Mark D. Scheuerell

Pacific salmon and the ecology of coastal ecosystems

One of the most spectacular phenomena in nature is the annual return of millions of salmon to spawn in their natal streams and lakes along the Pacific coast of North America. The salmon die after spawning, and the nutrients and energy in their bodies, derived almost entirely from marine sources, are deposited in the freshwater ecosystems. This represents a vital input to the ecosystems used as spawning grounds. Salmon-derived nutrients make up a substantial fraction of the plants and animals in aquatic and terrestrial habitats associated with healthy salmon populations. The decline of salmon numbers throughout much of their southern range in North America has prompted concern that the elimination of this “conveyor belt” of nutrients and energy may fundamentally change the productivity of these coastal freshwater and terrestrial ecosystems, and consequently their ability to support wildlife, including salmon. If progress is to be made towards understanding and conserving the connection between migratory sa...

DIEL VERTICAL MIGRATION BY JUVENILE SOCKEYE SALMON: EMPIRICAL EVIDENCE FOR THE ANTIPREDATION WINDOW

Diel vertical migration (DVM) is a widespread phenomenon in aquatic organisms, yet the adaptive significance of this behavior is still unclear. In particular, diel vertical migration by juvenile sockeye salmon (Oncorhynchus nerka) has received considerable attention. We studied how changes in the light environment affect juvenile sockeye DVM in Alaskan lakes through changes in foraging rates and predation risk. Using hydroacoustics to track temporal changes in fish distribution, we found clear patterns of DVM and a strong, significant correlation between the mean depth of the sockeye and the amount of light at the lake surface. However, we could not detect diel changes in the depth distribution of fish large enough to be sockeye predators. Given a lack of diel vertical migration in the zooplankton community, it appears that juvenile sockeye were not simply tracking their food supply. Calculations of the in situ light experienced by individual juvenile sockeye suggested that they migrate to maintain a constant light environment. This light environment allowed the sockeye to exploit an antipredation window whereby they could forage on zooplankton while reducing the odds of visual detection by their predators. Furthermore, this antipredation window was continuous in early summer, but was split into two discrete time periods during the crepuscular hours in late summer. These data support the hypothesis that changes in habitat use by juvenile sockeye salmon reflect a dynamic strategy to minimize the ratio of predation risk to foraging gain that changes dramatically over the course of diel cycles in pelagic ecosystems.

EFFECTS OF CHANGING CLIMATE ON ZOOPLANKTON AND JUVENILE SOCKEYE SALMON GROWTH IN SOUTHWESTERN ALASKA

Detecting and forecasting the effects of changing climate on natural and exploited populations represent a major challenge to ecologists and resource managers. These efforts are complicated by underlying density-dependent processes and the differ- ential responses of predators and their prey to changing climate. We explored the effects of density-dependence and changing climate on growth of juvenile sockeye salmon and the densities of their zooplankton prey in the Wood River system of southwestern Alaska. We fit dynamic time-series models to data collected between 1962 and 2002 describing growth of juvenile sockeye, timing of spring ice breakup, and summer zooplankton densities. The timing of spring breakup has moved about seven days earlier now than it was in the early 1960s. Our analyses suggest that most of this shift has been a response to the warm phase of the Pacific Decadal Oscillation that persisted from the mid-1970s to the late 1990s. This progression toward earlier spring breakup dates was associated with warmer summer water temperatures and increased zooplankton (especially Daphnia) densities, which translated into increased sockeye growth during their first year of life. The number of spawning adults that produced each year class of sockeye had a strong negative effect on juvenile sockeye growth rates, so that the size of the density-dependent effect was, on average, twice as large as the effect of spring breakup date. These results highlight the complexity of eco- logical responses to changing climate and suggest that climate warming may enhance growing conditions for juvenile salmonids in large lakes of Alaska.

Changes in the Spatial Distribution of Fishes in Lakes Along a Residential Development Gradient

As the human demand for freshwater natural resources such as fish and drinking water increases, we may rely more heavily on models to predict the response of aquatic ecosystems to natural and anthropogenic disturbance. Theses models in turn implicitly depend on the underlying spatial distribution of organisms. In terrestrial ecosystems, increased natural resource utilization has transformed habitat and changed the spatial distribution of organisms, with subsequent negative effects on biota. Recent studies in lakes demonstrate that human development of lakeshores alters the physical habitat and nutrient cycles. The impact of such disturbance by humans on the spatial distribution of aquatic organisms, however, remains unknown. Here we quantify the effect of lakeshore development on the spatial distribution of fishes in 23 lakes in the US Pacific Northwest. We found a significant decrease in the spatial aggregation of fishes with increased shoreline development by humans, reflecting a loss of refugia and resource heterogeneity that favors aggregation among fishes. We also found that lakes with a high perimeter–surface-area ratio and a relatively shallow littoral zone had much higher levels of fish aggregation, suggesting the importance of terrestrial inputs to lakes. Finally, we found a marginally significant decrease in fish spatial aggregation with increased total phosphorus concentration, but no effect of chlorophyll concentration, water transparency, the predator–prey ratio, or number of species on fish spatial distributions. These results suggest that anthropogenic modification of shorelines is significantly altering the spatial distribution of important aquatic organisms, and that these changes may have important implications for predictive modeling of ecosystem dynamics.

Disturbance of freshwater habitats by anadromous salmon in Alaska

High densities of habitat modifiers can dramatically alter the structure of ecosystems. Whereas spawning sockeye salmon (Oncorhynchus nerka) dig nests that cover over 2xa0m2 and are at least 20xa0cm deep, and can spawn at high densities, relatively little attention has been devoted to investigating the impacts of this disturbance. We hypothesized that this temporally and spatially predictable bioturbation has large impacts on the coastal aquatic habitats used by sockeye. We experimentally investigated the impacts of disturbance caused by spawning sockeye in two streams and two lakes in Alaska by excluding salmon from 2.25xa0m2 plots where they traditionally spawn. We sampled exclusions and control plots before, during, and after spawning. During sockeye spawning, fine sediment accumulated in areas where sockeye were excluded from spawning. In addition, sockeye spawning significantly decreased algal biomass by 80% compared to exclusion plots. We found mixed effects of spawning on the invertebrate assemblage. Tricladida and Chironomidae densities increased by 3x in exclusion plots relative to control plots in one creek site. However, for most taxa and sites, invertebrate densities declined substantially as spawning progressed, regardless of experimental treatment. Habitat modification by spawning salmon alters both community organization and ecosystem processes.

Habitat structure determines resource use by zooplankton in temperate lakes

While the importance of terrestrial linkages to aquatic ecosystems is well appreciated, the degree of terrestrial support of aquatic consumers remains debated. Estimates of terrestrial contributions to lake zooplankton have omitted a key food source, phytoplankton produced below the mixed layer. We used carbon and nitrogen stable isotope data from 25 Pacific Northwest lakes to assess the relative importance of particulate organic matter (POM) from the mixed layer, below the mixed layer and terrestrial detritus to zooplankton. Zooplankton and deep POM were depleted in ¹³C relative to mixed layer POM in lakes that can support deep primary production. A Bayesian stable isotope mixing model estimated that terrestrial detritus contributed <5% to zooplankton production, and confirms the role of lake optical and thermal properties; deep POM accounted for up to 80% of zooplankton production in the clearest lakes. These results suggest terrestrial support of lake zooplankton production is trivial.

Evolutionary responses by native species to major anthropogenic changes to their ecosystems: Pacific salmon in the Columbia River hydropower system

The human footprint is now large in all the Earths ecosystems, and construction of large dams in major river basins is among the anthropogenic changes that have had the most profound ecological consequences, particularly for migratory fishes. In the Columbia River basin of the western USA, considerable effort has been directed toward evaluating demographic effects of dams, yet little attention has been paid to evolutionary responses of migratory salmon to altered selective regimes. Here we make a first attempt to address this information gap. Transformation of the free‐flowing Columbia River into a series of slack‐water reservoirs has relaxed selection for adults capable of migrating long distances upstream against strong flows; conditions now favour fish capable of migrating through lakes and finding and navigating fish ladders. Juveniles must now be capable of surviving passage through multiple dams or collection and transportation around the dams. River flow patterns deliver some groups of juvenile salmon to the estuary later than is optimal for ocean survival, but countervailing selective pressures might constrain an evolutionary response toward earlier migration timing. Dams have increased the cost of migration, which reduces energy available for sexual selection and favours a nonmigratory life history. Reservoirs are a benign environment for many non‐native species that are competitors with or predators on salmon, and evolutionary responses are likely (but undocumented). More research is needed to tease apart the relative importance of evolutionary vs. plastic responses of salmon to these environmental changes; this research is logistically challenging for species with life histories like Pacific salmon, but results should substantially improve our understanding of key processes. If the Columbia River is ever returned to a quasinatural, free‐flowing state, remaining populations might face a Darwinian debt (and temporarily reduced fitness) as they struggle to re‐evolve historical adaptations.

Using Time Series Analysis to Characterize Evolutionary and Plastic Responses to Environmental Change: A Case Study of a Shift toward Earlier Migration Date in Sockeye Salmon

Environmental change can shift the phenotype of an organism through either evolutionary or nongenetic processes. Despite abundant evidence of phenotypic change in response to recent climate change, we typically lack sufficient genetic data to identify the role of evolution. We present a method of using phenotypic data to characterize the hypothesized role of natural selection and environmentally driven phenotypic shifts (plasticity). We modeled historical selection and environmental predictors of interannual variation in mean population phenotype using a multivariate state-space model framework. Through model comparisons, we assessed the extent to which an estimated selection differential explained observed variation better than environmental factors alone. We applied the method to a 60-year trend toward earlier migration in Columbia River sockeye salmon Oncorhynchus nerka, producing estimates of annual selection differentials, average realized heritability, and relative cumulative effects of selection and plasticity. We found that an evolutionary response to thermal selection was capable of explaining up to two-thirds of the phenotypic trend. Adaptive plastic responses to June river flow explain most of the remainder. This method is applicable to other populations with time series data if selection differentials are available or can be reconstructed. This method thus augments our toolbox for predicting responses to environmental change.

Exotic species in the Hudson River basin: A history of invasions and introductions

We compiled information about the distribution of exotic organisms in the fresh waters of the Hudson River basin. At least 113 nonindigenous species of vertebrates, vascular plants, and large invertebrates have established populations in the basin. Too little was known about the past or present distributions of algae and most small invertebrates to identify exotic species in these groups. Most established exotic species in the Hudson River basin originated from Eurasia or the Mississippi-Great Lakes basins, and were associated with vectors such as unintentional, releases (especially escapes from cultivation), shipping activities (especially, solid ballast or ballast water), canals, or intentional releases. Rates of species invasions of fresh and oligohaline waters in the basin have been high (ca. one new species per year) since about 1840. For many well-studied groups, introduced species constitute 4% to nearly 60% of the species now in the basin. Although the ecological impacts of the invaders in the Hudson River basin have not been well studied, we believe that about 10% of the exotic species, have had major ecological impacts in the basin. Since, the rates, of entry and composition of exotic species in the Hudson basin are similar to those observed., previously for the Laurentian Great Lakes, invasions tended to occur earlier in the Hudson basin, probably reflecting the earlier history of human commerce. While most exotics have had negative impacts on local flora and fauna, some fish species have provided unique angling opportunities and important economic benefits.

Big dams and salmon evolution: changes in thermal regimes and their potential evolutionary consequences

Dams designed for hydropower and other purposes alter the environments of many economically important fishes, including Chinook salmon (Oncorhynchus tshawytscha). We estimated that dams on the Rogue River, the Willamette River, the Cowlitz River, and Fall Creek decreased water temperatures during summer and increased water temperatures during fall and winter. These thermal changes undoubtedly impact the behavior, physiology, and life histories of Chinook salmon. For example, relatively high temperatures during the fall and winter should speed growth and development, leading to early emergence of fry. Evolutionary theory provides tools to predict selective pressures and genetic responses caused by this environmental warming. Here, we illustrate this point by conducting a sensitivity analysis of the fitness consequences of thermal changes caused by dams, mediated by the thermal sensitivity of embryonic development. Based on our model, we predict Chinook salmon likely suffered a decrease in mean fitness after the construction of a dam in the Rogue River. Nevertheless, these demographic impacts might have resulted in strong selection for compensatory strategies, such as delayed spawning by adults or slowed development by embryos. Because the thermal effects of dams vary throughout the year, we predict dams impacted late spawners more than early spawners. Similar analyses could shed light on the evolutionary consequences of other environmental perturbations and their interactions.