Climate change modifies reproduction phenology and investment in a lacustrine fish
Physical changes in high-latitude lakes have implications for diverse life history traits fishes and other taxa. Here, we use a five-decade time series from an Alaskan lake to explore the effects of climate change on growth and reproduction of a widely-distributed lacustrine fish, the threespine stickleback. We used multivariate autoregressive state-space (MARSS) models to describe trends in the mean length for multiple size modes and to explore the influence of physical (date of ice breakup, surface water temperature) and biological (density of con- and heterospecifics) factors. As predicted, the mean size of adult fish at the end of the growing season increased across years. In contrast, mean size of young of year fish declined over time. Overall, increased fish density predicted smaller mean size for all cohorts, warmer mean water temperature predicted larger size for all cohorts, and earlier ice breakup date predicted smaller size of young of year and larger size for adult fish. Early ice breakup was associated with earlier breeding, as evidenced by earlier capture of young of year fish, and an additional cohort of young of year fish was observed in early spring years. Our results suggest that early ice breakup altered both timing and frequency of breeding. While previous studies have documented the influence of changing conditions in northern lakes on breeding timing and growth, this is the first study to document increased breeding frequency, highlighting another pathway by which climate change can alter the ecology of northern lakes.
Environmental and phenological variability and the potential for mismatches
Survival of post-larval and juvenile organisms can depend on multiple seasonally-fluctuating features, including suitability of thermal conditions and food availability. The fitness of consumers depends on spatial and temporal overlap with prey production and on physical conditions conducive to growth, and a modification in phenology of either prey or consumers might decouple trophic relationships or reduce consumer growth and survival. This potential for mismatches is especially pronounced in organisms that migrate. Juvenile sockeye salmon (Oncorhynchus nerka) migrate from natal streams to lakes where they feed for a year before migrating to sea, and we hypothesized that their arrival must match suitable lake conditions if they are to survive and grow. We compared growth and survival from fry to seaward migrant stage for sockeye salmon with different lake entry dates, using mixed effects models to test the hypothesis that the most successful entry times correspond to favorable prey densities and lake thermal conditions, and that a mis-match in fish entry time and seasonal lake dynamics can have negative effects on fish growth and survival. Across years, fish entering the lake later in the season encountered higher zooplankton abundance and warmer water, and experienced higher average rates of growth and survival; however, the optimal date for lake entry ranged across years by up to a month. Zooplankton dynamics affected fish growth and survival more than lake temperature, and fish survival was best predictd by zooplankton abundance in the spring when fish enter the lake, while fish growth was best predicted by zooplankton abundance in the spring when fish migrated to the ocean. These results suggest that maintaining a wide window of migration times preserves the biological variability necessary in this life history event to avoid complete mismatches between juvenile fish and their resources, thereby enabling successful recruitment in an unpredictable environment.
Using physiological attributes to compare species responses to the environment
The threespine stickleback Gasterosteus aculeatus is widely distributed across ecosystems and commonly used as a model organism, but yet the species lacks a comprehensive model to describe physiological performance. This study parameterized a bioenergetics model for threespine sticklebacks, using laboratory measurements to determine mass- and temperature-dependent functions for maximum consumption and routine respiration costs. Model sensitivity was consistent with other comparable models, and growth estimates based on the stickleback bioenergetics model suggest 22 oC as the optimal temperature for growth when food is not limiting. This newly parameterized model was then applied to compare performance of threespine sticklebackwith that of sockeye salmon Oncorhynchus nerka, a commonly sympatric species around the Pacific Rim. This growth curve suggests that G. aculeatus maximizes growth at higher temperatures than sockeye salmon. Applied to a scenario where fish are experiencing changing thermal environments, bioenergetics models can be used to determine success of different species in altered habitats.