How fish cope in a world of feast and famine: behavioral and physiological adaptations to ecological heterogeneity
Armstrong, Jonny B.
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Predator-prey interactions are patchy in space and time due to a variety of interactions between predator behavior, prey behavior, and landscape heterogeneity. To survive in a patchy world, predators must capitalize on high quality foraging opportunities and store energy to survive periods of resource scarcity. In this dissertation, I explore the behavioral and physiological adaptations that enable predators to survive and thrive in a world of feast and famine. I begin with a broad analysis that asks: just how patchy are the foraging opportunities of predatory fishes? Since variation in foraging opportunities is extremely difficult to measure in the wild, I instead look to the digestive systems of fish, and explore what levels of variation would have led natural selection to favor the observed levels of excess digestive capacity seen across ~40 species of fish. I discovered high levels of excess digestive capacity that suggest variation in foraging opportunities may be 10-times higher than ecologists typically assume. This indicates that the ability of predators to exploit pulses of food is likely critical to the survival of individuals and the productivity of populations. In the subsequent chapters, I conduct case studies in the Bristol Bay region of Alaska to explore how animal behavior and physiology interact with landscape heterogeneity to influence the ability of predators to exploit an annual resource pulse. My field research explores the foraging ecology of juvenile coho salmon as they exploit pulsed sockeye salmon subsidies in thermally heterogeneous riverscapes. In Chapter 2, I explore how differences in water temperature among streams influence the potential for age-zero coho salmon to grow large enough to consume sockeye salmon eggs. The research in Chapter 3 occurs at the scale of a single stream, Bear Creek, and explores how coho salmon foraging behavior copes with trade-offs in the spatial distribution of thermal and trophic resources. I find that individuals actively exploit habitat heterogeneity to increase their digestive capacity, exhibiting feeding forays into cold habitats where sockeye salmon spawn, but spending the majority of their time residing in warmer habitats that provide higher digestive capacity. In Chapter 4, I explore how spatial patterns of water temperature vary at inter-annual timescales due to the episodic nature of summer precipitation events, and how the foraging behavior of juvenile coho salmon responds to a shifting mosaic of water temperature in Bear Creek. I find that precipitation events during wet summers trigger high flows that eliminate warm water habitat in the upstream thalweg of Bear Creek, but generate new warm water habitat in off-channel areas downstream. The spatial distribution of juvenile coho salmon tracked warm water habitat as it shifted across space, suggesting that their behavioral thermoregulatory behavior persists across highly variable flow conditions by exploiting the portfolio of habitat options provided by an intact floodplain. In summary, this dissertation seeks to understand how juvenile coho salmon (in particular) and aquatic predators (in general) capitalize on prey resources that are distributed heterogeneously in space and time. My research explores the interplay between intrinsic factors of behavior and physiology, and extrinsic factors of habitat conditions and their patterning across space and time.
- Fisheries