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FEATURE STORIES

Avian Predation in the Columbia River Basin, 2020Avian Predation in the Columbia River Basin, 2020

To address concerns about the impact of avian predation on the survival of Columbia River basin (CRB) juvenile salmonids (smolts), the U.S. Fish and Wildlife Service (USFWS), the U.S. Army Corps of Engineers (USACE), and their management partners developed and implemented three separate management plans to reduce predation rates on smolts by piscivorous waterbirds nesting at four breeding colonies in the CRB: The Caspian tern (Hydroprogne caspia; hereafter referred to as “tern” or “terns”) and double-crested cormorant (Phalacrocorax auratus; hereafter referred to as “cormorant” or “cormorants”) breeding colonies on East Sand Island in the Columbia River estuary (CRE); and the tern colonies on Crescent Island (McNary Reservoir) and on Goose Island (Potholes Reservoir) in the Columbia Plateau region (CPR). The primary goal of these management initiatives was to reduce predation rates (proportion of available smolts consumed) on Endangered Species Act (ESA)-listed anadromous salmonid (Oncorhynchus spp.) populations (referred to as Evolutionarily Significant Units [ESUs] or Distinct Population Segments [DPSs]) by reducing the size of or eliminating the breeding colonies at each of these four colony sites. Management initiatives implemented at these four colony sites have been primarily non-lethal strategies for terns (i.e. passive and active nest dissuasion) and primarily lethal (i.e. culling and egg-oiling) and non-lethal (i.e. nesting habitat management) strategies for cormorants. As part of the management plans for terns, the USACE created alternative nesting habitat for terns at various locations outside the CRB (i.e. northeastern California, southern Oregon, and San Francisco Bay) to compensate for reductions in tern nesting habitat on East Sand Island in the CRE and for elimination of tern nesting habitat on Crescent and Goose islands in the CPR.

Implementation of these management plans in 2020 was carried out by the USACE for the tern and cormorant colonies on East Sand Island and by the Bureau of Reclamation at the tern colony on Goose Island; active nest dissuasion was not conducted at the Crescent Island tern colony in 2020. The primary objectives of this study were to evaluate the efficacy of management to reduce avian predation on ESA-listed juvenile salmonids in the CRB, and to assess the magnitude of predation on smolts by piscivorous waterbirds nesting at unmanaged colonies, namely those of California gulls (Larus californicus) and ring-billed gulls (L. delawarensis; hereafter referred to 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 all piscivorous waterbird colonies within foraging range of juvenile salmonids in the middle Columbia River, lower Snake River, lower Columbia River, and CRE, including information on any new or incipient colonies; (2) estimate colony-specific predation rates on smolts by piscivorous waterbirds (with cost sharing from the USACE); and (3) measure the cumulative effects of predation by birds from multiple breeding colonies on salmonid survival, including an investigation of the additive effects of avian predation. The continued monitoring of colony sizes, locations, and predation rates on salmonids by piscivorous waterbirds nesting at both managed and un-managed colonies in the basin will help ensure that the intended benefits of management efforts are achieved and sustained, and that the accrued benefits from colony management are not offset by the emergence of new and growing colonies at other locations within the basin. In short, this research will guide managers in developing and monitoring long-term management initiatives for avian predators that are science-based, defensible, cost-effective, and have a high probability of success.

Colony Size
Columbia Plateau Region: The primary objective of the seventh year of implementation of the Inland Avian Predation Management Plan (IAPMP) was to limit the numbers of terns breeding on Goose Island and on other islands in Potholes Reservoir, and on Crescent Island in McNary Reservoir, to less than 40 breeding pairs each to reduce impacts from tern predation on ESA-listed juvenile salmonids in the CPR. To accomplish this task, the suitable tern nesting habitat at these sites was nearly eliminated by installing a variety of passive nest dissuasion materials on Goose Island and other islands in Potholes Reservoir prior to the 2020 breeding season, and by planting vegetation (planted in 2016) on Crescent Island. On both Goose and Crescent islands, passive nest dissuasion materials and/or vegetation covered all areas where terns had previously nested, as well as all areas of open, sparsely vegetated habitat that might be used by ground-nesting terns or gulls. Once terns arrived to initiate nesting in 2020, active nest dissuasion (i.e. human hazing) was used to dissuade terns from nesting on Goose Island and on other islands in Potholes Reservoir, activities described in detail in a separate report (see above). No hazing has been required to prevent terns from nesting on Crescent Island since the first year of management in 2015.

Despite the use of passive and active nest dissuasion techniques on Goose Island in each of the previous six years, some terns continued to display high fidelity to the island as a nesting site in 2020. As many as 6 breeding pairs of terns were successful in raising young (total of four fledglings) on Goose Island in 2020, the first year that terns were successful in establishing a breeding colony on Goose Island since 2015. Nesting success on Goose Island in 2020, although limited, was likely due to a decline in the frequency (i.e. number of days/week) and duration (i.e. number of hours/day), as well as changes in timing, of nest dissuasion activities (i.e. hazing) in 2020 relative to previous years. The continued fidelity of terns to Potholes Reservoir despite ongoing nest dissuasion activities is unexpected but is likely due to terns having a long nesting history at the site and the persistence of a large gull colony on the island, both before and during management, which continues to attract prospecting terns to the site. Another factor that might explain the strong fidelity of terns to the Potholes Reservoir is the paucity of alternative tern colony sites in the vicinity. As was the case in previous years during the management period, tern use of Goose Island for roosting and nesting in 2020 was largely limited to areas near the shoreline of the island that became exposed during the nesting season as reservoir levels receded. However, in 2020 terns were successful in establishing a small nesting colony on top of the island near the historic colony area, where we believe some, if not all, the fledglings were produced. Prior to management (2005–2013), an average of 367 breeding pairs of terns nested at the colony on Goose Island.

Willow and other native vegetation plantings on Crescent Island in 2016 have rendered the island unsuitable for tern nesting by eliminating all the open, bare-ground habitat that terns prefer for nesting. That, along with other passive nest dissuasion techniques used on Crescent Island (i.e. fencing, stakes, rope, flagging, and placement of woody debris), resulted in both terns and gulls abandoning the island as a nesting site during the 2015–2019 breeding seasons. In 2020, California gulls returned to Crescent Island to nest and established a breeding colony of approximately 400 nesting pairs at a site on and near the old tern colony site. Although terns were observed flying over Crescent Island during the 2020 breeding season, they did not attempt to nest at the site. The reason(s) for the return of a gull breeding colony on Crescent Island in 2020 are unknown but likely due to the removal of fencing and other passive nest dissuasion materials from the island and mortality of willows and other native vegetation on the island. Signs of intensive beaver herbivory have been observed on the island in previous years and was one of perhaps several factors affecting vegetation cover on the Crescent Island. If the gull colony on Crescent Island becomes more established in future years, we expect that the amount of open, unvegetated habitat on the island will also increase, which may ultimately lead to the reestablishment of a Caspian tern colony.

The complete abandonment of Crescent Island by nesting terns beginning in the first year of management (2015) was somewhat unexpected because 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 terns on Crescent Island was the successful dissuasion of gulls from nesting on Crescent Island during 2015–2019; terns are breeding associates of gulls and are attracted to nest at the periphery of gull colonies. At Goose Island, gull nesting could not be prevented using passive and active nest dissuasion techniques, 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 gulls established a new colony on Badger Island, located on the Columbia River just one kilometer upriver from Crescent Island, during 2015–2019. Similarly, most terns displaced from Crescent Island relocated to an unmanaged colony site in the Blalock Islands, John Day Reservoir on the Columbia River, 70 river kilometers downriver from Crescent Island, during 2015–2019. Small numbers of terns had nested in the Blalock Island during the previous decade. Prior to management (2005–2013), an average of 403 breeding pairs of terns had nested on Crescent Island.

Aerial, ground, and boat-based surveys were conducted in the CPR to determine where terns displaced from the managed colonies in Potholes Reservoir and at Crescent Island might attempt to nest. In 2020, terns attempted to nest at three different colony sites in the CPR: at one managed site (Goose Island) and at two unmanaged sites (Blalock Islands and Lenore Lake). In 2020, the tern colony at the Blalock Islands in John Day Reservoir was the largest in the CPR at 150 breeding pairs; this is down from 379 pairs that nested in the Blalock Islands in 2019 and down from the average colony size during the management period (391 pairs). After the Blalock Islands tern colony, the next largest tern colony in the CPR during 2020 was located on two islands (North Rock and Shoal Island) in Lenore Lake, at 53 breeding pairs; this is up from 48 pairs in 2019 and similar to the average colony size during the management period (52 breeding pairs). The Goose Island tern colony, at just 6 breeding pairs in 2020, was up from 0 pairs during 2016–2019. Only the tern colonies at Lenore Lake and on Goose Island were successful in rearing young to fledging in 2020, with a minimum of four fledglings produced at each colony. A total of 209 breeding pairs of terns nested in the CPR during 2020, the lowest number of terns to nest in the region since annual monitoring of Caspian tern colonies in the basin began in the late 1990’s. The cause(s) of the decline in the regional breeding population of terns in 2020 is unknown; perhaps management at formerly the two largest tern colonies in the CPR during each of the previous seven breeding seasons has resulted in most terns relocating to nesting colonies outside the region, as was expected.

As part of this work, we also estimated the colony size of other piscivorous waterbirds (see above) in the CPR during the 2020 breeding season. Because visits to these colonies were less frequent (1–2 times per month) as compared to our monitoring of tern colonies (2–4 times per month) in the region, our estimates of peak colony size at these other piscivorous waterbird colonies are less accurate (i.e. actual peak colony size could be somewhat higher or lower). There were a total of nine active gull colonies in the CPR region during the 2020 breeding season, ranging in size from about 400 breeding individuals (Crescent Island) to over 11,000 breeding individuals (Goose Island). Cormorants were confirmed nesting at five colony sites in the CPR during 2020, with colony size ranging from 1 pair (Miller Rocks) to 333 pairs (Harper Island in Sprague Lake). Finally, the size of the Badger Island pelican colony was estimated to be 3,165 breeding individuals in 2020. These data were used to identify which colony sites to scan for smolt PIT tags following the 2020 breeding season (see below).

Columbia River Estuary: In 2020, both the implementation of avian predation management initiatives and monitoring of the tern and cormorant colonies on East Sand Island and elsewhere in the estuary were conducted by the USACE and the Oregon Department of Fish and Wildlife (ODFW) and are only briefly summarized here. The USACE estimated that 2,387 breeding pairs of Caspian terns nested on the prepared 1-acre designated colony site on East Sand Island in 2020 (K. Tidwell, USACE-FFU, personal communication), which is below the target colony size for terns on East Sand Island specified in the management plan (i.e. 3,125–4,375 breeding pairs). No estimates of tern nesting success on the designated colony area at East Sand Island are available, but nesting success was apparently very low, and perhaps the colony failed to produce any young. Despite ongoing nest dissuasion efforts to prevent terns from nesting outside the designated 1-acre colony area, large numbers of terns (hundreds to thousands of adults per week) once again attempted to nest along the southeast and east beaches of East Sand Island. None of these tern nesting attempts outside the 1-acre designated colony site were successful in producing fledged young. In 2020, terns did not establish a colony on Rice Island in the upper Columbia River estuary (K. Tidwell, USACE-FFU, personal communication), a location where terns have attempted to nest since the area of suitable tern nesting habitat on East Sand Island was reduced. Cormorants did not nest on East Sand Island in 2020. Based on monitoring conducted by ODFW, the size of the double-crested cormorant colony on the Astoria-Megler Bridge was estimated to be about 5,080 breeding pairs (J. Lawonn, ODFW, personal communication), which is the largest cormorant colony size ever recorded on the bridge and the sixth consecutive year of growth in the size of that cormorant colony since management of the cormorant colony on East Sand Island was first implemented in 2015.

Predation Rates
To investigate the impacts of predation by piscivorous colonial waterbirds on survival of juvenile salmonids and to determine the efficacy of on-going management actions to reduce predation on smolts by terns in the CPR, we estimated ESU/DPS-specific predation rates based on recoveries of smolt PIT tags on bird colonies following the 2020 nesting season. Estimates were generated using previously published, standardized methods, providing a means to compare predation rates across avian predator species and colonies, salmonid species and ESUs/DPSs, and years. To help ensure that enough ESA-listed Upper Columbia River (UCR) steelhead – a population that is highly susceptible to avian predation and therefore a suitable population to evaluate the efficacy of management actions – were tagged and available for predation analyses in 2020, 6,294 steelhead smolts were captured, PIT-tagged, and released into the tailrace of Rock Island Dam (RIS) in the middle Columbia River as part of this study. Tagging at RIS commenced in 2008, resulting in a long-term dataset (2008–2020) with which to evaluate the impacts of terns and other piscivorous colonial waterbird species on the survival of ESA-listed steelhead and to evaluate relative changes in predation rates associated with management actions.

Efficacy of Avian Management Plans: A primary goal of the IAPMP was to reduce predation rates by terns on ESA-listed juvenile salmonids to less than 2% per salmonid ESU/DPS, per colony, and to less than 5% per salmonid ESU/DPS for all tern colonies in the CPR combined. Recoveries of smolt PIT tags on tern colonies in 2020 were used to compare predation rates prior to and during tern management actions associated with the IAPMP. Results indicated that tern predation rates on juvenile salmonids during 2020 were among the lowest ever recorded in the CPR, with estimates ranging from < 0.1% to 2.2% (95% credible interval = 0.7–5.4%) per colony, per ESU/DPS. For the first time since management actions were implemented in 2014, the goal of reducing predation rates to less 2% per colony, per ESU/DPS was achieved for nearly all ESUs/DPSs evaluated. Predation rates by terns nesting at the managed colony sites on Goose Island and Crescent Island were greatly reduced or eliminated in 2020. Predation rates by terns nesting at the unmanaged tern colony on Lenore Lake (North Rock) were all less than 2.0% per ESU/DPS, with the highest rate observed on UCR steelhead at 1.0% (0.6–1.5%). Predation rates by terns nesting at the unmanaged tern colony in the Blalock Islands were also less than 2% per salmonid ESU/DPS, apart from Snake River steelhead, where the estimated predation rate was slightly above the 2% threshold at 2.2% (0.7–5.4%).

Comparisons of tern predation rates on juvenile salmonids prior to and during implementation of the IAPMP indicate that there have been benefits to several salmonid ESUs/DPSs, especially UCR steelhead from management associated with Goose Island and other islands in Potholes Reservoir where average annual predation rates have been reduced from an estimated 15.7% (14.1–18.9%) prior to management (2007–2013) to 0.2% (0.1–0.5%) during the management period (2014–2020). There was also evidence that survival of UCR steelhead smolts has increased significantly in the river reach where most terns from the Goose Island and Crescent Island colonies previously foraged (RIS to McNary Dam) during implementation of the IAPMP; increases in steelhead survival rates were commensurate with reductions in tern predation rates. In 2020, tern predation rates were the lowest recorded to date, coincident with record high survival rates for UCR steelhead smolts. For example, during smolt passage from RIS to McNary Dam predation by all tern colonies combined was estimated to be 1.0% (0.6–1.5%) and smolt survival was estimated to be 82.9% (64.8–91.2%), the lowest estimated level of tern predation and the highest estimated level of smolt survival in this river reach since tagging studies at RIS commenced in 2008. Due to continued predation on juvenile salmonids by terns nesting in the Blalock Islands downstream of McNary Dam, however, impacts to some ESA-listed ESUs/DPSs in 2020, particularly Snake River steelhead, remained at or near the 2% threshold. As such, adaptive management actions at the Blalock Islands tern colony, actions that have been proposed for 2021, will likely be necessary to achieve the overall goal of the IAPMP in the future.

Estimates of predation rates on juvenile salmonids by terns nesting on East Sand Island in the Columbia River estuary in 2020 were the lowest recorded since terns began nesting on East Sand Island in 1999, with estimates ranging from 0.4% (0.2–0.9%) on Upper Columbia River spring Chinook salmon to 5.9% (4.5–8.1%) on Snake River steelhead (Evans et al. 2021). Results indicated that predation on steelhead smolts by terns nesting on East Sand Island has been reduced by 65% to 76%, depending on the DPS, reductions that meet or exceed those anticipated in the Estuary Caspian Tern Management Plan. Large numbers of terns (several hundred to several thousand) attempted to nest outside of the designated 1-acre nesting area on East Sand Island in 2020, as well as nesting attempts by terns on Rice Island in the upper Columbia River estuary. Thus, continued nest dissuasion efforts and continued monitoring at prospective tern colony sites throughout the estuary will be necessary to ensure predation rates by terns nesting in the estuary do not exceed levels stipulated in the Plan.

There was no established colony of cormorants on East Sand Island in 2020 and only a small number of smolt PIT tags (n = 38) were recovered on areas where cormorants briefly attempted to nest during the 2020 nesting season. No estimates of predation were available for the large cormorant colony on the Astoria-Megler Bridge or for the other smaller, cormorant colonies on navigational aids in the upper estuary during 2020. Based on higher per-capita (per bird) estimates of predation rates by cormorants nesting in the upper estuary in previous years, and the relatively large number of smolt PIT tags (n = 1,048) recovered from a small portion of the cormorant colony on the Astoria-Megler Bridge in 2020, cormorant predation rates on juvenile salmonids were likely substantial in 2020 but were not quantified and are thus unknown.

Other Piscivorous Colonial Waterbirds: An investigation of predation rates by piscivorous colonial waterbirds (gulls, pelicans, and cormorants) nesting at other colonies in the Columbia Basin indicated that consumption rates by gulls from colonies in the CPR were as high or higher than those measured at nearby tern colonies in 2020. Unlike terns, cormorants, and pelicans, gulls are known to consume dead or moribund fish and to kleptoparasitize (steal) fish from other piscivorous waterbirds (e.g., Caspian terns) so impacts from gull colonies may be more indicative of consumption rates, rather than predation rates. Consumption rates by gulls nesting on Badger Island in McNary Reservoir and on Miller Rocks in John Day Reservoir were among the highest of any bird colony evaluated in 2020. For example, estimates of consumption rates as high as 9.2% (2.0–18.4%) for Snake River steelhead and as high as 4.4% (1.8–9.2%) on UCR steelhead were documented for gulls nesting on Badger Island and on Miller Rocks, respectively, in 2020. Estimates of smolt consumption rates by gulls from these and other colonies in 2020, however, were generally lower than those in years past. In 2020, the cormorant colony on Foundation Island in McNary Reservoir was scanned for smolt PIT tags for the first time since 2014. Cormorant predation rates on smolts originating from the UCR were low (< 1.0% per ESU/DPS) but were significantly higher on smolts from Snake River ESUs/DPSs, with predation on Snake River steelhead and Snake River spring/summer Chinook salmon estimated at 4.0% (1.2–10.7%) and 2.5% (1.0–5.4%), respectively. Despite the high rates of consumption/predation on smolts observed at some gull and cormorant colonies in 2020, predation impacts by waterbirds nesting at several other colonies, particularly those located away from the Columbia River, were low to undetectable. For example, estimates of predation rates by cormorants nesting on Lenore Lake and at a small island in Hanford Reach were < 0.2% per salmonid ESU/DPS, as were estimates of consumption rates by gulls nesting at the large colony on Goose Island in Potholes Reservoir at < 0.1% per ESUs/DPSs. These results indicate that not all colonies of piscivorous waterbirds in the CPR posed a potential risk to smolt survival in 2020. Finally, accurate estimates of predation rates, those corrected for both on-colony PIT tag detection probabilities and deposition probabilities, were calculated for the first time for American white pelicans nesting on Badger Island in McNary Reservoir in 2020. Results indicated that smolt predation rates were < 0.5% per salmonid ESU/DPS; the highest estimated predation rate was on Snake River steelhead at 0.4% (0.1–4.2%). Significantly higher predation rates on other, non-ESA-listed juvenile salmonid stocks (e.g., subyearling Chinook salmon from the Upper River Bright stock) by Badger Island pelicans have been documented in other studies, and pelicans also consume some adult salmonids. Nevertheless, results of this study suggest that pelicans nesting at the colony on Badger Island posed only a small risk to actively migrating, ESA-listed UCR and Snake River juvenile salmonids in 2020.

Cumulative Predation and Smolt Survival: To investigate the cumulative effects of avian predation (predation by all avian predator species and colonies combined) and to determine what proportion of all sources of smolt mortality (1-survival) were due to predation by piscivorous colonial waterbirds, we conducted a mark-recapture-recovery analysis on UCR steelhead smolts that were PIT-tagged and released at RIS in 2020. We used previously published methods to jointly estimate predation and survival probabilities during smolt passage through multiple river reaches and we compared results from 2020 to those from previous years (2008–2019). Results from 2020 indicated that avian predation was a substantial source of mortality for UCR steelhead during out-migration from RIS to Bonneville Dam, with bird predation accounting for 56.1% (51.7–60.4%) of all mortality. In most previous years, the cumulative effects of avian predation and the proportion of total mortality that was due to predation were similar to or greater than those observed in 2020, with avian predation accounting for more than 50% of smolt mortality from all sources in 10 of the previous 12 years (2008–2019). Even after passage through the impounded sections of the middle and lower Columbia River upstream of Bonneville Dam, the impact of predation by piscivorous colonial waterbirds on UCR steelhead smolts in the CRE were substantial, with terns and cormorants breeding on East Sand Island annually consuming an average of 11.5% (10.3–12.9%) and 7.1% (5.9–8.6%), respectively, of available steelhead during 2008–2019. In 2020, cumulative predation/consumption rates (predation by all colonies combined) on UCR steelhead during smolt passage from RIS to the Pacific Ocean were estimated at 18.9% (15.0–22.8%), with the highest levels of predation/consumption by gulls at 14.0% (10.3–17.9%), followed by terns at 4.6% (3.5–5.9%), pelicans at 0.3% (0.1–1.0%), and cormorants at 0.1% (< 0.1–0.2%). These estimates exclude several colonies in the CRE (e.g., the large cormorant colony on the Astoria-Megler Bridge), however, and therefore underestimate total smolt losses due to predation by piscivorous colonial waterbirds in the estuary during 2020.

Additive Effects of Predation: To investigate to what degree avian predation on UCR steelhead smolts limited fish survival, we used a mark-recapture-recovery model to jointly estimate weekly and annual predation and survival probabilities among time-stratified cohorts to explicitly measure the strength, magnitude, and direction of the relationship between predation and survival. Data from 2019 and 2020 were used to update a previously published long-term dataset (2008–2018) and to evaluate more recent trends in predation and survival. Results indicated that the record low levels of tern predation on UCR steelhead in 2020 were associated with record high levels of survival during smolt outmigration from RIS to Bonneville Dam. An investigation of weekly and annual estimates of predation and survival probabilities suggested that a greater proportion of UCR steelhead smolts would have survived outmigration to Bonneville Dam in the absence of tern predation upstream of Bonneville Dam, with the estimated average annual difference in observed survival versus baseline survival (i.e. survival in the absence of tern predation) of 0.170 (0.097–0.227) during 2008–2020. Due to low levels of predation by cormorants upstream of Bonneville Dam (0.01 or 1%), differences in observed survival versus baseline survival were less than 0.01 (-0.005–0.008), indicating that only small increases in survival of UCR steelhead smolts to Bonneville Dam would be possible in the absence of cormorant predation upstream of Bonneville Dam. Although there was some evidence of a relationship between consumption probabilities by gulls and survival probabilities of UCR steelhead smolts, results were not statistically significant when considered across all years. The statistical power to accurately determine to what degree gull consumption influenced smolt survival was limited by a truncated time series, coupled with high levels of uncertainty in consumption and survival, and a lack of weekly variation in estimates of consumption probabilities. Since gulls are known to consume dead fish and to kleptoparasitize (steal) fish from other piscivorous waterbirds, it is likely that consumption of smolts by gulls was a more compensatory source of mortality compared to predation on smolts by terns and cormorants.

There was evidence that higher levels of tern predation on UCR steelhead smolts in the CRE were associated with lower returns of adults to Bonneville Dam, with increases in tern predation probabilities associated with statistically significant decreases in adult survival probabilities. These results suggest that in the absence of tern predation on UCR steelhead smolts, SARs for UCR steelhead would have nearly doubled, despite the fact that most smolts depredated by terns would have died from other causes before returning to Bonneville Dam. Results provide evidence that tern predation on UCR steelhead smolts was a partially additive source of mortality to the adult life-stage. There was some evidence that higher levels of cormorant predation on UCR steelhead smolts in the estuary were associated with lower adult returns to Bonneville Dam; however, results were not statistically significant when considered across all years. Furthermore, more recent data were not available for inclusion in this analysis due to the abandonment of the East Sand Island cormorant colony and the relocation of many of the cormorants that formerly nested at the large colony on East Sand Island to the Astoria-Megler Bridge. Impacts on smolt survival from predation by cormorants nesting on the Astoria-Megler Bridge are currently unknown but are likely greater on a per capita (per bird) basis. Collectively, these results suggest that efforts to reduce tern predation on UCR steelhead are increasing steelhead smolt survival in the CRB, particularly in years when predation rates are dramatically reduced as a result of management actions at Goose and Crescent islands, such as in 2020. More importantly from a conservation perspective, results suggest that in the absence of tern predation on UCR steelhead, significantly more adult steelhead would return to Bonneville Dam. Additional research is needed, however, to evaluate to what degree cormorant predation on, and gull consumption of, juvenile salmonids limits smolt survival and SARs in the CRB.

Biotic and Abiotic Factors: Previous research indicates that certain biotic and abiotic factors influence the susceptibility of juvenile salmonids to predation by colonial waterbirds during outmigration, as well as the probability of survival through the hydrosystem. As part of this study, and as recently recommended by the Independent Scientific Advisory Board (ISAB), we are investigating the influence of various covariates on the predation and survival probabilities of ESA-listed steelhead during outmigration. Covariates being investigated include biotic factors like fish size, rearing-type (hatchery, wild), abundance (density), run-timing, and environmental factors like spill, water transit time, and smolt arrival time in the estuary. The goal of this analysis is to describe those factors that best explain variation in survival and susceptibility to avian predation, and to identify potential “management relevant” variables, variables that resource managers may be able to control to some degree. Analyses of covariates are on-going, and results will be provided in our 2021 Annual Report to the Bonneville Power Administration and the Grant County PUD/PRCC.

Smolt Survival to Bonneville Dam
Due to restrictions imposed on travel/field work associated with the COVID-19 outbreak, several survival studies involving PIT-tagged smolts in the CRB were not conducted in 2020. One of those studies involved the operation of the National Marine Fisheries Service’s pair trawl net detection system in the lower Columbia River downstream of Bonneville Dam. To mitigate for the loss of pair trawl PIT tag detections in 2020, we scanned for smolt PIT tags at several piscivorous waterbird nesting, roosting, and loafing sites in the CRE with the goal of increasing sample sizes of PIT-tagged fish known to have survived outmigration to Bonneville Dam for use in survival models. This work was conducted in collaboration with the USACE. A total of 6,239 smolt PIT tags were detected at avian nesting, roosting, and loafing sites in the CRE, including 2,530 PIT tags that were recovered at sites that were beyond the original scope of predation studies funded by BPA, Grant PUD/PRCC, and USACE in 2020. Survival analyses indicated that without detections of tags on avian colonies in the estuary, accurate and precise estimates of reach-specific and a cumulative survival estimate for UCR steelhead released at RIS would not have been possible in 2020. For example, without PIT tag detections from bird colonies in the CRE, 95% credible bounds associated with estimates of UCR steelhead survival to Bonneville Dam were uninformative, ranging from 0.28 to 1.0 (i.e. 28 to 100%). Once detections from bird colonies in the CRE were added, credible bounds were 0.44 to 0.74 (i.e. 44 to 74%). This greater precision provided convincing evidence that a majority of UCR steelhead that were alive at RIS survived outmigration through the hydrosystem in 2020. Results of this and other studies indicate that recoveries of PIT tags on bird colonies in the CRE can be used to augment mark-recapture survival datasets to generate more accurate and precise estimates of smolt survival to Bonneville Dam.

-Bird Research Northwest

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