Earlier Migration Timing, Decreasing Phenotypic Variation, and Biocomplexity in Multiple Salmonid Species

Abstract

Climate-induced phenological shifts can influence population, evolutionary, and ecological dynamics, but our understanding of these phenomena is hampered by a lack of long-term demographic data. We use a multi-decade census of 5 salmonid species representing 14 life histories in a warming Alaskan stream to address the following key questions about climate change and phenology: How consistent are temporal patterns and drivers of phenology for similar species and alternative life histories? Are shifts in phenology associated with changes in phenotypic variation? How do phenological changes influence the availability of resource subsidies? For most salmonid species, life stages, and life histories, freshwater temperature influences migration timing – migration events are occurring earlier in time (mean = 1.7 days earlier per decade over the 3–5 decades), and the number of days over which migration events occur is decreasing (mean = 1.5 days per decade). Temporal trends in migration timing were not correlated with changes in intra-annual phenotypic variation, suggesting that these components of the phenotypic distribution have responded to environmental change independently. Despite commonalities across species and life histories, there was important biocomplexity in the form of disparate shifts in migration timing and variation in the environmental factors influencing migration timing for alternative life history strategies in the same population. Overall, adult populations have been stable during these phenotypic and environmental changes (λ ≈1.0), but the temporal availability of salmon as a resource in freshwater has decreased by nearly 30 days since 1971 due to changes in the median date of migration timing and decreases in intra-annual variation in migration timing. These novel observations advance our understanding of phenological change in response to climate warming, and indicate that climate change has influenced the ecology of salmon populations, which will have important consequences for the numerous species that depend on this resource.

Evolution of phenology in a salmonid population: a potential adaptive response to climate change

“…An advance in the optimal growth window, without a corresponding plastic response, would be expected to cause directional selection against late-migration time, thereby reducing phenotypic variation in this trait. Hence, this hypothesis is also consistent with the declines in phenotypic variation of migration time that have been observed in adults and fry in the even-and odd-year populations at Auke Creek (Kovach et al 2013b). Importantly, our study is limited to an observation of natural selection in a single generation, whereas the long-term trend in median migration time spans 17 generations at Auke Creek.…”

“…The shaded area depicts the 95% confidence interval. hypothesis provides a unifying explanation for the coinciding trends toward earlier migrations of adults and fry (Kovach et al 2013b) and warmer vernal sea-surface temperatures of Auke Bay (Fig. 6) that have been observed in this system over the past four decades.…”

“…Despite apparently strong natural selection against latemigrating fish and resulting demographic changes, the odd-year population has exhibited sustained levels of adult recruitment over the past 17 generations. Similarly, the even-year population, which has exhibited parallel changes in phenology, has also sustained its recruitment levels over the same period (Kovach et al 2013b). This indicates that genetically based variation in phenology has supported sustained productivity in Auke Creek during rapid climate change, an observation that underscores the importance of life history variation to the resilience of fish populations (Hilborn et al 2003;Greene et al 2010).…”

Importance of Riparian Zone: Effects of Resource Availability at Land-water Interface

“…There is a size threshold for age-0 coho salmon to consume salmon egg subsidy, which is regulated by water temperature [130]. Climate change can induce phenological shifts of keystone species to influence population, evolutionary, and ecological dynamics, and this shift will further affect species that depend on salmon resource subsidy [131]. Drought is another aspect of climate change and results in drying of streams and rivers, which has occurred in a higher frequency and a longer period in many parts of the world.…”

Annual Changes in Seasonal River Water Temperatures in the Eastern and Western United States

“…Changes in river water temperatures, and therefore thermal habitat, are projected to have particularly important effects on many freshwater communities. Predicted and recently-observed effects of increasing water temperature on riverine fish species include range shifts towards upstream, higher elevations [4], influences on migration timing [5,6] and changes in community structure [7]. In addition, because temperature is a key controller of physiological processes, changes in water temperatures have the potential to affect population vital rates, including growth and reproduction [8][9][10].…”