Widespread phenological shifts with temperature in Alaska’s marine fishes

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.