on Goose Island.
In 2017, egg laying by Caspian terns on Goose Island and elsewhere in Potholes Reservoir occurred between 29 April and 7 July. During this period, a total of 20 Caspian tern eggs were discovered at three different locations in Potholes Reservoir; 18 tern eggs were discovered on Goose Island and one tern egg was discovered on two different islands in northern Potholes Reservoir. Of the 20 tern eggs that were discovered, 18 were collected under permit and two were depredated by gulls soon after they were laid. By comparison, a total of 282 Caspian tern eggs were found on Goose Island and an incipient colony in northeastern Potholes Reservoir in 2016; 6 tern eggs were discovered and collected under permit on Goose Island and 276 tern eggs were discovered on an incipient colony in northeastern Potholes Reservoir after the colony was completely abandoned due to predation and disturbance caused by a mink early June. The eggs laid on the incipient colony in northeastern Potholes Reservoir were not collected.
Aerial, ground, and boat-based surveys were conducted in the Columbia Plateau region to determine where Caspian terns displaced from the managed colonies in Potholes Reservoir and at Crescent Island might attempt to re-nest. In 2017, Caspian terns attempted to nest at four different unmanaged colony sites in the Columbia Plateau region. Three of these colony sites had been used in previous years, and one site was new in 2017. The formerly occupied colony sites included the Blalock Islands complex in John Day Reservoir (449 breeding pairs in 2017, down from 483 breeding pairs in 2016), Harper Island in Sprague Lake (92 breeding pairs in 2017, up from three breeding pairs in 2016), and Badger Island in McNary Reservoir (41 breeding pairs in 2017, down from 60 breeding pairs in 2012; site not occupied by nesting terns in 2013-2016). The incipient colony site included a new island in Lenore Lake (123 breeding pairs in 2017) located approximately 0.4 km NNE from the former Lenore Lake colony site used by terns in 2014-2016. The former Caspian tern colony sites at Twinning Island in Banks Lake (6 breeding pairs in 2016) and an unnamed island in Lenore Lake (39 breeding pairs in 2016; see above) were not active in 2017. As was the case in 2015-2016, the largest Caspian tern colony in the Columbia Plateau region was on the Blalock Islands, where 64% of all the Caspian terns in the region nested during 2017. Compared to the average size of the Caspian tern colony on the Blalock Islands prior to management (2005-2013; 59 breeding pairs), the colony was 8-11 times larger during 2015-2017.
The total estimated breeding population of Caspian terns in the Columbia Plateau region during 2017 was 705 breeding pairs at four separate colonies. This represents a 19% 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), but was a slightly higher (4%) than the regional breeding population size for Caspian terns in 2016 (675 breeding pairs). Although nest dissuasion actions implemented on Goose and Crescent islands in 2017 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 due to the more than 8-fold increase in the number of Caspian terns nesting in the Blalock Islands and increases in colony size at three other colony sites (i.e. on an unnamed island in Lenore Lake, on Harper Island in Sprague Lake, and on Badger Island in the mid-Columbia River) in 2017, compared to the pre-management average for those colonies. The Blalock Islands colony during 2015-2017 was similar in size to the largest Caspian tern colonies recorded anywhere in the Columbia Plateau region since intensive monitoring began in 2005.
The over-all goal of the IAPMP is to reduce predation rates (percentage of available fish consumed) on juvenile salmonids by Caspian terns to less than 2% of each Endangered Species Act (ESA)-listed salmonid population (hereafter ESU/DPS), per colony, per year. Recoveries of smolt PIT tags on Caspian tern colonies were used to estimate predation rates and to compare smolt losses prior to and following tern management actions. To ensure adequate numbers of ESA-listed upper Columbia River steelhead – a population that is highly susceptible to tern predation and therefore a suitable population to evaluate the efficacy of management – were available for predation rate analyses, we intentionally PIT-tagged and released (n= 7,437) steelhead smolts into the tailrace of Rock Island Dam as part of this study in 2017.
Estimated predation rates indicated that the management goal of achieving rates of less than 2% were met for most, but not all, Caspian tern colonies and ESA-listed salmonid ESUs/DPSs in 2017. Predation rates were zero or close to zero for terns nesting in Potholes Reservoir (Goose and surrounding islands) and Crescent Island due to the complete (Crescent) or nearly complete (Potholes Reservoir) abandonment of these colony sites in 2017. Predation rates at the unmanaged Lenore Lake and Badger Island colonies were also less than 2% per colony and ESU/DPS, with the highest rate observed by Lenore Lake terns on upper Columbia River steelhead at 1.0% (95% credible interval [CRI] = 0.6-2.0). For the third consecutive year, predation rates for the large unmanaged colony in the Blalock Islands exceed the 2% threshold for Snake River and Upper Columbia River steelhead at 3.4% (95% CRI = 2.4-5.1) and 4.2% (95% CRI = 2.7-6.5), respectively. Rates were below the 2% threshold, however, for all other ESA-listed ESUs/DPSs evaluated. Due to lack of access to the colony site following the nesting season, predation rate estimates were not available for terns nesting on Harper Island in Sprague Lake in 2017.
Based on a comparison to historic predation rates by Caspian terns nesting in the Columbia Plateau region during 2007-2016, impacts were amongst the lowest ever recorded at managed colony sites in 2017. This was particularly true for predation on Upper Columbia River steelhead, where average pre-management predation rates of 15.7% (95% CRI = 14.1-18.9) by Goose Island terns were reduced to < 0.1% in 2017. Rates at the unmanaged Lenore Lake and Badger Island colonies were also low (≤ 1.0% per ESU/DPS and colony) in 2017. Impacts by terns nesting at the unmanaged Blalock Island colony, however, remained above the 2% goal for steelhead DPSs in 2017. Due to continued predation by Blalock Island terns, impacts to Snake River steelhead remain as high or higher than those observed prior to management due to the relocation of terns from Crescent Island to the Blalock Islands following implementation of IAPMP in 2015. Adaptive management actions at the Blalock Islands nesting sites may be needed before the over-all goal of reducing predation rates to less 2% per colony, ESU/DPS, and year can be achieved.
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 2017, the fourth 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 19%. Based on opportunistic resightings of banded Caspian terns in previous years, most terns that were displaced from colonies on Goose and Crescent islands have remained in the region, and many have attempted to nest at unmanaged colony sites. Most notable has been 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 2015-2017 have consumed sufficient numbers of juvenile salmonids to at least partially off-set 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. Based on results during the first four years of implementation of the IAPMP, the over-all goal of the Plan to reduce predation rates to less than 2% per tern colony, ESA-listed ESU/DPS, and year will not be fully realized until alternative nesting habitat is further reduced at unmanaged colony sites, especially in the Blalock Islands, the lone tern colony where predation rates exceeded management goals in 2017.
- Bird Research Northwest
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