Differential use of salmon by vertebrate consumers: implications for conservation

Carrion ecology in inland aquatic ecosystems: a systematic review

Carrion ecology, i.e. the decomposition and recycling of dead animals, has traditionally been neglected as a key process in ecosystem functioning. Similarly, despite the large threats that inland aquatic ecosystems (hereafter, aquatic ecosystems) face, the scientific literature is still largely biased towards terrestrial ecosystems. However, there has been an increasing number of studies on carrion ecology in aquatic ecosystems in the last two decades, highlighting their key role in nutrient recirculation and disease control. Thus, a global assessment of the ecological role of scavengers and carrion in aquatic ecosystems is timely. Here, we systematically reviewed scientific articles on carrion ecology in aquatic ecosystems to describe current knowledge, identify research gaps, and promote future studies that will deepen our understanding in this field. We found 206 relevant studies, which were highly biased towards North America, especially in lotic ecosystems, covering short time periods, and overlooking seasonality, a crucial factor in scavenging dynamics. Despite the low number of studies on scavenger assemblages, we recorded 55 orders of invertebrates from 179 families, with Diptera and Coleoptera being the most frequent orders. For vertebrates, we recorded 114 species from 40 families, with birds and mammals being the most common. Our results emphasise the significance of scavengers in stabilising food webs and facilitating nutrient cycling within aquatic ecosystems. Studies were strongly biased towards the assessment of the ecosystem effects of carrion, particularly of salmon carcasses in North America. The second most common research topic was the foraging ecology of vertebrates, which was mostly evaluated through sporadic observations of carrion in the diet. Articles assessing scavenger assemblages were scarce, and only a limited number of these studies evaluated carrion consumption patterns, which serve as a proxy for the role of scavengers in the ecosystem. The ecological functions performed by carrion and scavengers in aquatic ecosystems were diverse. The main ecological functions were carrion as food source and the role of scavengers in nutrient cycling, which appeared in 52.4% (N = 108) and 46.1% (N = 95) of publications, respectively. Ecosystem threats associated with carrion ecology were also identified, the most common being water eutrophication and carrion as source of pathogens (2.4%; N = 5 each). Regarding the effects of carrion on ecosystems, we found studies spanning all ecosystem components (N = 85), from soil or the water column to terrestrial vertebrates, with a particular focus on aquatic invertebrates and fish. Most of these articles found positive effects of carrion on ecosystems (e.g. higher species richness, abundance or fitness; 84.7%; N = 72), while a minority found negative effects, changes in community composition, or even no effects. Enhancing our understanding of scavengers and carrion in aquatic ecosystems is crucial to assessing their current and future roles amidst global change, mainly for water-land nutrient transport, due to changes in the amount and speed of nutrient movement, and for disease control and impact mitigation, due to the predicted increase in occurrence and magnitude of mortality events in aquatic ecosystems.

Ecological implications of the pink salmon invasion in northern Norway-Aggregative responses and terrestrial transfer by white-tailed eagles

Over the last 10 years, the spawning population of invasive pink salmon (Oncorhynchus gorbuscha) has increased in the river systems in northern Norway to a level that is causing concern about their impact on endemic fauna and ecosystem processes. The scale of transfer of pink salmon carcasses into the terrestrial ecosystem is likely to be a key determinant of terrestrial impact. Bears (Ursus sp.) are responsible for most such transfers in North America but are rare in Norway. The white-tailed eagle (Haliaeetus albicilla) is common however, and a candidate to be a main cause of such transfers. To evaluate this hypothesis, data on the abundance of white-tailed eagles and pink salmon were collected along the river Skallelv in northern Norway in 2021, a year the pink salmon spawned in the river, and in 2022, a year no pink salmon spawned in the river. The abundance of white-tailed eagles along the river was much higher the year pink salmon spawned in the river. Furthermore, white-tailed eagles were observed aggregating and catching pink salmon where and when pink salmon were present at the spawning and post-spawning stages. Based on our observations, we suggest that the white-tailed eagle is the main species involved in the transport of pink salmon from the river into the riparian zone in northern Norway and that other scavengers, in particular the red fox (Vulpes vulpes) and common raven (Corvus corax), play an important role in transporting pink salmon carcasses from the riparian zone to the wider terrestrial ecosystem.

Camera traps reveal that terrestrial predators are pervasive at riverscape cold-water thermal refuges

Perceived predation risks by terrestrial predators are major ecological forces in aquatic systems, particularly for aggregating fish. Riverscape thermal refuges are discrete, localized cold-water patches where fish temporarily aggregate to buffer against heat events. Predation pressures by terrestrial predators at thermal refuges may decrease the thermoregulatory benefits of refuge use, but quantifying such effects can be challenging and controversial when sampling can impose additional stress on fish. We passively monitored terrestrial predator visitation patterns and predation at four thermal refuges in the Housatonic River, Connecticut, USA, between May 18th and September 29th, 2022, with camera traps, a common wildlife monitoring method. Specifically, we (1) assessed diel visitation patterns by different categories of terrestrial predators at thermal refuges and determined if patterns varied among predator categories or with prevailing environmental conditions, and (2) estimated the probability of predation by hour of the day combined across all predator categories, quantifying general predation pressures at refuges. We detected at least one terrestrial predator at a thermal refuge each day, and mean hourly visitation rates (count/h) were highly variable across predator categories and sampling dates. The most supported generalized additive mixed model indicated that terrestrial predator visitation rates (count/h/day) varied with mean daily river discharge and water temperature differential, and relationships differed across categories of terrestrial predators. We observed 22 separate predation attempts on thermoregulating salmonids and predicted that the probability of predation by any terrestrial predator increased from 0.002 to 0.017 throughout a 24 h day (p = .004). Camera traps provided novel evidence that terrestrial predators are pervasive at riverine thermal refuges, which is relevant for refuge conservation and management globally. We recommend the implementation of a coordinated monitoring network across riverine thermal refuges using camera traps, further enriching our ecological understanding of cumulative predator effects in refuges across complex riverscapes.

The movement ecology of fishes

Movement of fishes in the aquatic realm is fundamental to their ecology and survival. Movement can be driven by a variety of biological, physiological and environmental factors occurring across all spatial and temporal scales. The intrinsic capacity of movement to impact fish individually (e.g., foraging) with potential knock-on effects throughout the ecosystem (e.g., food web dynamics) has garnered considerable interest in the field of movement ecology. The advancement of technology in recent decades, in combination with ever-growing threats to freshwater and marine systems, has further spurred empirical research and theoretical considerations. Given the rapid expansion within the field of movement ecology and its significant role in informing management and conservation efforts, a contemporary and multidisciplinary review about the various components influencing movement is outstanding. Using an established conceptual framework for movement ecology as a guide (i.e., Nathan et al., 2008: 19052), we synthesized the environmental and individual factors that affect the movement of fishes. Specifically, internal (e.g., energy acquisition, endocrinology, and homeostasis) and external (biotic and abiotic) environmental elements are discussed, as well as the different processes that influence individual-level (or population) decisions, such as navigation cues, motion capacity, propagation characteristics and group behaviours. In addition to environmental drivers and individual movement factors, we also explored how associated strategies help survival by optimizing physiological and other biological states. Next, we identified how movement ecology is increasingly being incorporated into management and conservation by highlighting the inherent benefits that spatio-temporal fish behaviour imbues into policy, regulatory, and remediation planning. Finally, we considered the future of movement ecology by evaluating ongoing technological innovations and both the challenges and opportunities that these advancements create for scientists and managers. As aquatic ecosystems continue to face alarming climate (and other human-driven) issues that impact animal movements, the comprehensive and multidisciplinary assessment of movement ecology will be instrumental in developing plans to guide research and promote sustainability measures for aquatic resources.

Scavenger guild and consumption patterns of an invasive alien fish species in a Mediterranean wetland

Invasive Alien Species (IAS) alter ecosystems, disrupting ecological processes and driving the loss of ecosystem services. The common carp Cyprinus carpio is a hazardous and widespread IAS, becoming the most abundant species in many aquatic ecosystems. This species transforms ecosystems by accumulating biomass to the detriment of other species, thus altering food webs. However, some terrestrial species, such as vertebrate scavengers, may benefit from dead carps, by incorporating part of the carp biomass into the terrestrial environment. This study describes the terrestrial vertebrate scavenger assemblage that benefits from carp carcasses in a Mediterranean wetland. We also evaluate the seasonal differences in the scavenger assemblage composition and carrion consumption patterns. Eighty carp carcasses (20 per season) were placed in El Hondo Natural Park, a seminatural mesohaline wetland in south-eastern Spain, and we monitored their consumption using camera traps. We recorded 14 scavenger species (10 birds and four mammals) consuming carp carcasses, including globally threatened species. Vertebrates consumed 73% of the carrion biomass and appeared consuming at 82% of the carcasses. Of these carcasses consumed, 75% were completely consumed and the mean consumption time of carcasses completely consumed by vertebrates was 44.4 h (SD = 42.1 h). We recorded differences in species richness, abundance, and assemblage composition among seasons, but we did not find seasonal differences in consumption patterns throughout the year. Our study recorded a rich and efficient terrestrial vertebrate scavenger assemblage benefitting from carp carcasses. We detected a seasonal replacement on the scavenger species, but a maintenance of the ecological function of carrion removal, as the most efficient carrion consumers were present throughout the year. The results highlight the importance of vertebrate scavengers in wetlands, removing possible infectious focus, and moving nutrients between aquatic and terrestrial environments.

The influence of human disturbance on Pacific salmon (Oncorhynchus spp.) in the diet of American black bears (Ursus americanus) in two areas of coastal British Columbia, Canada

Recent studies have highlighted the importance of salmon (genus Oncorhynchus Suckley, 1861) in the diet of bears, and of bears as consumers and key agents supporting the transport of salmon-derived nutrients to riparian ecosystems. Salmon abundance and human disturbance are known influences on bear ecology and behaviour, though few studies have quantified shifts in bear diet due to these effects. We used stable isotope analysis to investigate how salmon escapement and human presence influenced the proportion of salmon in the diet of American black bears ( Ursus americanus Pallas, 1780) in two locations in coastal British Columbia, Canada. We found that salmon constituted a small proportion of black bear diet across sexes and ecosystems, while bears appeared to gain a similar amount of energy and lean mass from terrestrial sources. Salmon consumption was not related to the total annual abundance of salmon in a watershed but was significantly lower in large streams with regular human presence, suggesting that human disturbance can cause a dietary shift in bears that could have important consequences to their fitness. We also observed that the isotopic signatures of key bear foods did not vary between foliage and fruit, simplifying data collection for future isotopic studies on bear diet.

Prevalence and patterns of scavenging by brown bears (Ursus arctos) on salmon (Oncorhynchus spp.) carcasses

Scavenging, an underappreciated mechanism of prey consumption for many predators, can contribute substantially to nutritional intake. Facultative scavengers such as brown bears (Ursus arctos Linnaeus, 1758) may both kill and scavenge Pacific salmon (genus Oncorhynchus Suckley, 1861), though the extent of scavenging and factors affecting this behavior are unclear. We tagged 899 sockeye salmon (Oncorhynchus nerka (Walbaum in Artedi, 1792)) carcasses and placed them on streambanks over 5 years at multiple sites in southwestern Alaska (USA) where brown bears annually prey on spawning sockeye salmon. Examination of carcasses revealed overall scavenging rates of 15% after 1 day and 54% after 3 days. Scavenging rate varied by site and year and increased throughout the salmon run. Contrary to predictions, scavenging was more frequent in senescent or bear-killed carcasses than ripe carcasses. Carcass consumption ranged from minimal to almost complete; body and brain tissues were most frequently consumed after 3 days (68% and 63% of carcasses, respectively). We also documented secondary scavenging (i.e., tissue consumption on two separate events) and delayed scavenging (i.e., scavenging observed after 3 days but not 1 day). Taken together, the results indicated that scavenging in these streams contributes significantly to total consumption of salmon by bears, with ramifications for other components of these salmon-dependent ecosystems.

Terrestrial and semi-aquatic scavengers on invasive Pacific pink salmon (Oncorhynchus gorbuscha) carcasses in a riparian ecosystem in northern Norway

Pacific pink salmon (Oncorhynchus gorbuscha) invasions, thought to originate from populations introduced and established in Russia, occurred along the Norwegian coast in 2017 and 2019. Despite several thousand pink salmon entering and establishing in northern Norwegian rivers, current understanding of the ecological effect of the species in northern Europe is limited. Scavengers feeding on pacific salmon carcasses are important vectors for the transport of marine derived energy and nutrients to terrestrial ecosystems in the Pacific Northwest, North America, where the salmon naturally occur. However the role of terrestrial and aquatic scavengers in the consumption and removal of pink salmon beyond the salmon’s native range is unknown. This study has identified terrestrial and sub-aquatic vertebrate scavengers on pink salmon carcasses in a sub-arctic river in northern Norway. Avian scavengers filmed by a camera placed near sites baited with pink salmon carcasses included the Eurasian magpie (Pica pica), hooded crow (Corvus cornix), common raven (Corvus corax), the European herring gull (Larus argentatus), redwing (Turdus iliacus) and goosander (Mergus merganser). However, the largest carcass weight was removed by red foxes (Vulpes vulpes). Carcasses entering Vesterelv River in 2019 were estimated to provide energy and nutrients to the river ecosystem an order of magnitude lower than in the Pacific Northwest. This study provides some of the first information in northern Europe on the mechanisms and quantification of energy and nutrient transfer from the ocean to riparian environments via introduced Pacific pink salmon. Results help to begin to determine the ecological effect of pink salmon and the development of appropriate management strategies.

Community Ecology and Conservation of Bear-Salmon Ecosystems

Apex predators play keystone roles in ecosystems through top-down control, but the effects of apex omnivores on ecosystems could be more varied because changes in the resource base alter their densities and reverberate through ecosystems in complex ways. In coastal temperate ecosystems throughout much of the Northern Hemisphere, anadromous salmon once supported abundant bear populations, but both taxa have declined or been extirpated from large parts of their former ranges with limited research on the consequences of diminished or absent interactions among species. Here we review the biogeography of bear-salmon interactions and the role of salmon-subsidized bears in (1) resource provisioning to plants and scavengers through the distribution of salmon carcasses, (2) competition among bears and other large carnivores, (3) predation of ungulate neonates, (4) seed dispersal, and (5) resource subsidies to rodents with seed-filled scats. In addition to our review of the literature, we present original data to demonstrate two community-level patterns that are currently unexplained. First, deer densities appear to be consistently higher on islands with abundant brown bears than adjacent islands with black bears and wolves, and moose calf survival is higher at low bear densities (<∼25 bears per 100 km2) but is constant across the vast majority of bear densities found in the wild (i.e., ∼>25 bears per 100 km2). Our review and empirical data highlight key knowledge gaps and research opportunities to understand the complex ecosystem effects related to bear-salmon interactions.

Managing salmon for wildlife: Do fisheries limit salmon consumption by bears in small Alaskan streams?

Ecosystem-based management requires consideration of overlapping resource use between humans and other consumers. Pacific salmon are an important resource for both fisheries and populations of wildlife around the Pacific rim, including coastal brown bears (Ursus arctos); salmon consumption has been positively linked to bear density, body size, and reproductive rate. As a case study within the broader context of human-wildlife competition for food, we used 16-22 yr of empirical data in four different salmon-bearing systems in southwestern Alaska to explore the relationship between sockeye salmon (Oncorhynchus nerka) availability and consumption by bears. We found a negative relationship between the annual biomass of salmon available to bears and the fraction of biomass consumed per fish, and a saturating relationship between salmon availability and the total annual biomass of salmon consumed by bears. Under modeled scenarios, bear consumption of salmon was predicted to increase only with dramatic (on the order of 50-100%) increases in prey availability. Even such large increases in salmon abundance were estimated to produce relatively modest increases in per capita salmon consumption by bears (2.4-4.8 kg·bear^-1 ·d^-1 , 15-59% of the estimated daily maximum per capita intake), in part because bears did not consume salmon entirely, especially when salmon were most available. Thus, while bears catching salmon in small streams may be limited by salmon harvest in some years, current management of the systems we studied is sufficient for bear populations to reach maximum salmon consumption every 2-4 yr. Consequently, allocating more salmon for brown bear conservation would unlikely result in an ecologically significant response for bears in these systems, though other ecosystem components might benefit. Our results highlight the need for documenting empirical relationships between prey abundance and consumption, particularly in systems with partial consumption, when evaluating the ecological response of managing prey resources for wildlife populations.

Report on the Short-Term Scavenging of Decomposing Native and Non-Native Trout in Appalachian Streams

Trout fishing is one of the primary recreational activities in the southern Appalachians, with large amounts of fish stocked on a regular basis. However, very little is known regarding the fate of hatchery reared carcasses not captured by anglers, representing a likely important ecological resource to local communities. We tested the efficacy of underwater video to characterize short term decomposition and consumption by aquatic scavengers of native brook and non-native rainbow trout, Salvelinus fontinalis and Oncorhychus mykiss. This study took place on the Cherokee Qualla Boundary in North Carolina, a location with one of the highest riverine stocked trout densities in the eastern United States. During May 2017, 10 waterproof cameras were deployed for 1-hour intervals on each carcass twice daily for a period of 5 days. We observed that 75.3% of recorded video contained river chub, Nocomis micropogon, with only 24.7% visited by crayfish, with a maximum of 9 and a mean of 1.93 for N. micropogon. Half of the carcasses were removed within 2 days. Based on natural history evidence and some trail cameras, we believe that otters were removing carcasses. Otters showed no preference for either trout species. Underwater video allowed us to characterize initial decomposition within stream diurnal scavengers in a short period using a visual, non-destructive low-cost method. Future studies should monitor large mammalian scavengers to further elucidate the role of fish stocking on aquatic communities.

Migratory coupling between predators and prey

Animal migrations act to couple ecosystems and are undertaken by some of the world's most endangered taxa. Predators often exploit migrant prey, but the movements taken by these consumers are rarely studied or understood. We define such movements, where migrant prey induce large-scale movements of predators, as migratory coupling. Migratory coupling can have ecological consequences for the participating prey, predators and the communities they traverse across the landscape. We review examples of migratory coupling in the literature and provide hypotheses regarding conditions favourable for their occurrence. We also provide a framework for interactions induced by migratory coupling and demonstrate their potential community-level impacts by examining other forms of spatial shifts in predators. Migratory coupling integrates the fields of landscape, movement, food web and community ecologies, and represents an understudied frontier in ecology.

Alaskan brown bears (Ursus arctos) aggregate and display fidelity to foraging neighborhoods while preying on Pacific salmon along small streams

The interaction between brown bears (Ursus arctos) and Pacific salmon (Oncorhynchus spp.) is important to the population dynamics of both species and a celebrated example of consumer-mediated nutrient transport. Yet, much of the site-specific information we have about the bears in this relationship comes from observations at a few highly visible but unrepresentative locations and a small number of radio-telemetry studies. Consequently, our understanding of brown bear abundance and behavior at more cryptic locations where they commonly feed on salmon, including small spawning streams, remains limited. We employed a noninvasive genetic approach (barbed wire hair snares) over four summers (2012-2015) to document patterns of brown bear abundance and movement among six spawning streams for sockeye salmon, O. nerka, in southwestern Alaska. The streams were grouped into two trios on opposite sides of Lake Aleknagik. Thus, we predicted that most bears would forage within only one trio during the spawning season because of the energetic costs associated with swimming between them or traveling around the lake and show fidelity to particular trios across years because of the benefits of familiarity with local salmon dynamics and stream characteristics. Huggins closed-capture models based on encounter histories from genotyped hair samples revealed that as many as 41 individuals visited single streams during the annual 6-week sampling season. Bears also moved freely among trios of streams but rarely moved between these putative foraging neighborhoods, either during or between years. By implication, even small salmon spawning streams can serve as important resources for brown bears, and consistent use of stream neighborhoods by certain bears may play an important role in spatially structuring coastal bear populations. Our findings also underscore the efficacy of noninvasive hair snagging and genetic analysis for examining bear abundance and movements at relatively fine spatial and temporal scales.

Environmental DNA from Residual Saliva for Efficient Noninvasive Genetic Monitoring of Brown Bears (Ursus arctos)

Noninvasive genetic sampling is an important tool in wildlife ecology and management, typically relying on hair snaring or scat sampling techniques, but hair snaring is labor and cost intensive, and scats yield relatively low quality DNA. New approaches utilizing environmental DNA (eDNA) may provide supplementary, cost-effective tools for noninvasive genetic sampling. We tested whether eDNA from residual saliva on partially-consumed Pacific salmon (Oncorhynchus spp.) carcasses might yield suitable DNA quality for noninvasive monitoring of brown bears (Ursus arctos). We compared the efficiency of monitoring brown bear populations using both fecal DNA and salivary eDNA collected from partially-consumed salmon carcasses in Southeast Alaska. We swabbed a range of tissue types from 156 partially-consumed salmon carcasses from a midseason run of lakeshore-spawning sockeye (O. nerka) and a late season run of stream-spawning chum (O. keta) salmon in 2014. We also swabbed a total of 272 scats from the same locations. Saliva swabs collected from the braincases of salmon had the best amplification rate, followed by swabs taken from individual bite holes. Saliva collected from salmon carcasses identified unique individuals more quickly and required much less labor to locate than scat samples. Salmon carcass swabbing is a promising method to aid in efficient and affordable monitoring of bear populations, and suggests that the swabbing of food remains or consumed baits from other animals may be an additional cost-effective and valuable tool in the study of the ecology and population biology of many elusive and/or wide-ranging species.

A time-lapse photography method for monitoring salmon (Oncorhynchus spp.) passage and abundance in streams

Accurately estimating population sizes is often a critical component of fisheries research and management. Although there is a growing appreciation of the importance of small-scale salmon population dynamics to the stability of salmon stock-complexes, our understanding of these populations is constrained by a lack of efficient and cost-effective monitoring tools for streams. Weirs are expensive, labor intensive, and can disrupt natural fish movements. While conventional video systems avoid some of these shortcomings, they are expensive and require excessive amounts of labor to review footage for data collection. Here, we present a novel method for quantifying salmon in small streams (<15 m wide, <1 m deep) that uses both time-lapse photography and video in a model-based double sampling scheme. This method produces an escapement estimate nearly as accurate as a video-only approach, but with substantially less labor, money, and effort. It requires servicing only every 14 days, detects salmon 24 h/day, is inexpensive, and produces escapement estimates with confidence intervals. In addition to escapement estimation, we present a method for estimating in-stream salmon abundance across time, data needed by researchers interested in predator--prey interactions or nutrient subsidies. We combined daily salmon passage estimates with stream specific estimates of daily mortality developed using previously published data. To demonstrate proof of concept for these methods, we present results from two streams in southwest Kodiak Island, Alaska in which high densities of sockeye salmon spawn.