Importance of salmon to wildlife: Implications for integrated management

An updated end-to-end ecosystem model of the Northern California Current reflecting ecosystem changes due to recent marine heatwaves

The Northern California Current is a highly productive marine upwelling ecosystem that is economically and ecologically important. It is home to both commercially harvested species and those that are federally listed under the U.S. Endangered Species Act. Recently, there has been a global shift from single-species fisheries management to ecosystem-based fisheries management, which acknowledges that more complex dynamics can reverberate through a food web. Here, we have integrated new research into an end-to-end ecosystem model (i.e., physics to fisheries) using data from long-term ocean surveys, phytoplankton satellite imagery paired with a vertically generalized production model, a recently assembled diet database, fishery catch information, species distribution models, and existing literature. This spatially-explicit model includes 90 living and detrital functional groups ranging from phytoplankton, krill, and forage fish to salmon, seabirds, and marine mammals, and nine fisheries that occur off the coast of Washington, Oregon, and Northern California. This model was updated from previous regional models to account for more recent changes in the Northern California Current (e.g., increases in market squid and some gelatinous zooplankton such as pyrosomes and salps), to expand the previous domain to increase the spatial resolution, to include data from previously unincorporated surveys, and to add improved characterization of endangered species, such as Chinook salmon (Oncorhynchus tshawytscha) and southern resident killer whales (Orcinus orca). Our model is mass-balanced, ecologically plausible, without extinctions, and stable over 150-year simulations. Ammonium and nitrate availability, total primary production rates, and model-derived phytoplankton time series are within realistic ranges. As we move towards holistic ecosystem-based fisheries management, we must continue to openly and collaboratively integrate our disparate datasets and collective knowledge to solve the intricate problems we face. As a tool for future research, we provide the data and code to use our ecosystem model.

The Importance of Environmental Variability and Transient Population Dynamics for a Northern Ungulate

The pathways through which environmental variability affects population dynamics remain poorly understood, limiting ecological inference and management actions. Here, we use matrix-based population models to examine the vital rate responses to environmental variability and individual traits, and subsequent transient dynamics of the population in response to the environment. Using Sitka black-tailed deer (Odocoileus hemionus sitkensis) in Southeast Alaska as a study system, we modeled effects of inter-annual process variance of covariates on female survival, pregnancy rate, and fetal rate, and summer and winter fawn survival. To examine the influence of environmental variance on population dynamics, we compared asymptotic and transient perturbation analysis (elasticity analysis, a life-table response experiment, and transience simulation). We found that summer fawn survival was primarily determined by black bear (Ursus americanus) predation and was positively influenced by mass at birth and female sex. Winter fawn survival was determined by malnutrition in deep-snow winters and was influenced by an interaction between date of birth and snow depth, with late-born fawns at greater risk in deep-snow winters. Adult female survival was the most influential vital rate based on classic elasticity analysis, however, elasticity analysis based on process variation indicated that winter and summer fawn survival were most variable and thus most influential to variability in population growth. Transient dynamics produced by non-stable stage distributions produced realized annual growth rates different from predicted asymptotic growth rates in all years, emphasizing the importance of winter perturbations to population dynamics of this species.

Significance of anecdotes for historical perspective: black bear predation on sea turtle eggs

In his April 2010 TED talk on the shifting baseline syndrome, Daniel Pauly warned us that ‘we transform the world, but we don’t remember it.’ This lapse is the greatest obstacle to understanding and restoring the structure and function of ecosystems transformed by anthropogenic effects over past centuries or even over the past few decades. Historical anecdotes can be a powerful tool to address gaps in our knowledge of the past. I present a case study to demonstrate the use of anecdotes to reveal the extensive predation by black bears Ursus americanus on sea turtle eggs in Florida, USA. Until the late 1800s, bears were major predators on eggs deposited by the large sea turtle aggregations nesting on the east coast of Florida. However, this past source of mortality, and the resulting substantial transport of nutrients from marine to terrestrial habitats via the bears, are largely unknown today. By the early 1900s, the great influx of humans to the east coast of Florida quickly decimated the bear populations by hunting and habitat degradation. Without historical anecdotes, knowledge of the extensive predation by black bears on sea turtle eggs in Florida would have been lost.

Relationships between Pacific salmon and aquatic and terrestrial ecosystems: implications for ecosystem-based management

Pacific salmon influence temperate terrestrial and freshwater ecosystems through the dispersal of marine‐derived nutrients and ecosystem engineering of stream beds when spawning. They also support large fisheries, particularly along the west coast of North America. We provide a comprehensive synthesis of relationships between the densities of Pacific salmon and terrestrial and aquatic ecosystems, summarize the direction, shape, and magnitude of these relationships, and identify possible ecosystem‐based management indicators and benchmarks. We found 31 studies that provided 172 relationships between salmon density (or salmon abundance) and species abundance, species diversity, food provisioning, individual growth, concentration of marine‐derived isotopes, nutrient enhancement, phenology, and several other ecological responses. The most common published relationship was between salmon density and marine‐derived isotopes (40%), whereas very few relationships quantified ecosystem‐level responses (5%). Only 13% of all relationships tended to reach an asymptote (i.e., a saturating response) as salmon densities increased. The number of salmon killed by bears and the change in biomass of different stream invertebrate taxa between spawning and nonspawning seasons were relationships that usually reached saturation. Approximately 46% of all relationships were best described with linear or curved nonasymptotic models, indicating a lack of saturation. In contrast, 41% of data sets showed no relationship with salmon density or abundance, including many of the relationships with stream invertebrate and biofilm biomass density, marine‐derived isotope concentrations, or vegetation density. Bears required the highest densities of salmon to reach their maximum observed food consumption (i.e., 9.2 kg/m² to reach the 90% threshold of the relationship’s asymptote), followed by freshwater fish abundance (90% threshold = 7.3 kg/m² of salmon). Although the effects of salmon density on ecosystems are highly varied, it appears that several of these relationships, such as bear food consumption, could be used to develop indicators and benchmarks for ecosystem‐based fisheries management.

Dietary plasticity and the importance of salmon to brown bear (Ursus arctos) body size and condition in a low Arctic ecosystem

Ecological flexibility within animal populations can allow for variation in resource use and foraging decisions. We estimated brown bear (Ursus arctos) diet composition in Gates of the Arctic National Park and Preserve, Alaska from 2013 to 2015 to evaluate how variation in foraging behavior influences body condition and size. We used stable carbon (δ13C) and nitrogen (δ15N) isotope analyses of sectioned brown bear hair samples to evaluate assimilated diet. We then developed a set of a priori linear models to evaluate differences in the diet composition of brown bears (n = 80) in relation to body fat (%) and mass. The proportion of meat (salmon [Oncorhynchus keta] and terrestrial meat combined) in the diet from July through late September varied between male and female bears, with males ($$\stackrel{-}{x}$$ x - = 62%, SD = 30) assimilating significantly more meat than females ($$\stackrel{-}{x}$$ x - = 40%, SD = 29). Most of the meat consumed came from marine-derived resources for males (53% of the total diet or 86% of the meat) and females (31% of the total diet or 77% of the meat). As we found the range of observed diets was unrelated to physiological outcomes (i.e., percentage body fat), we suggest that ecological flexibility within populations may provide an adaptive advantage by allowing individuals to reduce competition with conspecifics by foraging on alternate food resources. Identifying variable foraging behaviors within a population can allow for a better understanding of complex behaviors and, ultimately, lead to more informed management decisions related to habitat use, development, and harvest.

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.

Direct and indirect effects of temperature and prey abundance on bald eagle reproductive dynamics

Understanding the mechanisms by which populations are regulated is critical for predicting the effects of large-scale perturbations. While discrete mortality events provide clear evidence of direct impacts, indirect pathways are more difficult to assess but may play important roles in population and ecosystem dynamics. Here, we use multi-state occupancy models to analyze a long-term dataset on nesting bald eagles in south-central Alaska with the goal of identifying both direct and indirect mechanisms influencing reproductive output in this apex predator. We found that the probabilities of both nest occupancy and success were higher in the portion of the study area where water turbidity was low, supporting the hypothesis that access to aquatic prey is a critical factor limiting the reproductive output of eagles in this system. As expected, nest success was also positively related to salmon abundance; however, the negative effect of spring warmth suggested that access to salmon resources is indirectly diminished in warm springs as a consequence of increased glacial melt. Together, these findings reveal complex interrelationships between a critical prey resource and large-scale weather and climate processes which likely alter the accessibility of resources rather than directly affecting resource abundance. While important for understanding bald eagle reproductive dynamics in this system specifically, our results have broader implications that suggest complex interrelationships among system components.

A Critical Assessment of the Ecological Assumptions Underpinning Compensatory Mitigation of Salmon-Derived Nutrients

We critically evaluate some of the key ecological assumptions underpinning the use of nutrient replacement as a means of recovering salmon populations and a range of other organisms thought to be linked to productive salmon runs. These assumptions include: (1) nutrient mitigation mimics the ecological roles of salmon, (2) mitigation is needed to replace salmon-derived nutrients and stimulate primary and invertebrate production in streams, and (3) food resources in rearing habitats limit populations of salmon and resident fishes. First, we call into question assumption one because an array of evidence points to the multi-faceted role played by spawning salmon, including disturbance via redd-building, nutrient recycling by live fish, and consumption by terrestrial consumers. Second, we show that assumption two may require qualification based upon a more complete understanding of nutrient cycling and productivity in streams. Third, we evaluate the empirical evidence supporting food limitation of fish populations and conclude it has been only weakly tested. On the basis of this assessment, we urge caution in the application of nutrient mitigation as a management tool. Although applications of nutrients and other materials intended to mitigate for lost or diminished runs of Pacific salmon may trigger ecological responses within treated ecosystems, contributions of these activities toward actual mitigation may be limited.

Using grizzly bears to assess harvest-ecosystem tradeoffs in salmon fisheries

Using grizzly bears as surrogates for “salmon ecosystem” function, the authors develop a generalizable ecosystem-based management framework that enables decision-makers to quantify ecosystem-harvest tradeoffs between wild and human recipients of natural resources like fish.

Implementation of ecosystem-based fisheries management (EBFM) requires a clear conceptual and quantitative framework for assessing how different harvest options can modify benefits to ecosystem and human beneficiaries. We address this social-ecological need for Pacific salmon fisheries, which are economically valuable but intercept much of the annual pulse of nutrient subsidies that salmon provide to terrestrial and aquatic food webs. We used grizzly bears, vectors of salmon nutrients and animals with densities strongly coupled to salmon abundance, as surrogates for “salmon ecosystem” function. Combining salmon biomass and stock-recruitment data with stable isotope analysis, we assess potential tradeoffs between fishery yields and bear population densities for six sockeye salmon stocks in Bristol Bay, Alaska, and British Columbia (BC), Canada. For the coastal stocks, we find that both bear densities and fishery yields would increase substantially if ecosystem allocations of salmon increase from currently applied lower to upper goals and beyond. This aligning of benefits comes at a potential cost, however, with the possibility of forgoing harvests in low productivity years. In contrast, we detect acute tradeoffs between bear densities and fishery yields in interior stocks within the Fraser River, BC, where biomass from other salmon species is low. There, increasing salmon allocations to ecosystems would benefit threatened bear populations at the cost of reduced long-term yields. To resolve this conflict, we propose an EBFM goal that values fisheries and bears (and by extension, the ecosystem) equally. At such targets, ecosystem benefits are unexpectedly large compared with losses in fishery yields. To explore other management options, we generate tradeoff curves that provide stock-specific accounting of the expected loss to fishers and gain to bears as more salmon escape the fishery. Our approach, modified to suit multiple scenarios, provides a generalizable method to resolve conflicts over shared resources in other systems.

Commercial fisheries that harvest salmon for human consumption can end up diverting nutrients that would normally be directed to terrestrial and aquatic ecosystems. We examined this problem for Pacific salmon fisheries by using grizzly bears as indicators of salmon ecosystem function. Bear densities vary enormously depending on salmon availability, and by leaving uneaten salmon carcass remains beside spawning streams, bears play an important role in dispersing marine nutrients to plants, invertebrates, and other wildlife. By relating the number of spawning fish to bear diet and density, we developed a model to quantify “ecosystem-harvest” tradeoffs; i.e., how bear density changes with the amount of fish harvested (fishery yields). We estimated this tradeoff between yields and bear density for six sockeye salmon stocks in Alaska and British Columbia (BC) across a range of management options that varied the number of salmon allowed to escape from the fishery. Our model shows that bear densities will increase substantially with more spawning fish at all sites. Notably, in most study systems, fishery yields are also expected to increase as the number of spawning fish increases. There is one exception, however, in the Fraser River (BC), where bears are threatened and sockeye salmon are nearly the only species of salmon available. Here, releasing more salmon to spawn would result in lower fishery yields. To resolve such conflicts in this and other systems, we propose a generalizable ecosystem-based fisheries management framework, which allows decision-makers (such as fisheries managers and conservation scientists) to evaluate different allocation options between fisheries and other ecosystem recipients.

Major components of grizzly bear diet across North America

We measured stable carbon and nitrogen isotope ratios in guard hair of 81 populations of grizzly bears (Ursus arctos L., 1758) across North America and used mixing models to assign diet fractions of salmon, meat derived from terrestrial sources, kokanee (Oncorhynchus nerka (Walbaum in Artedi, 1792)), and plants. In addition, we examined the relationship between skull size and diet of bears killed by people in British Columbia. The majority of carbon and nitrogen assimilated by most coastal grizzly bear populations was derived from salmon, while interior populations usually derived a much smaller fraction of their nutrients from salmon, even in areas with relatively large salmon runs. Terrestrial prey was a large part of the diet where ungulates were abundant, with the highest fractions observed in the central Arctic, where caribou (Rangifer tarandus (L., 1758)) were very abundant. Bears in some boreal areas, where moose (Alces alces (L., 1758)) were abundant, also ate a lot of meat. Bears in dryer areas with low snowfall tended to have relatively high meat diet fractions, presumably because ungulates are more abundant in such environments. Kokanee were an important food in central British Columbia. In areas where meat was more than about a third of the diet, males and females had similar meat diet fractions, but where meat was a smaller portion of the diet, males usually had higher meat diet fractions than females. Females reached 95% of their average adult skull length by 5 years of age, while males took 8 years. Skull width of male grizzly bears increased throughout life, while this trend was slight in females. Skull size increased with the amount of salmon in the diet, but the influence of terrestrial meat on size was inconclusive. We suggest that the amount of salmon in the diet is functionally related to fitness in grizzly bears.