Caspian Terns Hydroprogne Caspia Research Articles

Caspian Tern Management to Increase Survival of Juvenile Salmonids in the Columbia River Basin: Progress and Adaptive Management Considerations

Predation by Caspian terns Hydroprogne caspia is a factor limiting the recovery of some Endangered Species Act‐listed populations of salmonids from the Columbia River basin, especially steelhead Oncorhynchus mykiss. This prompted the development and implementation of two separate management plans, one in the Columbia River estuary and the other in the Columbia Plateau region, to reduce the impact of Caspian tern predation on smolt survival. Caspian terns nesting at managed breeding colonies within the basin were relocated to alternative nesting islands created for terns outside the basin. Both plans were successful in significantly reducing smolt losses to Caspian terns nesting at managed sites; however, new developments have led to regression in smolt survival gains associated with tern management. Adaptive management to prevent terns at managed colonies from relocating to nest elsewhere in the basin and to improve nesting opportunities for Caspian terns outside the basin are needed to maximize the survival benefits to Endangered Species Act‐listed salmonids from management. Adaptive management is also necessary to ensure the long‐term viability of the Pacific Flyway population of Caspian terns, whose breeding population has declined by more than 50% since the two management plans were implemented.

Estimating Cause‐Specific Mortality and Survival of Juvenile Fall Chinook Salmon: An Investigation of Avian Predation across Large Spatial Scales

AbstractTo investigate the effects of piscivorous colonial waterbird predation on juvenile salmonids (smolts) and to determine what proportion of all sources of smolt mortality was due to predation, we analyzed a data set describing fall Chinook Salmon Oncorhynchus tshawytscha that were marked with PIT tags in the Columbia River, USA. We used a state‐space Bayesian model that incorporated live detections of tagged fish and recoveries of tags from dead fish to jointly estimate predation and survival over multiple river reaches and years (2008–2019). By incorporating recoveries of tags on bird colonies, the model was also able to increase the precision and spatial extent of survival estimates compared with a more traditional capture–recapture model. Results indicated that the avian predation probability (proportion of available fish consumed) varied depending on the predator species, colony location, and the fish's rearing type. Estimates of cumulative predation probability ranged annually from 0.066 (95% credible interval = 0.049–0.089) to 0.283 (0.210–0.419), and predation accounted for 7.3% (4.3–11.9%) to 29.1% (19.7–40.2%) of all sources of smolt mortality during out‐migration to Bonneville Dam. Probabilities of predation on wild smolts were significantly higher than those for hatchery smolts in most river reaches and years. Predation probabilities were the highest for colonies of American white pelicans Pelecanus erythrorhynchos and double‐crested cormorants Nannopterum auritum, which were located near where wild smolts rear and congregate. Estimates of predation by Caspian terns Hydroprogne caspia, California gulls Larus californicus, and ring‐billed gulls L. delawarensis were often <0.03 of available smolts annually. Collectively, results indicated that the cumulative effects of predation on Chinook Salmon smolts were substantial in some but not all river reaches and years. Future models that consider biological and environmental factors influencing smolt susceptibility to avian predation may provide a more holistic understanding of the degree to which predation limits fish survival.

A Review of Factors Affecting the Susceptibility of Juvenile Salmonids to Avian Predation

AbstractWe reviewed studies of piscivorous colonial waterbird predation on juvenile salmonids to synthesize current knowledge of factors affecting fish susceptibility to avian predators. Specifically, we examined peer‐reviewed publications and reports from academic, governmental, and nongovernmental agencies to identify commonalities and differences in susceptibility of salmonids to avian predation, with a focus on mark–recovery studies in the Columbia River basin. Factors hypothesized to influence salmonid susceptibility to avian predation were grouped into four general categories: (1) salmonid species and populations, (2) environmental factors, (3) prey density, predator density, and migration timing, and (4) prey characteristics. Our review focused on predation by Caspian terns Hydroprogne caspia, double‐crested cormorants Nannopterum auritum, and gull species Larus spp. as these are the most well‐studied avian predators of salmonids. Results indicated that predator–prey interactions varied across salmonid species and populations and species of avian predator. Inferences across studies supported multiple hypotheses regarding predator–prey dynamics, including environmental factors that influence prey exposure to predators (e.g., river flows, turbidity, alternative prey), variation in predator and prey abundances, predator characteristics (e.g., foraging behavior, colony location), and prey characteristics (e.g., fish length, condition). Mark–recovery studies of avian predation on fish populations have greatly improved our understanding of the factors affecting fish susceptibility to avian predation, the relative contributions of abiotic and biotic factors to predation susceptibility, and the extent to which avian predation affects fish survival and the viability of prey populations. Future studies that jointly model predation and survival and the factors affecting those processes will further broaden our understanding of predator–prey dynamics and directly evaluate the effects of predation on prey population dynamics.

Conservation measures for the Caspian Tern Hydroprogne caspia at the largest colony in Sweden

We present conservation actions during 2007–2020 as part of the national Caspian Tern Hydroprogne caspia species action plan at Stenarna in the Björn archipelago, Uppland, the largest colony in Sweden. We applied a combination of monitoring, research, and management measures conducted within an adaptive approach framework, using both established and novel techniques. The implementation of conservation measures led to increased breeding success, from 0 fledglings per pair in 2007 to 1.3 in 2020. A surveillance video camera installed in 2009 aided in monitoring efforts and also revealed predation by Herring Gull Larus argentatus and White-tailed Eagle Haliaeetus albicilla, leading to species-specific control strategies. Management of the island and surrounding archipelago, including hunting of invasive American mink Mustela vison, vegetation removal, and habitat restoration after a severe storm, have also been instrumental to the success of the project. Implementation of projects such as this have the potential to improve conditions for continued viability of endangered species in a changing world and are likely to be useful to other conservation practitioners.

Paternal transmission of migration knowledge in a long-distance bird migrant

While advances in biologging have revealed many spectacular animal migrations, it remains poorly understood how young animals learn to migrate. Even in social species, it is unclear how migratory skills are transmitted from one generation to another and what implications this may have. Here we show that in Caspian terns Hydroprogne caspia family groups, genetic and foster male parents carry the main responsibility for migrating with young. During migration, young birds stayed close to an adult at all times, with the bond dissipating on the wintering grounds. Solo-migrating adults migrated faster than did adults accompanying young. Four young that lost contact with their parent at an early stage of migration all died. During their first solo migration, subadult terns remained faithful to routes they took with their parents as young. Our results provide evidence for cultural inheritance of migration knowledge in a long-distance bird migrant and show that sex-biased (allo)parental care en route shapes migration through social learning.

Second Records of the Caspian Tern Hydroprogne caspia in Lebanon 2022

Caspian Tern Hydroprogne caspia is a monotypic, cosmopolitan species occurring in North America, North Europe, Africa, Madagascar, Asia, Australia, and New Zealand. There are only three European populations of approximately 4,500 breeding pairs along the coasts of the Caspian, Black, and Baltic Seas. It’s the largest of the terns, with a wingspan averaging. It has a body length of 48 - 56 cm, a weight of 570-780 gram and a wing span of 127-140 cm. Its size, stout red bill, and lack of a deeply forked tail distinguishes it from other white terns found in the state. Its black cap, large red bill, and tern-like habit of flying slowly with its bill pointed downward separates it from the gulls. The low harsh call of the Caspian tern sounds similar to karrr or kraa-ah and is given frequently while in flight. The orange feet of immature birds distinguish them from fall-plumaged adults which have black feet. Recently it was recorded for the first time in Lebanon in 15th of September 2021 and the second record was made in 18th of January 2022, both records were made by Michel Sawan in north Lebanon.

First Records of the Caspian Tern Hydroprogne caspia in Lebanon 2021

Caspian Tern Hydroprogne caspia is a monotypic, cosmopolitan species occurring in North America, North Europe, Africa, Madagascar, Asia, Australia, and New Zealand. There are only three European populations of approximately 4,500 breeding pairs along the coasts of the Caspian, Black, and Baltic Seas. It’s the largest of the terns, with a wingspan averaging. It has a body length of 48 - 56 cm, a weight of 570-780 gram and a wing span of 127-140 cm. Its size, stout red bill, and lack of a deeply forked tail distinguishes it from other white terns found in the state. Its black cap, large red bill, and tern-like habit of flying slowly with its bill pointed downward separates it from the gulls. The low harsh call of the Caspian tern sounds similar to karrr or kraa-ah and is given frequently while in flight. The orange feet of immature birds distinguish them from fall-plumaged adults which have black feet. Recently it was recorded for the first time in Lebanon in 15th of September 2021 at Cheikh Zennad, Akkar District, North Lebanon by Michel Sawan.

First tracking of declining Caspian terns Hydroprogne caspia breeding in the Baltic Sea reveals high migratory dispersion and disjunct annual ranges as obstacles to effective conservation

The conservation of migratory species poses significant challenges that may be countered by detailed knowledge about the sites used by migrants throughout the annual cycle. We present the first GPS‐tracking data on the migration of declining Caspian terns Hydroprogne caspia breeding in the Baltic Sea. For 39 Caspian terns from colonies along a latitudinal gradient from 57 to 65°N, we identified key migratory routes, stopovers and wintering areas. In autumn these seabirds migrated using coastal and freshwater stopovers along six routes to reach their wintering areas across the Sahel, the Nile River Basin and the southern Iberian Peninsula. In spring, adults returned to the breeding grounds in the Baltic using a time optimizing strategy by reducing time at stopover by 78%, whereas most subadults remained sedentary and some performed only partial return migrations. Of the stopover sites used in both seasons, 58% are protected and have a reported management plan. Conservation strategies in wintering areas, stopover sites that are not protected or had not been previously recognized, and the inclusion of the species in important migratory flyways across Europe and Africa will be important to prevent further population declines of a species that depends on aquatic habitats.

Cumulative Effects of Avian Predation on Upper Columbia River Steelhead

AbstractTo investigate the cumulative effects of colonial waterbird predation on fish mortality and to determine what proportion of all sources of fish mortality (1 − survival) was due to bird predation, we conducted a mark–recapture–recovery study with upper Columbia River steelhead Oncorhynchus mykiss that were PIT‐tagged and released (N = 78,409) at Rock Island Dam on the Columbia River, USA. We used a state–space Bayesian model that incorporated live detections and dead recoveries of tagged fish to jointly estimate predation and survival probabilities during smolt out‐migration to the Pacific Ocean over an 11‐year study period. Estimated cumulative (all colonies combined) avian predation probabilities ranged from 0.31 (95% credible interval [CRI] = 0.27–0.38) to 0.53 (95% CRI = 0.42–0.64) annually, indicating that avian predation was a substantial source of mortality. Of the predator species evaluated, predation by Caspian terns Hydroprogne caspia was often the highest, with predation probabilities ranging from 0.11 (95% CRI = 0.09–0.14) to 0.38 (95% CRI = 0.29–0.47). Probabilities of predation by double‐crested cormorants Phalacrocorax auritus and mixed colonies of California gulls Larus californicus and ring‐billed gulls L. delawarensis were generally lower than the probabilities for terns but were also substantial, with upwards of 0.04 (95% CRI = 0.03–0.07; cormorants) and 0.31 (95% CRI = 0.25–0.39; gulls) of steelhead consumed. Comparisons of total smolt mortality with mortality due to avian predation indicated that avian predation accounted for 42% (95% CRI = 30–56%) to 70% (95% CRI = 53–87%) of total mortality, suggesting that more steelhead were consumed by avian predators than died from all other mortality sources combined. Results indicate that avian predation, although not the original cause of steelhead declines in the basin, is now a factor limiting the survival of upper Columbia River steelhead. Using the analytical framework developed in this study, future studies can consider the cumulative impact of multiple mortality sources across large spatial and temporal scales to more fully understand the extent to which they limit fish survival.

Anthropogenic debris accumulated in nests of seabirds in an uninhabited island in West Africa

Anthropogenic debris is rapidly spreading in marine environments and poses major problems to biodiversity and humans. However, mitigation strategies are limited by the lack of information on the accumulation patterns of these pollutants both at sea and in marine wildlife. Using observations from an uninhabited island in Delta du Saloum National Park, Senegal, we investigate here the prevalence and abundance of debris in nests of two sympatric seabird species, Caspian terns Hydroprogne caspia and great cormorants Phalacrocorax carbo, and its potential as an indicator of ocean pollution. We recorded debris in 15% of the Caspian terns' nests, and 40% of the great cormorants' nests. Plastics accounted for >95% of items recorded in the nests of both species. Caspian terns collected a diverse array of debris, both in terms of types and colours, with a preference for hard plastics and white items. The great cormorants accumulated in nests mainly greenish fishing gears resembling the vegetation typically used as nest materials. Nests of both bird species contained soft plastic in proportions lower than those observed in the environment. Our results suggest that nests of Caspian terns and great cormorants are not useful indicators of pollution by anthropogenic debris in the surrounding waters of the Delta du Saloum National Park. Nests of great cormorants, however, have the potential to indicate areas vulnerable to plastic pollution associated to fishery. The high amount of debris in nests of this species demonstrates the need for a better implementation of waste management strategies in Senegal.

Selecting Piscivorous Bird Species for Monitoring Environmental Change in the Banc d'Arguin, Mauritania

This study aims to select piscivorous bird species to be used as indicators to monitor changes in fish diversity at the Banc d'Arguin, Mauritania. Such species should have a high diet diversity, low dietary overlap and data should be easy to collect. We compared the diets of five colony nesting bird species at the Banc d'Arguin: the Slender-billed Gull Chroicocephalus genei, Caspian Tern Hydroprognecaspia and Royal Tern Thalasseus maximus albididorsalis, which are part of an ongoing monitoring program, and the White-breasted Cormorant Phalacrocorax carbo lucidus and Mauritanian Spoonbill Platalea leucorodiabalsaci as potential new additions. All species considerably differed in prey species composition, prey species diversity and prey size. Diet overlap indices varied between 0.05 and 0.69 with the highest overlap values between Mauritanian Spoonbill and Slender-billed Gull. There are major differences between the species in ecological and behavioural mechanisms promoting diet segregation, such as foraging habitat use, foraging ranges and prey-catching techniques. We conclude that the species investigated serve our monitoring goal well. Practical recommendations for the implementation of a monitoring program are given.

Avian Predation on Juvenile Salmonids: Spatial and Temporal Analysis Based on Acoustic and Passive Integrated Transponder Tags

AbstractWe evaluated the impact of predation on juvenile steelhead Oncorhynchus mykiss and yearling and subyearling Chinook Salmon O. tshawytscha by piscivorous waterbirds from 11 different breeding colonies in the Columbia River basin during 2012 and 2014. Fish were tagged with both acoustic tags and PIT tags and were tracked via a network of hydrophone arrays to estimate total smolt mortality (1 – survival) at various spatial and temporal scales during out‐migration. Recoveries of PIT tags on bird colonies, coupled with the last known detections of live fish passing hydrophone arrays, were used to estimate the impact of avian predation relative to total smolt mortality. Results indicated that avian predation was a substantial source of steelhead mortality, with predation probability (proportion of available fish consumed by birds) ranging from 0.06 to 0.28 for fish traveling through the lower Snake River and the lower and middle Columbia River. Predation probability estimates ranged from 0.03 to 0.09 for available tagged yearling Chinook Salmon and from 0.01 to 0.05 for subyearlings. Smolt predation by gulls Larus spp. was concentrated near hydroelectric dams, while predation by Caspian terns Hydroprogne caspia was concentrated within reservoirs. No concentrated areas of predation were identified for double‐crested cormorants Phalacrocorax auritus or American white pelicans Pelecanus erythrorhynchos. Comparisons of total smolt mortality relative to mortality from colonial waterbirds indicated that avian predation was one of the greatest sources of mortality for steelhead and yearling Chinook Salmon during out‐migration. In contrast, avian predation on subyearling Chinook Salmon was generally low and constituted a minor component of total mortality. Our results demonstrate that acoustic and PIT tag technologies can be combined to quantify where and when smolt mortality occurs and the fraction of mortality that is due to colonial waterbird predation relative to non‐avian mortality sources.Received November 4, 2015; accepted February 1, 2016 Published online June 27, 2016

New bird records from the Fortescue Marsh and nearby claypans, Pilbara bioregion, Western Australia

The Fortescue Marsh in the Pilbara bioregion, Western Australia, is an extensive ephemeral wetland that fills episodically. It is considered as a potential Ramsar site and is recognised as a nationally important wetland and an Important Bird Area. We surveyed birds at 21 sites on the Fortescue Marsh and a further 23 sites (44 sites in total), including nearby claypans Coondiner Pool and Mungthannannie Pool, in the Fortescue Valley over 12 days in March–April and July 2012. A total of 100 bird species (34 waterbird and 66 landbird species) was recorded during the survey. A further 86 bird species (including 28 waterbird species and 58 landbird species) were recorded for this area from searches of databases and the literature (total of 187 species; 62 waterbirds). New and significant observations during the survey included the first breeding record of Australian Shelduck Tadorna tadornoides for the Pilbara, the first breeding records on the Marsh of Black-tailed Native-hen Tribonyx ventralis and Caspian Tern Hydroprogne caspia, and the first record of Australian Painted Snipe Rostratula australis for the Fortescue Valley. Despite this area’s importance to breeding and visiting waterbirds, the birds of the Fortescue Marsh remain surprisingly under-studied. We highlight some significant but overlooked literature records of waterbirds on the Marsh. Further ground and aerial surveys, and ongoing monitoring of this region would be valuable.

Quantifying Avian Predation on Fish Populations: Integrating Predator‐Specific Deposition Probabilities in Tag Recovery Studies

AbstractAccurate assessment of specific mortality factors is vital to prioritize recovery actions for threatened and endangered species. For decades, tag recovery methods have been used to estimate fish mortality due to avian predation. Predation probabilities derived from fish tag recoveries on piscivorous waterbird colonies typically reflect minimum estimates of predation due to an unknown and unaccounted‐for fraction of tags that are consumed but not deposited on‐colony (i.e., deposition probability). We applied an integrated tag recovery modeling approach in a Bayesian context to estimate predation probabilities that accounted for predator‐specific tag detection and deposition probabilities in a multiple‐predator system. Studies of PIT tag deposition were conducted across three bird species nesting at seven different colonies in the Columbia River basin, USA. Tag deposition probabilities differed significantly among predator species (Caspian terns Hydroprogne caspia: deposition probability = 0.71, 95% credible interval [CRI] = 0.51–0.89; double‐crested cormorants Phalacrocorax auritus: 0.51, 95% CRI = 0.34–0.70; California gulls Larus californicus: 0.15, 95% CRI = 0.11–0.21) but showed little variation across trials within a species or across years. Data from a 6‐year study (2008–2013) of PIT‐tagged juvenile Snake River steelhead Oncorhynchus mykiss (listed as threatened under the Endangered Species Act) indicated that colony‐specific predation probabilities ranged from less than 0.01 to 0.17 and varied by predator species, colony location, and year. Integrating the predator‐specific deposition probabilities increased the predation probabilities by a factor of approximately 1.4 for Caspian terns, 2.0 for double‐crested cormorants, and 6.7 for California gulls compared with traditional minimum predation rate methods, which do not account for deposition probabilities. Results supported previous findings on the high predation impacts from strictly piscivorous waterbirds nesting in the Columbia River estuary (i.e., terns and cormorants), but our findings also revealed greater impacts of a generalist predator species (i.e., California gulls) than were previously documented. Approaches used in this study allow for direct comparisons among multiple fish mortality factors and considerably improve the reliability of tag recovery models for estimating predation probabilities in multiple‐predator systems.Received August 1, 2014; accepted November 11, 2014

Seasonal distributions of Caspian Terns Hydroprogne caspia from Swedish populations, revealed by recoveries and resightings of ringed birds

The aim of this study was, by analysing recoveries and resightings of Caspian Terns Hydroprogne caspia ringed in Sweden, (1) to identify areas of importance during their migration and wintering, (2) to investigate the annual routines of migration and wintering of different age categories, and (3) to investigate how human activities, especially shooting, have affected survival of Caspian Terns over the 80-year period. In total, 2,048 metal-ring recoveries and resightings from 1,420 birds and 101 colour-ring recoveries and resightings since 2011 from 85 Caspian Terns ringed in Sweden were analysed for spatio-temporal patterns, age-dependent differences and mortality due to human activities. Ring recoveries beyond the breeding areas were widely dispersed southward but concentrated in several locations, such as coastal areas in the Netherlands, Italy and Germany during migration seasons and in Mali and Tunisia during winter. Distances to their wintering areas of up to more than 6,000 km were longer than for other populations of this species. Age-dependent differences in seasonal distributions seemed to be due mainly to young Caspian Terns remaining in wintering areas or otherwise well south of the breeding areas during their first years of life. Although shooting appeared to have been an important cause of death in former years, it has declined and recently seems to have had less influence on survival of European Caspian Terns than in the 1940s–2000s. Other recent changes in the distribution of recoveries are also discussed from ecological and social perspectives.

Nutritional stress affects corticosterone deposition in feathers of Caspian tern chicks

Stressful environmental conditions affect the adrenocortical function of developing animals, which can have consequences for their fitness. Discovery of the avian stress hormone corticosterone (CORT) in feathers has the potential to broaden the application of endocrine research in ecological and evolutionary studies of wild birds by providing a long‐term measure of CORT secretion. Mechanisms of CORT deposition in feathers are not well known and few studies have related feather CORT to circulating plasma CORT during feather growth. Our objective was to experimentally test the validity of using feather CORT as a measure of CORT secretion in developing birds experiencing nutritional stress. Caspian tern Hydroprogne caspia chicks were fed ad libitum or restricted (35% less than ad libitum) diets for four weeks. We measured CORT in feathers from these chicks to examine the relationship between feather CORT concentrations and nutritional limitation, circulating plasma CORT, and feather development. We found that feather CORT was higher in controls fed ad libitum than in restricted individuals, despite higher levels of plasma CORT in restricted chicks compared to controls. Feather mass and growth rates were strongly and positively related to feather CORT concentrations in both treatments. This is the first experimental study to show that feather CORT concentrations can be lower in response to nutritional stress, even when plasma CORT concentrations are elevated. Our results indicate that CORT deposition in feathers may be confounded when feather mass and growth rates are compromised by nutritional stress. We conclude that feather CORT can be used for assessing nutritional stress in growing birds, but the direction of response depends on how strongly stress affects feather development.

Relative Vulnerability of PIT‐Tagged Subyearling Fall Chinook Salmon to Predation by Caspian Terns and Double‐Crested Cormorants in the Columbia River Estuary

AbstractWe quantified the percentage of PIT‐tagged subyearling fall Chinook Salmon Oncorhynchus tshawytscha that were consumed by Caspian terns Hydroprogne caspia and double‐crested cormorants Phalacrocorax auritus nesting on East Sand Island in the Columbia River estuary by electronically recovering PIT tags that were deposited on the bird colonies. We released 23 groups of PIT‐tagged subyearling fall Chinook Salmon from hatcheries in the lower Columbia River downstream of Bonneville Dam from 2002 to 2010. Vulnerability to avian predation was compared between PIT‐tagged subyearlings of two Columbia River basin stocks: tule and upriver bright (URB). Recoveries of PIT tags revealed that overall predation rates were significantly different between the tule stock (22%) and URB stock (3%); for fish that were detected as entering the lower Columbia River during the same week, predation rates also differed between stocks (tule: 21%; URB: 2%). Minimum predation rates on tule subyearlings originating from hatcheries downstream of Bonneville Dam were among the highest documented for any salmonid species in the Columbia River basin to date, occasionally exceeding 35% of the available fish. The ratio of URB fish consumed by the two avian predators indicated that the percentages were nearly equal (cormorant [%]: tern [%] = 51:49), whereas the ratio for tule‐stock fish consumed by the two avian species was not uniform (cormorant: tern = 81:19). Differences in predation rates between the tule stock and the URB stock may be attributable to migration behaviors exhibited in the estuary. We estimate that more than 8 million tule fall Chinook Salmon subyearlings released from hatcheries annually are consumed by double‐crested cormorants and Caspian terns nesting on East Sand Island; ongoing management actions by multiple federal, state, and tribal governments, if successful, will decrease predation on fall Chinook Salmon stocks.

Potential Effects of Management on Caspian Tern Hydroprogne caspia Predation on Juvenile Salmonids at a Colony in San Francisco Bay, California

AbstractSan Francisco Bay is a proposed relocation site for some of the Caspian terns Hydroprogne caspia currently nesting at the world's largest colony for the species in the Columbia River estuary and consuming salmonids listed under the U.S. Endangered Species Act (ESA). However, several runs of salmonids listed under the ESA occur in San Francisco Bay and managers are concerned that increased Caspian tern predation may pose a threat to the recovery of these fish. We used a bioenergetics modeling approach, employing estimates of tern energy requirements and proportions of energy supplied by various prey types, to estimate the consumption of juvenile salmonids by Caspian terns nesting on Brooks Island in central San Francisco Bay during 2008 and 2009. Estimated salmonid consumption was ∼205,000 smolts (95% confidence interval, 175,000–245,000 smolts) in 2008 and ∼167,000 smolts (144,000–191,000 smolts) in 2009. The interannual difference in smolt consumption was due to the smaller size of the tern colony and lower nesting success in 2009. Estimated predation rates on ESA‐listed Central Valley spring‐run Chinook salmon Oncorhynchus tshawytscha (0.1%) were lower than those on unlisted fall‐run Chinook salmon (1.0%). Continuation of the current downward trend in the number of Caspian terns nesting on Brooks Island and the resulting reductions in salmonid predation would not be sufficient to reverse salmonid declines in San Francisco Bay. The proposed enhancement of the Brooks Island Caspian tern colony to 3,000 individuals would at most cause declines in annual population growth rates of 0.28% for fall‐run Chinook salmon and 0.02% for threatened spring‐run Chinook salmon, assuming that the mortality from tern predation is 100% additive.

Susceptibility of Juvenile Steelhead to Avian Predation: the Influence of Individual Fish Characteristics and River Conditions

AbstractIdentification of the factors that influence susceptibility to predation can aid in developing management strategies to recover fish populations of conservation concern. Predator–prey relationships can be influenced by numerous factors, including prey condition, prey size, and environmental conditions. We investigated these factors by using juvenile steelhead Oncorhynchus mykiss from the Snake River (Pacific Northwest, USA), a distinct population segment that is listed as threatened under the U.S. Endangered Species Act. During 2007–2009, steelhead smolts (n = 25,909) were captured, examined for external condition characteristics (e.g., body injuries, descaling, external signs of disease, fin damage, and ectoparasite infestations), marked with passive integrated transponder (PIT) tags, and released to continue their out‐migration. Recoveries of PIT tags on a downstream colony of Caspian terns Hydroprogne caspia (n = 913 tags) indicated that steelhead susceptibility to Caspian tern predation increased significantly with decreases in steelhead external condition, decreased water discharge, and decreased water clarity. Susceptibility to Caspian tern predation also increased with increasing steelhead fork length up to 202 mm but then decreased for longer steelhead. Recoveries of PIT tags on a downstream colony of double‐crested cormorants Phalacrocorax auritus (n = 493 tags) indicated that steelhead susceptibility to double‐crested cormorant predation increased significantly with declining external condition of steelhead, and that steelhead of hatchery origin were more susceptible than their wild counterparts. Results indicate that steelhead susceptibility to avian predation is dependent on fish condition and length and is influenced by river conditions and rearing environment.

Systemwide Evaluation of Avian Predation on Juvenile Salmonids from the Columbia River Based on Recoveries of Passive Integrated Transponder Tags

AbstractWe recovered passive integrated transponder (PIT) tags from nine piscivorous waterbird colonies in the Columbia River basin to evaluate avian predation on Endangered Species Act (ESA)‐listed salmonid Oncorhynchus spp. populations during 2007–2010. Avian predation rates were calculated based on the percentage of PIT‐tagged juvenile salmonids that were detected as passing hydroelectric dams and subsequently were consumed and deposited by birds on their nesting colonies. Caspian terns Hydroprogne caspia (hereafter, “terns”) and double‐crested cormorants Phalacrocorax auritus (hereafter, “cormorants”) nesting on East Sand Island in the Columbia River estuary consumed the highest proportions of available PIT‐tagged salmonids, with minimum predation rates ranging from 2.5% for Willamette River spring Chinook salmon O. tshawytscha to 16.0% for Snake River steelhead O. mykiss. Estimated predation rates by terns, cormorants, gulls of two species (California gull Larus californicus and ring‐billed gull L. delawarensis), and American white pelicans Pelecanus erythrorhynchos nesting near the confluence of the Snake and Columbia rivers were also substantial; minimum predation rates ranged from 1.4% for Snake River fall Chinook salmon to 13.2% for upper Columbia River steelhead. Predation on ESA‐listed salmonids by gulls and American white pelicans were minor (<2.0% per ESA‐listed salmonid population) relative to predation by terns and cormorants. Cumulative impacts were greater for Snake River and upper Columbia River salmonids than for salmonids originating closer to the estuary because upriver salmonids must migrate past more bird colonies to reach the ocean. Predation rates adjusted for colony size (per capita rates) were significantly higher for terns and cormorants nesting at inland colonies (upstream of Bonneville Dam) than for those nesting in the estuary, suggesting that inland colonies have a greater reliance on salmonids as a food source. Management actions to increase salmonid survival by reducing avian predation in the estuary could be offset if birds that disperse from the estuary relocate to inland nesting sites on or near the Columbia River.