Avian Predation in the Columbia River Basin, 2022
Currently, there are three different management plans underway to reduce the impacts of predation by piscivorous colonial waterbirds on the survival of Endangered Species Act (ESA)- listed juvenile salmonids (smolts; Oncorhynchus spp.) in the Columbia River Basin (CRB). Two separate management plans target the breeding colonies of Caspian terns (Hydroprogne caspia; hereafter referred to as “tern” or “terns”) and double-crested cormorants (Nannopterum auritum; hereafter referred to as “cormorant” or “cormorants”) on East Sand Island (ESI) in the Columbia River estuary (CRE) and one plan targets the tern colonies on Crescent Island (McNary Reservoir) and Goose Island (Potholes Reservoir) in the Columbia Plateau region (CPR). As part of the management plans for terns both in the CRE and CPR, adaptive management actions have been conducted at various other colony locations where terns displaced from the managed colonies have relocated to nest. The primary objectives of this study were to evaluate the efficacy of management actions to reduce predation by terns and cormorants on juvenile salmonids in the CRB, and to assess the magnitude of predation on smolts by other piscivorous colonial waterbirds, namely California gulls (Larus californicus) and ring-billed gulls (L. delawarensis; hereafter referred to collectively as “gull” or “gulls”) and American white pelicans (Pelecanus erythrorhynchos; hereafter referred to as “pelican” or “pelicans”). Specifically, we sought to (1) locate and estimate peak colony size for piscivorous colonial waterbird species (terns, cormorants, gulls, and pelicans) at colonies within foraging range of juvenile salmonids out-migrating in the middle Columbia River, lower Snake River, lower Columbia River, and CRE; (2) estimate colony-specific predation rates on smolts by piscivorous colonial waterbirds; and (3) estimate the cumulative effects of predation by piscivorous waterbirds from multiple breeding colonies, including an investigation into the additive effects of avian predation on salmonid mortality.
There was a total of 34 active piscivorous waterbird colonies detected in the CRB in 2022. Of those, cormorant and gull colonies were the most prevalent (13 and 11 colonies, respectively), followed by tern colonies (8), and pelican colonies (2). Most of these colonies were in the CPR (24 colonies), with 8 and 2 colonies being in the CRE and the lower Columbia River, respectively. Tern and cormorant colonies continue to be managed as part of ongoing management plans and management has resulted in a shift in the nesting distribution of these birds, prompting adaptive management at several previously unmanaged colony sites. As has been the case in the past, gulls were the most numerous (ca. 48,000 individuals) of all the piscivorous colonial waterbirds in the CRB, followed by cormorants (ca. 6,200 breeding pairs), pelicans (ca. 4,500 individuals), and terns (ca. 2,500 breeding pairs). Recent data suggest that the numbers of terns and cormorants nesting in the CRB have declined, the objective of management, but in the case of both terns and cormorants has apparently resulted in significant declines in the Pacific Flyway breeding populations, raising concerns about their conservation status, especially for the rapidly declining Pacific Flyway population of Caspian terns.
In the CRE, efforts to prevent large numbers of terns from nesting outside the designated 1-acre colony area on ESI were successful. However, the designated tern colony on ESI once again failed to produce any young, with complete nesting failure being observed at that colony in 5 out of the last 7 years. The reason(s) for the nesting failure at the ESI tern colony in 2022 is unknown but is likely due to gull predation on tern eggs, especially during disturbances of the colony by bald eagles (Haliaeetus leucocephalus). During the period when terns mostly abandoned the ESI colony in late May, terns were counted in the thousands on Rice Island in the upper CRE. Adaptive management conducted to prevent nesting by terns on Rice Island were eventually successful in dispersing the terns from that site, but only after 2.5 weeks coinciding with the peak of the steelhead outmigration period when thousands of terns were present on Rice Island.
As was the case in the CRE, management implemented at the tern colonies in the CPR (Goose and Crescent islands) has resulted in a shift in the nesting distribution of terns in the region. Efforts to dissuade terns from nesting on Goose Island and elsewhere in Potholes Reservoir were largely successful; only 16 breeding pairs were observed on Goose Island in 2022. Management to dissuade terns from nesting on Crescent Island in McNary Reservoir ceased in 2021, however, and terns re-established a nesting colony of 149 breeding pairs on the island in 2022. Previously, most of the terns that were dissuaded from nesting on Crescent Island starting in 2015 had relocated to nest on the Blalock Islands in John Day Reservoir. Beginning in 2021, adaptive management to raise the elevation of John Day Reservoir was implemented to inundate all tern nesting habitat in the Blalock Islands and in 2021-2022 no nesting by terns was observed in the Blalock Islands. This adaptive management apparently resulted in a shift of terns back to the former colony site on Crescent Island, where tern dissuasion had ceased, and to Badger Island, located on the Columbia River just upstream of Crescent Island. Despite adaptive management implemented in 2022 to reduce tern nesting habitat on Badger Island (via placement of large woody debris on one of two nesting areas used by terns in 2021), the largest tern colony in the CPR during 2022 was on Badger Island (267 breeding pairs on a site without woody debris located on the northeast shoreline). A total of 511 breeding pairs of terns nested in the CPR in 2022, the highest regional population of terns observed since 2017. These trends indicate that monitoring and adaptive management will be necessary in the future if the goals and objectives of the management plans for terns in the CRE and the CPR are to be realized.
Ongoing management at the ESI cormorant colony has also resulted in the dispersal of birds away from ESI, with most nesting cormorants now located further upstream in the CRE on the Astoria-Megler Bridge. This year marks the fourth year in a row when there has been little to no cormorant nesting on ESI, with any nesting attempts on ESI failing early in incubation. Meanwhile, cormorant colony size on the Astoria-Megler Bridge increased exponentially during implementation of the cormorant management plan during 2015-2019, culminating in over 5,000 breeding pairs nesting on the bridge in 2020. This is a concern for fisheries managers because piscivorous waterbirds nesting in the freshwater or mixing zones of the CRE can consume a far greater number of juvenile salmonids compared to conspecifics nesting at sites in the marine zone of the CRE, such as ESI (see below).
To investigate the effects of predation by piscivorous colonial waterbirds on the survival of smolts, and to determine the efficacy of ongoing management actions to reduce avian predation, we estimated population (Evolutionarily Significant Unit [ESU] or Distinct Population Segment [DPS])-specific predation rates based on recoveries of smolt PIT tags on bird colonies following the 2022 nesting season. One of the primary objectives of the Inland Avian Predation Management Plan (IAPMP) was to reduce predation rates by terns in the CPR to less than 2% per salmonid ESU/DPS, per colony, per year. In 2022, predation rates by terns nesting on Shoal Island in Lenore Lake, WA, exceeded this threshold for Upper Columbia River (UCR) steelhead, with an estimated 2.1% (95% credible interval = 1.4–3.4%) of available smolts depredated by terns. Also, predation rates by terns nesting on Badger Island in McNary Reservoir exceeded this threshold for Snake River (SR) steelhead with an estimated 2.8% (2.0–4.4%) of available smolts depredated by terns. Predation rates on all other ESA-listed ESUs/DPSs were less than 2% per tern colony, meeting the objective of the IAPMP for those salmonid populations. Although predation rates by terns nesting on Crescent Island in 2022 were < 2% per salmonid ESU/DPS, predation rates were substantially higher compared to 2021, commensurate with the increase in size of the Crescent Island tern colony from just 1 pair in 2021 to 149 pairs in 2022; the highest estimated predation rate by Crescent Island terns was on SR steelhead at 1.3% (0.9–2.1%). Due to adaptive management actions that resulted in higher reservoir elevations in John Day Reservoir, no terns nested on the Blalock Islands in 2021-2022, a nesting site where predation rates exceeded the 2% threshold for several ESUs/DPSs in previous years (2015-2020).
Average annual predation rates on UCR steelhead by terns nesting on Goose Island and elsewhere in Potholes Reservoir have been reduced from an estimated 15.7% (14.1–18.9%) prior to management (2007–2013) to 1.3% (0.8–2.3%) during the management period (2014-2022). There was also evidence that survival of UCR steelhead smolts has increased significantly in the river reach where terns from Goose Island and Crescent Island forage following implementation of the IAPMP. Average annual predation rates on SR steelhead and other SR ESUs/DPSs, however, have remained largely unchanged since management actions at Crescent Island commenced in 2015. This is due to terns relocating from Crescent Island in McNary Reservoir downstream to the Blalock Islands in John Day Reservoir during 2015-2020 and then from the Blalock Islands back upstream to Crescent Island and Badger Island during 2021-2022. Due to increases in predation on smolts by terns nesting on Crescent Island, Badger Island, and Lenore Lake in 2022, adaptive management actions will likely be necessary in the future to achieve the goals and objectives of the IAPMP for reducing tern predation rates.
Comparable estimates of predation rates on ESA-listed salmonid ESUs/DPSs by terns nesting on ESI in the CRE during the 2021-2022 nesting seasons were not available, estimates that were generated with support from the U.S. Army Corps of Engineers in previous years. In 2020, estimates of predation rates by terns nesting on ESI were the lowest recorded since 1999, with estimates ranging from 0.4% (0.2–0.9%) for UCR spring Chinook salmon to 5.9% (4.5–8.1%) for SR steelhead. Results from 2020 indicated that predation by terns nesting on ESI had been reduced by 65% to 76% for steelhead DPSs, reductions that met or exceeded those anticipated in the Caspian Tern Management Plan for the Columbia River Estuary. In 2022, terns attempted to nest on Rice Island in the upper CRE, in addition to nesting at the designated colony site on ESI. Estimated predation rates by terns that attempted to nest on Rice Island ranged from 0.1% (<0.1–0.5%) on SR sockeye salmon to 2.9% (1.6–5.1%) on Middle Columbia River steelhead. Our results indicate that continued implementation of nest dissuasion activities, continued monitoring of tern nesting sites throughout the CRE, and empirically derived estimates of ESU/DPS-specific predation rates will be necessary to ensure that predation impacts in the future do not exceed the levels stipulated in the estuary tern management plan.
For the third consecutive year there was no evidence of persistent nesting by double-crested cormorants on ESI in 2022. An unintended consequence of management actions at the cormorant colony on ESI was the abandonment of that colony and the subsequent rapid expansion of the cormorant colony on the Astoria-Megler Bridge (AMB). The AMB is located upstream of ESI in the freshwater mixing zone of the CRE, an aquatic environment where fewer alternative prey fish are available and juvenile salmonids are consumed in greater proportion on a per capita basis relative to cormorants nesting on ESI in the marine zone of the CRE. Estimates of per capita (per breeding pair) predation rates on salmonid smolts by cormorants nesting on the AMB in 2022 were 2–4 times greater (depending on salmonid ESU/DPS) than average annual per capita predation rate by cormorants that nested on ESI prior to implementation of management actions in 2015. Estimates of colony-wide (all breeding pairs) predation rates by cormorants nesting on the AMB in 2022 ranged from 3.1% (2.1–7.9%) for SR fall Chinook salmon to 8.6% (3.2–15.1%) for UCR steelhead. Colony-wide estimates of predation rates by cormorants nesting on the AMB in 2022 were similar to, or greater than, those of cormorants that formerly nesting on ESI, despite the much smaller size of the cormorant colony on the AMB (4,054 pairs in 2022) compared with the cormorant colony on ESI (12,787 pairs on average during 2003–2014).
For the first time since the colony was discovered in 2012, predation rates were estimated for cormorants nesting at a colony on transmission towers near the town of Troutdale, OR in 2022. The colony has rapidly increased in size, from 26 breeding pairs in 2013 to 353 breeding pairs in 2022. The Troutdale transmission tower (TRT) colony is located in the freshwater zone of the lower Columbia River, 44 river kilometers (Rkm) downstream of Bonneville Dam. Per capita predation rates on salmonid smolts by TRT cormorants were significantly higher compared to those of cormorants nesting on ESI (10–20 times higher, depending on the ESU/DPS) and compared to those of cormorants nesting on the AMB (4–8 times higher, depending on ESU/DPS); colony-wide predation rates for cormorants nesting at the TRT colony ranged from 0.7% (0.1–2.0%) for SR fall Chinook to 4.4% (1.2–9.5%) for SR sockeye in 2022. Collectively, our results indicate that colony location is closely related to the magnitude of cormorant predation on salmonid smolts and its effects on smolt survival in the CRE and lower Columbia River. Due to the movement of cormorants from the marine-zone to the mixing- and freshwater-zones of the CRE and lower Columbia River, the impact of cormorant predation on smolt survival has remained unchanged or has increased since cormorant management actions on ESI were first implemented in 2015.
An investigation of predation on salmonid smolts by piscivorous colonial waterbirds nesting at other unmanaged colonies has indicated that smolt consumption rates by gulls and smolt predation rates by cormorants nesting at certain colonies in the CPR were higher than those of nearby managed tern colonies in 2022. Unlike terns and cormorants, gulls are scavengers and are known to consume dead or moribund fish and to steal (kleptoparasitize) fish from other waterbirds, like terns. Consequently, we use the term “gull consumption rates” rather than “gull predation rates.” Estimates of consumption rates as high as 5.7% (2.7–11.8%) for SR steelhead and 6.6% (3.3–11.4%) for UCR steelhead were documented for gulls nesting at Badger Island in McNary Reservoir and Miller Rocks in The Dalles Reservoir, respectively. Estimates of cormorant predation rates as high as 3.7% (2.0–7.0%) for SR spring/summer Chinook were documented at the Foundation Island colony in McNary Reservoir. Estimates of consumption/predation rates by gulls and cormorants nesting at colonies in the CPR during 2022 were generally lower than those observed in 2021, but similar to those in several other years dating back to 2008. Despite the relatively high predation/consumption rates on smolts observed at some gull and cormorant colonies in the CPR during 2022, impacts from piscivorous waterbirds nesting at several other colonies in the CPR were low to non-existent. For example, estimates of predation/consumption rates by cormorants nesting on Shoal Island in Lenore Lake, by gulls nesting on Goose Island in Potholes Reservoir, by American white pelicans nesting on Miller Sands Spit in the CRE, and by Brandt’s cormorants (Phalacrocorax penicillatus) nesting on the AMB in the CRE were often < 0.2% per colony, per salmonid ESU/DPS in 2022. These were also all relatively large colonies (hundreds to thousands of breeding adults) and results indicate that not all piscivorous waterbird colonies in the CRB pose a threat to smolt survival.
An investigation of cumulative predation/consumption rates for SR and UCR salmonid smolts by piscivorous waterbirds from all colonies combined indicated that avian predation/consumption rates represented a substantial proportion of available steelhead smolts; an estimated 25.3% (21.9–30.2%) of SR steelhead and 29.4% (24.1–36.2%) of UCR steelhead were consumed by piscivorous colonial waterbirds in 2022. Cumulative predation/consumption rates were significantly lower for salmon ESUs, with an estimated 8.2% (6.2–10.6%) of SR sub-yearling Chinook, 11.1% (9.1–14.3%) of SR yearling Chinook, and 13.4% (10.5–18.8%) of SR sockeye consumed by birds in 2022. By river reach, predation/consumption rates were highest for smolt between Bonneville Dam and the Pacific Ocean, followed by Rock Island Dam (for UCR smolts) or Lower Monumental Dam (for SR smolts) to McNary Dam for most of the salmonid ESUs/DPSs evaluated in 2022. Reach-specific predation/consumption rates in 2022 were similar to those of previous years, although estimates in previous years often did not include predation/consumption rates for all active piscivorous waterbird colonies, resulting in minimum estimates of the cumulative level of predation/consumption in some years. Collectively, results indicate that the cumulative effects of avian predation/consumption remain a substantial source of smolt mortality, particularly steelhead mortality, during outmigration to the Pacific Ocean.
Previously published research has demonstrated that predation by terns was largely an additive source of mortality for steelhead smolts, with significantly more steelhead estimated to survive outmigration in the absence of tern predation. Results of the relationship between gull consumption and smolt survival in these same studies, however, were inclusive as of 2018. In 2022, we updated the time-series available for these analyses to include data from 2019-2022 and we investigated the relationship at a smaller spatial-scale to evaluate to what degree consumption by gulls was associated with lower steelhead survival during smolt outmigration from McNary Dam to Bonneville Dam. Our results indicated that higher levels of gull consumption of both UCR and SR steelhead smolts were, on average, associated with lower levels of steelhead survival. The relationship was statistically significant for both steelhead DPSs when data from all years were considered. The statistical power to accurately determine to what degree smolt consumption by gulls influenced smolt survival was, however, limited by uncertainty in estimates of gull consumption rates and a lack of weekly variation in estimates of gull consumption rates in some years. As a result, estimates of the relationship between gull consumption and steelhead survival, although statistically significant, were imprecise. With these caveats in mind, our results provide evidence that some proportion of the steelhead smolts consumed by gulls that forage between McNary and Bonneville dams would have survived outmigration to Bonneville Dam in the absence of gull consumption. Because gulls will readily consume stunned/disoriented, dead, and moribund fish, however, additional research to understand to what degree gull consumption of smolts in the tailrace of dams and elsewhere represents additive mortality, and therefore limits smolt survival, is warranted.
Lastly, as part of an ongoing analysis, we investigated the effects of biotic and abiotic factors or covariates on steelhead smolt predation by terns and smolt survival from Rock Island Dam to McNary Dam during 2008-2021. Analyses represent a novel approach to assess a covariate-parameterized capture-recapture-recovery model and to assess covariates in the additive-mortality framework. We used the additive-mortality framework to evaluate possible covariate associations with respect to not only measures of tern mortality, but also their potential association with “baseline” levels of morality (i.e. mortality in the absence of tern predation). Analyses also uniquely investigated data on a daily basis, necessitating an estimation of smolt passage times (days until exposure) to better characterize the environmental conditions experienced by smolts during outmigration. Results indicated that hatchery smolts were more likely to be consumed by terns and were less likely to survive outmigration to McNary Dam. Larger-sized smolts were also more likely to be consumed by terns than smaller-sized smolts. Other, non-tern sources of mortality were also associated with steelhead smolt survival to McNary Dam, including a fish’s rear-type, size, and fish condition (injured or diseased). No statistically significant relationship, however, was identified amongst the abiotic factors investigated, including river discharge, % spill, elevation, and temperature. A lack of an identifiable relationship with abiotic factors could be due to uncertainty associated with estimated smolt exposure times or due to other, unidentified factors that influenced smolt survival, but that were not considered as part of the analysis. As part of future covariate studies we will build on the modelling effort developed herein to explore associations between smolt migration speed, survival, and predation and to better investigate the degree to which a fish’s passage route at a dam and across multiple dams, influence downstream recapture, predation, and survival probabilities.
-Bird Research Northwest
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