Tern Management on the Columbia Plateau, 2018
In 2018, the U.S. Army Corps of Engineers - Walla Walla District and the U.S. Bureau of Reclamation (BOR) completed 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 salmonid (Oncorhynchus spp.) populations from the Columbia River basin (USACE 2014). The primary objective of management in the fifth year of implementation of the plan was to limit the numbers of Caspian terns breeding at Goose Island and surrounding islands in Potholes Reservoir and on Crescent Island in McNary Reservoir to less than 40 breeding pairs each to reduce predation impacts of terns on ESA-listed juvenile salmonids in the Columbia Plateau region. 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 2018 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, 4.1 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 2018; additionally, all open areas on Crescent Island had been planted with willows and other native vegetation prior to the 2016 nesting season. Finally, an island in northeastern Potholes Reservoir that was used by Caspian terns for nesting in 2016 (0.15 acre) and one additional nearby island where terns were observed prospecting in 2018 (0.10 acre) were covered in passive dissuasion to prevent terns from nesting at those sites. Once Caspian terns arrived to initiate nesting, active nest dissuasion (i.e. human hazing) was used to try to dissuade terns from nesting on Goose Island and other islands in Potholes Reservoir. No hazing has been required to prevent Caspian terns from nesting on Crescent Island since the onset of management in 2015.
As was the case in 2015-2017, 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 2018 nesting season. 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; 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 during 2015-2018; 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 some, if not most, of these birds likely established a new colony on Badger Island located on the Columbia River just one kilometer upriver from Crescent Island in 2015-2018. 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 2015-2018 and Badger Island in 2017-2018, 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 in 2018, some Caspian terns continued to display high fidelity to Potholes Reservoir as a nesting area in 2018, the fifth 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 2015-2017, Caspian tern use of Goose Island for roosting and nesting in 2018 was largely limited to areas near the island’s shoreline that became exposed during the nesting season as reservoir levels receded. Despite high fidelity of terns to the area, active nest dissuasion (hazing) and collection (under permit) of any Caspian tern eggs discovered were factors in preventing the formation of a Caspian tern colony in Potholes Reservoir in 2018. This is the third 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 a small rocky islet (i.e., Northwest Rocks) immediately adjacent to 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 2018, egg laying by Caspian terns on Goose Island and elsewhere in Potholes Reservoir occurred between 30 April and 16 July. During this period, a total of 11 Caspian tern eggs were discovered at two different locations in Potholes Reservoir; 10 tern eggs were discovered on Goose Island and one tern egg was discovered on a previously unused island in northern Potholes Reservoir. All 11 tern eggs discovered were collected under permit. By comparison, a total of 20 Caspian tern eggs were found on Goose Island and other islands in northeastern Potholes Reservoir in 2017.
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 2018, Caspian terns attempted to nest at four extant colony sites in the Columbia Plateau region that are currently unmanaged. All four of these sites have been used for breeding by Caspian terns previously, including the Blalock Islands complex in John Day Reservoir (313 breeding pairs in 2018; up from the pre-management average [59 breeding pairs] and down from the average during the management period [393 breeding pairs]), Badger Island in McNary Reservoir (8 breeding pairs in 2018; down from the pre-management average [10 breeding pairs] and down from the average during the management period [10 breeding pairs]), Harper Island in Sprague Lake (79 breeding pairs in 2018; up from the pre-management average [8 breeding pairs] and up from the average during the management period [38 breeding pairs]), and an unnamed island in Lenore Lake (91 breeding pairs in 2018; up from the pre-management average [0 breeding pairs] and up from the average during the management period [54 breeding pairs]). The former Caspian tern colony site at Twinning Island in Banks Lake was not active in 2017-2018, with the average colony size during the pre-management and management periods both totaling 27 breeding pairs. As was the case in 2015-2017, 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 2018. 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 2015-2018.
The total estimated breeding population of Caspian terns in the Columbia Plateau region during 2018 was 491 breeding pairs at four separate colonies. This represents a 44% decline in the regional breeding population size for Caspian terns compared pre-management average (874 breeding pairs), and a 27% decline when compared to the average during the management period (679 breeding pairs). Although nest dissuasion actions implemented on Goose and Crescent islands in 2018 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 continued use and increase in the colony size at unmanaged sites (i.e., Blalock Islands, Lenore Lake, and Harper island) when compared to pre-management averages. While smaller in 2018, the average Blalock Islands colony size during 2015-2018 (480 breeding pairs) was similar in size to the largest Caspian tern colonies recorded anywhere in the Columbia Plateau region since intensive monitoring began in 2005.
The overall goal of the IAPMP is to reduce predation on juvenile salmonids in the Columbia Plateau region at Goose and Crescent Islands while implementing adaptive management actions to limit the formation of incipient colonies within the basin, where feasible. The target metric is for a predation rate of less than 2% on any ESA-listed salmonid stock (hereafter ESA/DPS), per colony, per year. Recoveries of smolt PIT tags on Caspian tern colonies in 2018 were used to estimate predation rates and to compare smolt losses prior to and following tern management actions associated with the IAPMP. 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 actions – were available for predation rate analyses, we intentionally PIT-tagged and released (n = 7,366) steelhead smolts into the tailrace of Rock Island Dam as part of this study in 2018.
Predation rates indicated that the 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 2018. 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 Island) or nearly complete (Potholes Reservoir) abandonment of these colony sites in 2018. Predation rates per ESU/DPS at unmanaged Caspian tern colonies varies due to colony location as it relates to the availability of juvenile salmonids and alternative prey sources. Predation rates at the unmanaged Lenore Lake tern colony were also less than 2% per ESU/DPS, with the highest rate observed on Upper Columbia River steelhead at 0.8% (95% credible interval [CRI] = 0.4–1.7). Predation rates for the large unmanaged tern colony in the Blalock Islands, however, exceed the 2% threshold for three ESA-listed ESUs/DPSs in 2018; (1) Upper Columbia River steelhead at 2.9% (95% CRI = 1.5–5.2), (2) Snake River steelhead at 2.5% (95% CRI = 1.4–4.5), and (3) Snake River sockeye at 2.0% (95% CRI = 0.4–6.1). Due to a lack of access to the colony site following the nesting season, predation rate estimates were not available for Caspian terns nesting on Harper Island in Sprague Lake in 2018. Based on limited data from years past, Caspian terns nesting on Harper Island forage on juvenile salmonids in lower Snake River but impacts by the colony in 2018 were presumably less than 2% per ESA-listed salmonid population based on the relatively small number of terns (79 breeding pairs) that nested on Harper Island in 2018. Predation rate estimates at the Badger Island tern colony were also not available in 2018, but impacts were presumably close to zero given the small number (8 pairs) and brief (approximately one week) existence of a colony on Badger Island in 2018.
Based on a comparison to historic predation rates by Caspian terns nesting in the Columbia Plateau region during 2007–2017, impacts were amongst the lowest ever recorded in 2018. 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 in Potholes Reservoir were reduced to < 0.1% in 2018. Adaptive management at incipient colony sites in northern Potholes Reservoir also reduced or eliminated predation on Upper Columbia River steelhead from 4.1% (95% CRI = 2.9–6.3) in 2016 to < 0.1% in 2018. Historic predation rates at the unmanaged Lenore Lake tern colony were also low (≤ 1.0% per ESU/DPS), suggesting that at its current size (16 to 91 nesting pairs, per year), the colony poses only a minor threat to Upper Columbia River steelhead survival. Impacts by terns nesting at the unmanaged Blalock Island colony in 2018, however, remained above the 2% minimum goal or threshold for numerous ESUs/DPSs, as was the case during 2015–2017. Due to continued high rates of predation by Blalock Island terns, impacts to some ESA-listed ESUs/DPSs, particularly those originating from Snake River, remain as high or higher than those observed prior to implementation of management actions as part of the IAPMP.
In summary, management aimed at eliminating 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, were successful in 2018. 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 44% in 2018, the most appreciable decline since the onset of management in 2014. However, based on opportunistic resightings of banded Caspian terns in previous years, many terns that were displaced from colonies on Goose and Crescent islands have remained in the region, and 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 2015-2018 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 at the Blalock Islands remains heavily dependent on water levels, with tern nesting habitat typically only being available when reservoir levels are below full reservoir levels. Changes in water levels due to weather related events (e.g., high spring flows and/or high wind events) have occurred that limit colony size and productivity at this site. Based on results collected during this five-year study (2014-2018), the IAPMP objective of preventing Caspian terns from nesting on Goose and Crescent islands, thereby reducing predation rates by terns nesting at these two sites on ESA-listed salmonid stocks to less than 2%, has been achieved. The adaptive management objective of the IAPMP, to limit predation on ESA-listed salmonid stocks at other colonies in the Columbia Plateau region to less than 2%, will not be realized until the size of the Caspian tern colony at the Blalock Islands is reduced from its current size and there are no further substantive increases in the colony size at other tern colonies in the Columbia Plateau region.
-Bird Research Northwest
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