Recent evidence suggests that foraging opportunities for fish are far more variable than ecologists typically assume. To maintain positive energy balance in this hyper-patchy world, fish must rapidly store energy during periods of food abundance. Water temperature regulates the digestive capacity of fish and limits rates of energy gain during resource pulses. Spatial heterogeneity in water temperature creates the potential for fish to manipulate their digestive capacity through post-feeding movements. A continuing challenge is to understand how behavioral strategies exploit landscape heterogeneity in thermal and trophic resources. Here we present research on juvenile coho salmon that rear in a thermally heterogeneous stream and gorge on the eggs of spawning sockeye salmon. We mapped spatial patterns in sockeye salmon abundance and water temperature, monitored coho salmon foraging behavior, and measured thermal effects on rates of digestion.
Results/Conclusions
Groundwater springs 1300 m upstream divided the focal stream into a cold lower section (mean temperature = 6.5°C) and a warm upper section (mean temperature = 9.5 °C). Sockeye salmon spawning peaked in the lower 600 m of the stream, and never extended into the warmer region upstream. This generated strong spatial trade-offs between trophic and thermal resources. An in situ experiment revealed that gastric evacuation of sockeye eggs would be roughly 2-times faster in the warmer waters upstream. PIT tag antenna arrays 800m and 1300m upstream recorded coho salmon making feeding forays into the cold downstream reaches at night and returning to the warm upstream reaches during the day, a round trip of roughly 1 km or more. Coho salmon sampled upstream consistently exhibited eggs in their diets. Feeding trails showed coho salmon could ingest up to 18% of their body mass in a single meal and required 3 days to evacuate a full stomach in 10°C water. This explains why some individuals waited 3 days in between feeding forays. In summary, we show that extreme stomach capacity supports long-distance feeding forays in coho salmon, enabling them to exploit spatial heterogeneity in trophic and thermal resources.