Inter-Tributary Movements by Resident Salmonids across a Boreal Riverscape.

Stream-dwelling fishes inhabit river networks where resources are distributed heterogeneously across space and time. Current theory emphasizes that fishes often perform large-scale movements among habitat patches for reproduction and seeking refugia, but assumes that fish are relatively sedentary during growth phases of their life cycle. Using stationary passive integrated transponder (PIT)-tag antennas and snorkel surveys, we assessed the individual and population level movement patterns of two species of fish across a network of tributaries within the Wood River basin in southwestern Alaska where summer foraging opportunities vary substantially among streams, seasons, and years. Across two years, Arctic grayling (Thymallus arcticus) and rainbow trout (Oncorhynchus mykiss) exhibited kilometer-scale movements among streams during the summer growing season. Although we monitored movements at a small fraction of all tributaries used by grayling and rainbow trout, approximately 50% of individuals moved among two or more streams separated by at least 7 km within a single summer. Movements were concentrated in June and July, and subsided by early August. The decline in movements coincided with spawning by anadromous sockeye salmon, which offer a high-quality resource pulse of food to resident species. Inter-stream movements may represent prospecting behavior as individuals seek out the most profitable foraging opportunities that are patchily distributed across space and time. Our results highlight that large-scale movements may not only be necessary for individuals to fulfill their life-cycle, but also to exploit heterogeneously spaced trophic resources. Therefore, habitat fragmentation and homogenization may have strong, but currently undescribed, ecological effects on the access to critical food resources in stream-dwelling fish populations.

Predator avoidance during reproduction: diel movements by spawning sockeye salmon between stream and lake habitats.

Daily movements of mobile organisms between habitats in response to changing trade-offs between predation risk and foraging gains are well established; however, less in known about whether similar tactics are used during reproduction, a time period when many organisms are particularly vulnerable to predators. We investigated the reproductive behaviour of adult sockeye salmon (Oncorhynchus nerka) and the activity of their principal predator, brown bears (Ursus arctos), on streams in south-western Alaska. Specifically, we continuously monitored movements of salmon between lake habitat, where salmon are invulnerable to bears, and three small streams, where salmon spawn and are highly vulnerable to bears. We conducted our study across 2 years that offered a distinct contrast in bear activity and predation rates. Diel movements by adult sockeye salmon between stream and lake habitat were observed in 51.3% ± 17.7% (mean ± SD) of individuals among years and sites. Fish that moved tended to hold in the lake for most of the day and then migrated into spawning streams during the night, coincident with when bear activity on streams tended to be lowest. Additionally, cyclic movements between lakes and spawning streams were concentrated earlier in the spawning season. Individuals that exhibited diel movements had longer average reproductive life spans than those who made only one directed movement into a stream. However, the relative effect was dependent on the timing of bear predation, which varied between years. When predation pressure primarily occurred early in the spawning run (i.e., during the height of the diel movements), movers lived 120-310% longer than non-movers. If predation pressure was concentrated later in the spawning run (i.e. when most movements had ceased), movers only lived 10-60% longer. Our results suggest a dynamic trade-off in reproductive strategies of sockeye salmon; adults must be in the stream to reproduce, but must also avoid predation long enough to spawn. Given the interannual variation in the timing and intensity of predation pressure, the advantages of a particular movement strategy will likely vary among years. Regardless, movements by salmon allowed individuals to exploit fine-scale habitat heterogeneity during reproduction, which appears to be a strategy to reduce predation risk on the spawning grounds.

Riding the crimson tide: mobile terrestrial consumers track phenological variation in spawning of an anadromous fish.

When resources are spatially and temporally variable, consumers can increase their foraging success by moving to track ephemeral feeding opportunities as these shift across the landscape; the best examples derive from herbivore-plant systems, where grazers migrate to capitalize on the seasonal waves of vegetation growth. We evaluated whether analogous processes occur in watersheds supporting spawning sockeye salmon (Oncorhynchus nerka), asking whether seasonal activities of predators and scavengers shift spatial distributions to capitalize on asynchronous spawning among populations of salmon. Both glaucous-winged gulls and coastal brown bears showed distinct shifts in their spatial distributions over the course of the summer, reflecting the shifting distribution of spawning sockeye salmon, which was associated with variation in water temperature among spawning sites. By tracking the spatial and temporal variation in the phenology of their principal prey, consumers substantially extended their foraging opportunity on a superabundant, yet locally ephemeral, resource. Ecosystem-based fishery management efforts that seek to balance trade-offs between fisheries and ecosystem processes supported by salmon should, therefore, assess the importance of life-history variation, particularly in phenological traits, for maintaining important ecosystem functions, such as providing marine-derived resources for terrestrial predators and scavengers.

Temperature-associated population diversity in salmon confers benefits to mobile consumers.

Habitat heterogeneity can generate intraspecific diversity through local adaptation of populations. While it is becoming increasingly clear that population diversity can increase stability in species abundance, less is known about how population diversity can benefit consumers that can integrate across population diversity in their prey. Here we demonstrate cascading effects of thermal heterogeneity on trout-salmon interactions in streams where rainbow trout rely heavily on the seasonal availability of anadromous salmon eggs. Water temperature in an Alaskan stream varied spatially from 5 degrees C to 17.5 degrees C, and spawning sockeye salmon showed population differentiation associated with this thermal heterogeneity. Individuals that spawned early in cool regions of the 5 km long stream were genetically differentiated from those spawning in warmer regions later in the season. Sockeye salmon spawning generates a pulsed resource subsidy that supports the majority of seasonal growth in stream-dwelling rainbow trout. The spatial and temporal structuring of sockeye salmon spawn timing in our focal stream extended the duration of the pulsed subsidy compared to a thermally homogeneous stream with a single population of salmon. Further, rainbow trout adopted movement strategies that exploited the multiple pulses of egg subsidies in the thermally heterogeneous stream. Fish that moved to track the resource pulse grew at rates about 2.5 times higher than those that remained stationary or trout in the reference stream with a single seasonal pulse of eggs. Our results demonstrate that habitat heterogeneity can have important effects on the population diversity of dominant species, and in turn, influence their value to species that prey upon them. Therefore, habitat homogenization may have farther-reaching ecological effects than previously considered.