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USGS2013

Snake River fall Chinook salmon life history investigations: Annual report 2011 (April 2011 - March 2012)

Executive Summary Chapter One – This chapter was published in the Transactions of the American Fisheries Society in 2012. We conducted a three-year radiotelemetry study in the lower Snake River to answer the questions: do fall Chinook salmon juveniles pass dams during winter when bypass systems and structures designed to prevent mortality are not operated; does downstream movement rate vary annually, seasonally, and from reservoir to reservoir; and, what are some of the factors that contribute to annual, seasonal, and spatial variation in downstream movement rate? Fall Chinook salmon juveniles moved downstream up to 169 km and fast enough (7.5 km/d) such that large percentages (up to 93%) of the fish passed one or more dams during winter. Mean downstream movement rate varied annually (9.2-11.3 km/d), increased from winter (7.5 km/d) to spring (16.4 km/d), and increased (6.9-16.8 km/d) as fish moved downstream from reservoir to reservoir. Fish condition factor at tagging explained some of the annual variation ( P ≤ 0.01) in downstream movement rate, whereas water particle velocity ( P ≤0.0001) and temperature ( P ≤0.0001) explained portions of the seasonal variation. An increase in migrational disposition as fish moved downstream helped explain the spatial variation ( P =0.05-0.07). The potential cost of winter movement might be reduced survival due to turbine passage when the bypass systems and spillway passage structures are not operated. Efforts to understand and increase passage survival of winter migrants in large impoundments might help to rehabilitate some imperiled anadromous salmonid populations. Chapter Two – Natural juvenile fall Chinook salmon in the Snake and Clearwater rivers exhibit two life history strategies. “Ocean-type” fish migrate out to the ocean in their first summer of life as subyearlings, but “reservoir-type” fish delay seaward migration during the summer, and some overwinter in reservoirs before continuing their migration the following spring as yearlings. Earlier emerging fish produced in the Snake River tend to adopt the ocean-type life history whereas many of the later emerging fish from the Clearwater River tend to adopt the reservoirtype life history. The underlying cause of the reservoir-type life history is poorly understood, but we believe there may be link to physiological development. We used traditional markers of the parr-smolt transformation (smoltification), including gill Na + /K + -ATPase activity and thyroid hormone levels, along with gene expression microarrays to assess the development of ocean-type juvenile fall Chinook salmon and then compared it to that of juvenile fall Chinook salmon from the Clearwater River. We showed that parr in the Snake River are physiologically distinct from actively-migrating smolts but smolts migrating early and late in the summer and fall are physiologically similar. Juvenile fall Chinook salmon collected from the Clearwater River were similar in size to early-migrating smolts in the Snake River but were most physiologically similar to Snake River parr. Genes differentially expressed between Snake River parr and smolts and between fish from the Clearwater River and smolts from the Snake River were involved in the cell cycle, steroid metabolism and other metabolic pathways, and DNA repair and packaging. Many of the genes differentially expressed in these comparisons had expression patterns that correlated with gill Na + /K + -ATPase activity, suggesting that they were related to smoltification and migration status. Chapter Three – Natural subyearlings produced in the Clearwater River are exposed to cool (~10-12°C) temperatures when water is released from Dworshak Reservoir for summer flow augmentation. Total dissolved gas (TDG) levels range from 100-110% in the lower Clearwater iv River. When fish move into the Snake River, they encounter temperatures up to 24°C at the surface which have the potential to incur gas bubble disease (GBD) in fish as dissolved gases in their bodies expand under warmer temperatures. This may result in both direct and indirect mortality, but this situation has been little studied. We conducted laboratory experiments to examine subyearling mortality rates and incidence and severity of GBD in fish that were moved between waters that varied in TDG and temperature. Fish experienced significant mortality only at temperatures of 25°C, which increased with exposure time. However there was no significance difference in mortality between fish acclimated to 100% TDG and 110% TDG. Fish that died did show signs of GBD. Generally, signs of GBD such as bubbles in the lateral line and unpaired fins were higher in fish acclimated at 110% TDG than in fish acclimated at 100% TDG, but there were few trends related to exposure temperature. Field measurements of TDG showed that TDG ranged from about 100% to 122.5% at some locations. Generally, TDG fluctuated daily, up to 8% during August and early September, and was highest late in the afternoon and lowest in the early morning. Laboratory results and field monitoring demonstrated that emigrating juvenile salmon can potentially be at risk from elevated temperatures, TDG, and GBD albeit to an unknown extent, which may increase their vulnerability to predation. Chapter Four – We conducted monthly beam trawling in Lower Granite and Little Goose reservoirs to describe the seasonal abundance of benthic epifauna that are potentially important as prey to juvenile fall Chinook salmon. The predominant taxa collected were Siberian prawns, the opossum shrimp Neomysis mercedis , and the amphipod Corophium sp. Prawns were relatively abundant at shallow sites in both reservoirs in June, but were more abundant at deep sites in lower and middle reservoir reaches in autumn. Prawn densities were commonly <0.2/m2. Prawn length-frequency data indicated that there were at least two size classes. Juvenile prawns present in shallow water more often than adult prawns, which were generally only found in deep water by autumn. Ovigerous prawns had an average of 171 eggs, which represented about 11.5% of their body weight. Limited diet analyses suggested that prawns consumed Corophium , Neomysis , and aquatic insects. Neomysis dominated all catches both in terms of abundance and biomass, and they were more abundant in Lower Granite compared to Little Goose reservoir. Neomysis were more abundant at shallow sites than at deep sites. Corophium were present in our collections but were never abundant, probably because our trawl was not effective at capturing them. The caloric content of prawns (4,782 Kcal), Neomysis (4,962 Kcal), and Corophium (4,926 Kcal) indicates that these prey would be energetically profitable for juvenile salmon. Subyearling fall Chinook salmon prey heavily on Neomysis and Corophium at times, but the importance of prawns as prey is uncertain.

Oregon, Washington↗

USGS2009

Osprey: Worldwide sentinel species for assessing and monitoring environmental contamination in rivers, lakes, reservoirs, and estuaries

In the United States, many fish and wildlife species have been used nationwide to monitor environmental contaminant exposure and effects, including carcasses of the bald eagle ( Haliaeetus leucocephalus ), the only top avian predator regularly used in the past. Unfortunately, bald eagles are sensitive to investigator intrusion at the nest. Thus, the osprey ( Pandion haliaetus ) is evaluated as a potential sentinel species for aquatic ecosystems. Several characteristics support the choice of the osprey as a sentinel species, including: (1) fish-eating diet atop the aquatic food web, (2) long-lived with strong nest fidelity, (3) adapts to human landscapes (potentially the most contaminated), (4) tolerates short-term nest disturbance, (5) nests spatially distributed at regular intervals, (6) highly visible nests easily located for study, (7) ability to accumulate most, if not all, lipophilic contaminants, (8) known sensitivity to many contaminants, and (9) nearly a worldwide distribution. These osprey traits have been instrumental in successfully using the species to understand population distribution, abundance, and changes over time; the effects of various contaminants on reproductive success; how contaminants in prey (fish on biomass basis) contribute to egg concentrations (i.e., biomagnification factors); and spatial residue patterns. Data summarized include nesting population surveys, detailed nesting studies, and chemical analyses of osprey egg, organ, blood, and feather samples for contaminants that bioaccumulate and/or biomagnify in aquatic food webs; and biochemical evaluations of blood and various organs. Studies in the United States, Canada, Mexico, Europe, and elsewhere have shown the osprey to be a useful sentinel species for monitoring selected environmental contaminants, including some emerging contaminants in lakes, reservoirs, rivers, and estuaries.

Journal of Toxicology and Environmental Health, Pa↗

USGS2019

Changes in adult Chinook salmon (Oncorhynchus tshawytscha) survival within the lower Columbia River amid increasing pinniped abundance

Significant effort towards conservation has contributed to the recovery of historically depleted pinniped populations world-wide. However, in several locations where pinnipeds have increased, they have been blamed for preventing the recovery of commercially valuable fish species through predation. Prompted by increasing pinniped abundance within the Columbia River (CR) USA, over a six year period, we used Passive Integrated Transponder tags to measure the survival of adult spring-run Chinook salmon (Oncorhynchus tshawytscha) through the estuary and lower CR to Bonneville Dam (Rkm 234). We estimated 51 751 - 224 705 salmon died annually from sources other than harvest. Mixed-effects logistic regression modelling identified pinniped predation as the most likely source. The odds of survival was estimated to decrease by 32% (95% CI: 6%-51% decrease) for every additional 467 sea lions, and to increase by 32% (95% CI: 8%-61% increase) for every increase of 1.5 in the log of American shad (Alosa sapidissima), a potential prey item for sea lions. A third covariate was the adipose clip status of the fish, indicating whether it was eligible for harvest.

Oregon, Washington↗

USGS1992

Functional response and capture timing in an individual-based model: predation by northern squawfish (Ptychocheilus oregonensis) on juvenile salmonids in the Columbia River

The behavior of individual northern squawfish ( Ptychocheilus oregonensis ) preying on juvenile salmonids was modeled to address questions about capture rate and the timing of prey captures (random versus contagious). Prey density, predator weight, prey weight, temperature, and diel feeding pattern were first incorporated into predation equations analogous to Holling Type 2 and Type 3 functional response models. Type 2 and Type 3 equations fit field data from the Columbia River equally well, and both models predicted predation rates on five of seven independent dates. Selecting a functional response type may be complicated by variable predation rates, analytical methods, and assumptions of the model equations. Using the Type 2 functional response, random versus contagious timing of prey capture was tested using two related models. ln the simpler model, salmon captures were assumed to be controlled by a Poisson renewal process; in the second model, several salmon captures were assumed to occur during brief "feeding bouts", modeled with a compound Poisson process. Salmon captures by individual northern squawfish were clustered through time, rather than random, based on comparison of model simulations and field data. The contagious-feeding result suggests that salmonids may be encountered as patches or schools in the river.

Canadian Journal of Fisheries and Aquatic Sciences↗

USGS2000

Effects of feeding ration on larval swimming speed and responsiveness to predator attacks: Implications for cohort survival

We conducted laboratory experiments to examine the effects of feeding ration on the routine swimming speed of larval striped bass (Morone saxatilis) and their responsiveness to simulated-predator attacks. Striped bass were reared in low (7 prey ?? L-1), medium (354 prey ?? L-1), or high (740 prey ?? L-1) prey treatments from age 4 to 14 days posthatch. Larvae reared in the low-prey treatment had slower routine swimming speeds and shorter reactive distances and were less responsive to simulated-predator attacks. These differences were most pronounced after age 10 and appeared to be an effect of deteriorating larval condition rather than an effect of size. Simulation models were constructed for two potential fish predators, Alosa aestivalis and Pomoxis nigromaculatus, to examine how variation in growth rate, swimming speed, and responsiveness to predator attacks might influence mortality rate. Our simulations predicted that cohort mortality rate would decrease with increasing larval growth rates, even though faster routine swimming speed and growth rate increased encounter rates with predators. The influence of larval growth rate and responsiveness on mortality rate varied between the two predators, but cohorts experiencing no growth always had the greatest mortality rate.

Canadian Journal of Fisheries and Aquatic Sciences↗

USGS2017

Incorporating food web dynamics into ecological restoration: A modeling approach for river ecosystems

Restoration is frequently aimed at the recovery of target species, but also influences the larger food web in which these species participate. Effects of restoration on this broader network of organisms can influence target species both directly and indirectly via changes in energy flow through food webs. To help incorporate these complexities into river restoration planning we constructed a model that links river food web dynamics to in-stream physical habitat and riparian vegetation conditions. We present an application of the model to the Methow River, Washington (USA), a location of on-going restoration aimed at recovering salmon. Three restoration strategies were simulated: riparian vegetation restoration, nutrient augmentation via salmon carcass addition, and side-channel reconnection. We also added populations of nonnative aquatic snails and fish to the modeled food web to explore how changes in food web structure mediate responses to restoration. Simulations suggest that side-channel reconnection may be a better strategy than carcass addition and vegetation planting for improving conditions for salmon in this river segment. However, modeled responses were strongly sensitive to changes in the structure of the food web. The addition of nonnative snails and fish modified pathways of energy through the food web, which negated restoration improvements. This finding illustrates that forecasting responses to restoration may require accounting for the structure of food webs, and that changes in this structure—as might be expected with the spread of invasive species—could compromise restoration outcomes. Unlike habitat-based approaches to restoration assessment that focus on the direct effects of physical habitat conditions on single species of interest, our approach dynamically links the success of target organisms to the success of competitors, predators, and prey. By elucidating the direct and indirect pathways by which restoration affects target species, dynamic food web models can improve restoration planning by fostering a deeper understanding of system connectedness and dynamics.

Washington↗

USGS2007

Consumption estimates of walleye stocked as fry to suppress fathead minnow populations in west-central Minnesota wetlands

Fisheries managers throughout the Prairie Pothole Region of Minnesota often use semi-permanent and permanent wetland basins to extensively culture walleye Sander vitreus fry. Waterfowl managers have expressed concern over this practice because of the potential influence that fish have on food resources used by waterfowl during development and migration. It is well known that native fathead minnows Pimephales promelas can have detrimental effects on macroinvertebrates, zooplankton, water clarity, epiphyton, and macrophytes in wetlands. Because walleye commonly become piscivorous as soon as mouth gape allows, walleye fry may suppress fathead minnow populations and improve wetland conditions for waterfowl. In this study, we quantify consumption estimates, specifically predation on fathead minnows, by age-0 and age-1 walleye reared in natural wetland basins. Six wetlands were stocked in mid-May 2001 and 2002 at a rate of 12,000 walleye fry ha-1. Age-0 walleye were sampled bi-weekly from mid-June through mid-September 2001. Age-0 and age-1 walleye were sampled monthly from mid-May through mid-September 2002. A generalised diet shift from zooplankton to fish to macroinvertebrates was observed in 2001, whereas diets of juvenile walleye contained primarily macroinvertebrates in 2002. Stocked walleye quickly reduced fathead minnow populations in 2001 and suppression was maintained throughout 2002. Although walleye consumed primarily macroinvertebrates once prey fish populations became suppressed, consumption estimates of invertebrates by walleye were substantially less than those documented for fathead minnow populations. Thus, stocking age-0 walleye was an effective biomanipulation tool that substantially reduced fathead minnow densities and influenced lower trophic levels in these aquatic communities. ?? 2007 Blackwell Munksgaard.

Ecology of Freshwater Fish↗

USGS2013

Seventy-five years of science—The U.S. Geological Survey’s Western Fisheries Research Center

As of January 2010, 75 years have elapsed since Dr. Frederic Fish initiated the pioneering research program that would evolve into today’s Western Fisheries Research Center (WFRC). Fish began his research working alone in the basement of the recently opened Fisheries Biological Laboratory on Lake Union in Seattle, Washington. WFRC’s research began under the aegis of the U.S. Fish and Wildlife Service and ends its first 75 years as part of the U.S. Geological Survey with a staff of more than 150 biologists and support personnel and a heritage of fundamental research that has made important contributions to our understanding of the biology and ecology of the economically important fish and fish populations of the Nation. Although the current staff may rarely stop to think about it, WFRC’s antecedents extend many years into the past and are intimately involved with the history of fisheries conservation in the Western United States. Thus, WFRC Director Lyman Thorsteinson asked me to write the story of this laboratory “while there are still a few of you around who were here for some of the earlier years” to document the rich history and culture of WFRC by recognizing its many famous scientists and their achievements. This historyalso would help document WFRC’s research ‘footprint’ in the Western United States and its strategic directions. Center Director Thorsteinson concluded that WFRC’s heritage told by an emeritus scientist also would add a texture of legitimacy based on personal knowledge that will all-to-soon be lost to the WFRC and to the USGS. The WFRC story is important for the future as well as for historical reasons. It describes how we got to the place we are today by documenting the origin, original mission, and our evolving role in response to the constantly changing technical information requirements of new environmental legislation and organizational decision-making. The WFRC research program owes its existence to the policy requirements of Federal conservation legislation originating with the construction of Grand Coulee Dam in 1933. The research program was shaped by laws enacted in subsequent years such as the Federal Water Pollution Control Act (1972), National Environmental Policy Act (1973), Endangered Species Act (1974), and Northwest Power Planning Act (1980), to name only a few. The WFRC has not been constrained by direct management or regulatory responsibility for a particular fishery (such as providing sustainable catch limits data to a resource management structure). Thus, WFRC has been able to concentrate on scientific pursuits and information needs required by contemporary environmental legislation. Over the years, we have pioneered in several important areas of fisheries research including the diagnoses and control of diseases in economically important fish, effects of environmental alterations on the physiological quality and survival of Pacific salmon released from federal mitigation hatcheries, applications in biotelemetry, and the bioenergetics of predator-prey interactions in the Columbia River. The WFRC of today is a widely distributed organization in the Western United States. Knowledge of the historical connections and accomplishments of our predecessors is important beyond the sense of pride and unity it instills in the WFRC family of today. For example, a discerning reader will note the evolution of WFRC’s research from a single disciplinary focus (early era—hatchery disease problems), to multiple disciplines (middle to late era—species, populations, habitats; threatened and endangered species), to the present era (multidisciplinary and with increasing process focus). For the benefit of the current WFRC staff, more emphasis has been placed on the early years rather than on the present day because people are quite naturally more familiar with the recent past than with the research done during the first decades of WFRC’s existence. By every rational measure, the WFRC has evolved into a fisheries research organization well positioned to provide the biological information needed to support the continued conservation and management of our Nation’s living aquatic natural resources. The high standard of excellence that connects WFRC’s past to our present research program provides a firm foundation on which to base the work yet to be done. In another 75 years, WFRC will undoubtedly be a very different place than it is today, but its evolution will be forever rooted in the story of the research and of the people related here. More about the diverse fisheries research projects WFRC scientists are conducting today is available at WFRC’s website: http://wfrc.usgs.gov/.

Washington↗

USGS2006

Patterns of fish use and piscivore abundance within a reconnected saltmarsh impoundment in the northern Indian River Lagoon, Florida

Nearly all saltmarshes in east-central, Florida were impounded for mosquito control during the 1960s. The majority of these marshes have since been reconnected to the estuary by culverts, providing an opportunity to effectively measure exchange of aquatic organisms. A multi-gear approach was used monthly to simultaneously estimate fish standing stock (cast net), fish exchange with the estuary (culvert traps), and piscivore abundance (gill nets and bird counts) to document patterns of fish use in a reconnected saltmarsh impoundment. Changes in saltmarsh fish abundance, and exchange of fish with the estuary reflected the seasonal pattern of marsh flooding in the northern Indian River Lagoon system. During a 6-month period of marsh flooding, resident fish had continuous access to the marsh surface. Large piscivorous fish regularly entered the impoundment via creeks and ditches to prey upon small resident fish, and piscivorous birds aggregated following major fish movements to the marsh surface or to deep habitats. As water levels receded in winter, saltmarsh fish concentrated into deep habitats and emigration to the estuary ensued (200% greater biomass left the impoundment than entered). Fish abundance and community structure along the estuary shoreline (although fringed with marsh vegetation) were not analogous to marsh creeks and ditches. Perimeter ditches provided deep-water habitat for large estuarine predators, and shallow creeks served as an alternative habitat for resident fish when the marsh surface was dry. Use of the impoundment as nursery by transients was limited to Mugil cephalus Linnaeus, but large juvenile and adult piscivorous fish used the impoundment for feeding. In conclusion, the saltmarsh impoundment was a feeding site for piscivorous fish and birds, and functioned as a net exporter of forage fish to adjacent estuarine waters. ?? Springer 2006.

Wetlands Ecology and Management↗

USGS1985

Lake Michigan's capacity to support lake trout (Salvelinus namaycush) and other salmonines: an estimate based on the status of prey populations in the 1970s

We used a mass balance equation relating total mortality of age II and older alewives ( Alosa pseudoharengus ) to their removals by predatory fish and other sources of mortality as the basis for estimating that the forage base in Lake Michigan could support an additional 13&ensp;000 to 21&ensp;000&ensp; t of lake trout ( Salvelinus namaycush ) or a total lake trout biomass between 15&ensp;000 and 23&ensp;000&ensp; t . This biomass estimate is projected with biomasses of other trout and salmon held at 1979 levels. Major premises of this approach are that (1) the sustained availability of age II and older alewives to salmonines will ultimately limit the expansion of salmonine stocks, (2) the alewife population was oscillating without trend during 1973–80, and (3) additional limited predation on alewives would be compensated by a reduction in natural mortality caused by physiological stress and disease.

Canadian Journal of Fisheries and Aquatic Sciences↗

USGS2017

Behavioral response of giant gartersnakes (Thamnophis gigas) to the relative availability of aquatic habitat on the landscape

Most extant giant gartersnake ( Thamnophis gigas ) populations persist in an agro-ecosystem dominated by rice, which serves as a surrogate to the expansive marshes lost to flood control projects and development of the Great Central Valley of California. Knowledge of how giant gartersnakes use the rice agricultural landscape, including how they respond to fallowing, idling, or crop rotations, would greatly benefit conservation of giant gartersnakes by informing more snake-friendly land and water management practices. We studied adult giant gartersnakes at 11 sites in the rice-growing regions of the Sacramento Valley during an extended drought in California to evaluate their response to differences in water availability at the site and individual levels. Although our study indicated that giant gartersnakes make little use of rice fields themselves, and avoid cultivated rice relative to its availability on the landscape, rice is a crucial component of the modern landscape for giant gartersnakes. Giant gartersnakes are strongly associated with the canals that supply water to and drain water from rice fields; these canals provide much more stable habitat than rice fields because they maintain water longer and support marsh-like conditions for most of the giant gartersnake active season. Nonetheless, our results suggest that maintaining canals without neighboring rice fields would be detrimental to giant gartersnake populations, with decreases in giant gartersnake survival rates associated with less rice production in the surrounding landscape. Increased productivity of prey populations, dispersion of potential predators across a larger landscape, and a more secure water supply are just some of the mechanisms by which rice fields might benefit giant gartersnakes in adjacent canals. Results indicate that identifying how rice benefits giant gartersnakes in canals and the extent to which the rice agro-ecosystem could provide these benefits when rice is fallowed would inform the use of water for other purposes without harm to giant gartersnakes. Our study also suggests that without such understanding, maintaining rice and associated canals in the Sacramento Valley is critical for the sustainability of giant gartersnake populations.

California↗

USGS2006

Long-term trends of bloater ( Coregonus hoyi ) recruitment in Lake Michigan: evidence for the effect of sex ratio

Long-term population trends are generally explained by factors extrinsic (e.g., climate, predation) rather than intrinsic (e.g., genetics, maternal effects) to the population. We sought to understand the long-term population dynamics of an important native Lake Michigan prey fish, the bloater Coregonus hoyi . Over a 38-year time series, three 10- to 15-year phases occurred (poor, excellent, and then poor recruitment) without high interannual variability within a particular phase. We used dynamic linear models to determine whether extrinsic (winter and spring temperature, alewife predator densities) or intrinsic factors (population egg production, adult condition, adult sex ratio) explained variation in recruitment. Models that included population egg production, sex ratio, winter and spring temperature, and adult bloater condition explained the most variation. Of these variables, sex ratio, which ranged from 47% to 97% female across the time series, consistently had the greatest effect: recruitment declined with female predominance. Including biomass of adult alewife predators in the models did not explain additional variation. Overall our results indicated that bloater recruitment is linked to its sex ratio, but understanding the underlying mechanisms will require additional efforts.

Canadian Journal of Fisheries and Aquatic Sciences↗

USGS2012

Structure of marine predator and prey communities along environmental gradients in a glaciated fjord

Spatial patterns of marine predator communities are influenced to varying degrees by prey distribution and environmental gradients. We examined physical and biological attributes of an estuarine fjord with strong glacier influence to determine the factors that most influence the structure of predator and prey communities. Our results suggest that some species, such as walleye pollock ( Theragra chalcogramma ), black-legged kittiwake ( Rissa tridactyla ), and glaucous-winged gull ( Larus glaucescens ), were widely distributed across environmental gradients, indicating less specialization, whereas species such as capelin ( Mallotus villosus ), harbor seal ( Phoca vitulina ), and Kittlitz's murrelet ( Brachyramphus brevirostris ) appeared to have more specialized habitat requirements related to glacial influence. We found that upper trophic level communities were well correlated with their mid trophic level prey community, but strong physical gradients in photic depth, temperature, and nutrients played an important role in community structure as well. Mid-trophic level forage fish communities were correlated with the physical gradients more closely than upper trophic levels were, and they showed strong affinity to tidewater glaciers. Silica was closely correlated with the distribution of fish communities, the mechanisms of which deserve further study.

Alaska↗

USGS2010

Variation in active and passive resource inputs to experimental pools: mechanisms and possible consequences for food webs

1. Cross-ecosystem movements of resources, including detritus, nutrients and living prey, can strongly influence food web dynamics in recipient habitats. Variation in resource inputs is thought to be driven by factors external to the recipient habitat (e.g. donor habitat productivity and boundary conditions). However, inputs of or by ‘active’ living resources may be strongly influenced by recipient habitat quality when organisms exhibit behavioural habitat selection when crossing ecosystem boundaries. 2. To examine whether behavioural responses to recipient habitat quality alter the relative inputs of ‘active’ living and ‘passive’ detrital resources to recipient food webs, we manipulated the presence of caged predatory fish and measured biomass, energy and organic content of inputs to outdoor experimental pools of adult aquatic insects, frog eggs, terrestrial plant matter and terrestrial arthropods. 3. Caged fish reduced the biomass, energy and organic matter donated to pools by tree frog eggs by ∼70%, but did not alter insect colonisation or passive allochthonous inputs of terrestrial arthropods and plant material. Terrestrial plant matter and adult aquatic insects provided the most energy and organic matter inputs to the pools (40–50%), while terrestrial arthropods provided the least (7%). Inputs of frog egg were relatively small but varied considerably among pools and over time (3%, range = 0–20%). Absolute and proportional amounts varied by input type. 4. Aquatic predators can strongly affect the magnitude of active, but not passive, inputs and that the effect of recipient habitat quality on active inputs is variable. Furthermore, some active inputs (i.e. aquatic insect colonists) can provide similar amounts of energy and organic matter as passive inputs of terrestrial plant matter, which are well known to be important. Because inputs differ in quality and the trophic level they subsidise, proportional changes in input type could have strong effects on recipient food webs. 5. Cross-ecosystem resource inputs have previously been characterised as donor-controlled. However, control by the recipient food web could lead to greater feedback between resource flow and consumer dynamics than has been appreciated so far.

Freshwater Biology↗

USGS2024

An invasive predator substantially alters energy flux without changing food web functional state or stability

Understanding how invasive species affect the stability and function of ecosystems is critical for conserving ecosystems. Here, we quantified the effect of an actively suppressed invasive species on the Yellowstone Lake, U.S.A. ecosystem using a food-web energetics approach. 2. We compared energy flux, functional state, and stability of four food web states: a pre-invasion network, and three post-invasion networks undergoing active invasive species suppression: initial invasion; expansion; decline. 3. Invasion caused > 25% change (±) in energy flux for most consumers, and total flux increased twofold post-invasion. Flux to the species of conservation concern, Yellowstone cutthroat trout ( Oncorhynchus virginalis bouvieri ), was 2.8-times less post-invasion vs pre-invasion while invasive lake trout ( Salvelinus namaycush ) flux was up to 17.3-times higher compared to the initial invasion network. The dominant functional state and food web stability did not change post-invasion, likely due to introduction of a generalist predator and the stabilizing effect of suppression. 4. Lake trout invasion in Yellowstone Lake caused large changes to energy flux, shifting dominant fluxes away from the species of conservation concern, despite not changing functional state or stability. We demonstrate that changes in energy flux may signal invasions in ecosystems, but functional state or stability may not necessarily reflect the magnitude of invasion influences. 5. Implications for conservation: For invaded fish communities, a better understanding of how the invasive species controls the food web beyond just the direct influence on prey results can be achieved by investigating energy flux, functional state, and food-web stability. Furthermore, evaluating the effect of suppression beyond the invasive species can demonstrate the far-reaching value of suppression management actions for conservation.

Wyoming↗

USGS2003

Predation on stocked Atlantic salmon (Salmo salar) fry

We studied predator-prey interactions between juvenile Atlantic salmon (Salmo salar) and trout in three Massachusetts, U.S.A., streams and in artificial streams. We sampled stomach contents of age-1+ and older salmon and trout (Salvelinus fontinalis, Salmo trutta) following salmon fry stocking in the spring of 1997 and 1998. Between 4.3 and 48.6% of the stocked fry were consumed within the first 2 days after stocking, and total fry mortality from predation varied from 4.3 to 60.7%. No significant differences were found between stomach weights of predators (without fry weight) that consumed fry and those that did not. Artificial stream experiments testing effects of habitat complexity and predator species on predator consumption rates revealed that consumption rates were not different between brook (S. fontinalis) and brown (S. trutta) trout (p = 0.59). Predation rate tended to decrease as the percentage of riffle habitat increased but the decrease was not significant (p = 0.22). Our results indicate that predation on stocked Atlantic salmon fry can be substantial (up to 60%), appears to be short lived (2 days), and is not related in a simple way to abiotic and biotic factors.

Canadian Journal of Fisheries and Aquatic Sciences↗

USGS2022

Response of alewife abundance to the bacterial kidney disease outbreak in the Chinook salmon population of Lake Michigan: Importance of predation

Prey fish abundance can be influenced by predation (top-down) and food limitation (bottom-up) effects. I characterized temporal trends in yearling and older (YAO) alewife ( Alosa pseudoharengus ) biomass density, as estimated by a long-term bottom trawl survey, in Lake Michigan during 1973–2019. Special attention was given to the bacterial kidney disease (BKD) outbreak in the population of Chinook salmon ( Oncorhynchus tshawytscha ), the predominant predator on alewives in the lake. YAO alewife biomass density exhibited a steep and significant decline during 1973–1985, but then partially rebounded with a significant increase between the 1983–1985 and 1986–2003 (BKD years) periods, followed by a significant decrease between the 1986–2003 and 2004–2006 periods. YAO alewife biomass density showed another significant decline during 2004–2019. The BKD outbreak led to a partial relaxation of predation on the YAO alewife population, and YAO alewife biomass density responded by showing a moderate increase in 1986 and then fluctuating about this moderately higher level for more than 15 years. Overall, temporal trends in YAO alewife biomass density were primarily driven by predation.

Lake Michigan↗

USGS2020

Biological control of Aedes mosquito larvae with carnivorous aquatic plant, Utricularia macrorhiza

Background Biological controls with predators of larval mosquito vectors have historically focused almost exclusively on insectivorous animals, with few studies examining predatory plants as potential larvacidal agents. In this study, we experimentally evaluate a generalist plant predator of North America, Utricularia macrorhiza, the common bladderwort, and evaluate its larvacidal efficiency for the mosquito vectors Aedes aegypti and Aedes albopictus in no-choice, laboratory experiments. We sought to determine first, whether U. macrorhiza is a competent predator of container-breeding mosquitoes, and second, its predation efficiency for early and late instar larvae of each mosquito species. Methods Newly hatched, first instar Aedes albopictus and Aedes aegypti larvae were separately exposed in cohorts of 10 to field collected U. macrorhiza cuttings. Data on development time and larval survival were collected on a daily basis to ascertain the effectiveness of U. macrorhiza as a larval predator. Survival models were used to assess differences in larval survival between cohorts that were exposed to U. macrorhiza and those that were not. A permutation analysis was used to investigate whether storing U. macrorhiza in laboratory conditions for extended periods of time (1 month vs. 6 months) affected its predation efficiency. Results Our results indicated a 100% and 95% reduction of survival of Ae. aegypti and Ae. albopictus larvae respectively, in the presence of U. macrorhiza relative to controls within five days, with peak larvacidal efficiency in plant cuttings from ponds collected in August. Utricularia macrorhiza cuttings, which were prey-deprived, and maintained in laboratory conditions for 6 months were more effective larval predators than cuttings, which were maintained prey-free for 1 month. Conclusions Due to the combination of high predation efficiency and the unique biological feature of facultative predation, we suggest that U. macrorhiza warrants further development as a method for larval mosquito control.

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