Widespread Phenological Shifts With Temperature in Alaska's Marine Fishes

ABSTRACT Changes in the timing of fish spawning and early life stage development can affect the temporal match or mismatch of larvae with production of preferred prey as well as their availability to predators, with potential consequences for recruitment success, food‐web dynamics, and fisheries. Using > 370,000 observations from over four decades of spring ichthyoplankton surveys in the Gulf of Alaska and Bering Sea, we investigated long‐term changes in the phenology of 29 fish species, including commercially important taxa such as Pacific cod, walleye pollock, and Pacific halibut. Larval size on a standardized date (size‐at‐date) was used as a proxy for larval developmental timing in spring, and reflects a combination of hatch timing (larval age), growth, and mortality. Spatiotemporal generalized linear mixed models were used to account for variable sampling effort in space and time in order to isolate long‐term trends and thermal effects on larval size. For a majority of species, interannual variation in mean size‐at‐date was significantly and positively related to temperature, demonstrating widespread thermal effects on the phenology of fish early life stages. Despite the wide diversity of life history traits exhibited by the 29 species examined, patterns in size‐at‐date over time were similar across most species within each ecosystem, reflecting the common effect of temperature on phenology. While temperature affected size‐at‐date, there was little evidence of long‐term trends, likely due to the lack of a linear trend in winter–spring temperatures observed in recent decades. We demonstrate a novel analytical method to assess changes in phenology from larval size observations sampled at variable locations and times, and detect phenological shifts that were not necessarily identifiable from larval abundance data alone. Our results suggest that earlier spring phenology due to warming will be a common response among fishes to projected future climate change in high‐latitude ecosystems.