Food, Phenology, and Flow—How Prey Phenology and Streamflow Dynamics Affect the Behavior, Ecology, and Recovery of Pacific Salmon
- Author(s): Rossi, Gabriel J
- Advisor(s): Power, Mary E
- et al.
Pacific salmon (Oncorhynchus) are an iconic genus of anadromous fish whose decline over the last century has linked indigenous peoples, government agencies, fishers, advocates, and scientists in the common, albeit messy, cause of salmon recovery. While the history of ecology, and the earliest research on Pacific salmon, is grounded in the understanding that food web and prey dynamics control the abundance and distribution of predators, the last half-century of salmon management has focused primarily on hatchery production and physical habitat suitability. The continued decline of Pacific salmon, after decades of efforts to restore them, has prompted salmon ecologists to remember what Charles Elton (1927) understood nearly a century ago: “Food is the burning question in animal society, and the whole structure and activities of the community are dependent upon questions of food-supply.” My dissertation proceeds from the observation that both salmon management and salmon ecology have placed too little emphasis on aquatic food webs, animal behavior, and biotic interactions as key drivers of salmon production and resilience—and thus as chief mechanisms of salmon recovery.
This dissertation is comprised of a series of studies and experiments from 2016 to 2019, which explore how the seasonal interaction among physical habitat, phenology of salmon prey, and foraging behavior affects juvenile salmon growth and performance. The experiments described here take place near the southern end of the range of Pacific salmon—in Mediterranean streams along the Pacific Coast of California. These streams experience a characteristic recession of streamflow between spring and fall, which is an ideal natural gradient to investigate the dynamic interaction between physical habitat and prey phenology. The studies described here also focus on one species of Pacific salmon—Oncorhynchus mykiss (the “steelhead” or “rainbow trout”).
My first chapter starts from the observation that O. mykiss are typically considered drift foragers that occupy higher velocity habitat than other salmonids. Yet their persistence in Mediterranean coastal streams with seasonally low velocity and drift suggests an underappreciated flexibility in foraging behavior. Using novel 3-D videogrammetry along with measurements of invertebrate drift and stream hydraulics, I quantified the seasonality of foraging behaviors that O. mykiss employ during the spring-summer flow recession. I also explored the factors that trigger of foraging mode shifts, which are important for both ecological interactions and salmon management. I found that 80% of O. mykiss were drift foraging in May, 2018; while over 70% used alternative foraging modes by July. Foraging mode was significantly correlated to maximum water velocity and to riffle crest depth. While drift concentration was a poor univariate predictor, the top-ranked statistical models of foraging mode included both hydraulic variables and drift concentration. This study revealed that O. mykiss in Mediterranean streams express more diverse foraging behavior than previously suggested in the literature, and that changes in foraging mode could be predicted by simple hydraulic indicators. This study also lead us to speculate that non-drifting prey fluxes, as well as risk aversion, may influence late-summer foraging decisions in Mediterranean coastal streams.
In my second chapter I took advantage of an existing streamflow augmentation project to conduct a novel manipulative experiment to elevate dry-season flow in an intermittent, salmon-bearing stream. Using a Before-After-Control-Impact (BACI) study design, I directly measured the responses of O. mykiss behavior, their invertebrate prey, physical habitat, and water quality to flow enhancement. I found that elevating mid-summer baseflow for one week caused significant increases in dissolved oxygen, riffle width, depth, velocity, invertebrate drift, and benthic invertebrate standing crop – and these changes were evident at sites more than 1 km downstream from the point of flow augmentation. I also documented consistent behavioral changes indicating that pool-dwelling salmonids were able to exploit the increased prey abundance and improved metabolic environment in the treatment reach after flow augmentation. Counts of foraging salmonids near riffle-pool transition zones doubled, foraging movement remained steady (relative to decreasing movement in the control reach), and the proportion of drift foraging salmonids increased from an average of 26% to an average of 77% after augmentation. Collectively, these results suggest that dry season flow enhancement can significantly improve habitat profitability for salmonids, but that the timing and location of flow augmentation, relative to downstream points of interest, are critical to meeting ecological objectives.
In my third chapter I propose a conceptual model for how stream hydraulics and secondary production interact to affect foraging profitability for juvenile salmon during the streamflow recession in Mediterranean coastal streams. I evaluated the model by comparing two distinct salmonid rearing streams—one perennial, cool, and shaded; the other intermittent, seasonally warm, and sunny. In both streams, I conducted a photon-to-salmon food web study to document the seasonality of hydraulic habitat relative to the phenology of prey biomass for O. mykiss. I also estimated the seasonal change in O. mykiss growth potential using a drift foraging bioenergetics model. Both streams offered profitable foraging opportunities for O. mykiss, but modeled growth potential peaked at least two months earlier in the intermittent stream, where lipid storage measured in mid-July was nearly twice as high. In contrast, modeled and measured early-summer growth was higher in the perennial stream. By late summer, foraging profitability declined in both streams. However, abiotic conditions approached lethal tolerance levels in the intermittent stream, whereas the perennial stream maintained suitable abiotic conditions all summer. Seasonal changes in growth potential were explained by the timing of streamflow recession relative to the phenology of prey fluxes. But ecological interactions (especially intraspecific competition) and flexible foraging behavior both appear to have mediated realized foraging opportunity. These observations lead me to speculate that juvenile O. mykiss life histories should differ among streams, due to the timing of their spring-summer growth potential and to differing late summer survival rates. Collectively, these stream types could contribute to a stabilizing portfolio of juvenile salmonid life histories.