Local reflects global: Life-stage dependent changes in the phenology of coastal habitat use by North Sea herring

Climate warming is affecting the suitability and utilisation of coastal
habitats by marine fishes around the world. Phenological changes are an
important indicator of population responses to climate-induced changes but
remain difficult to detect in marine fish populations. The design of
large-scale monitoring surveys does not allow fine-grained temporal
inference of population responses, while the responses of ecologically and
economically important species groups such as small pelagic fish are
particularly sensitive to temporal resolution. Here, we use the longest,
highest-resolution time series of species composition and abundance of
marine fishes in northern Europe to detect possible phenological shifts in
the small pelagic North Sea herring. We detect a clear forward temporal
shift in the phenology of nearshore habitat use by small juvenile North
Sea herring. This forward shift can best be explained by changes in water
temperatures in the North Sea. We find that reducing the temporal
resolution of our data to reflect the resolution typical of larger surveys
makes it difficult to detect phenological shifts and drastically reduces
the effect sizes of environmental covariates such as seawater temperature.
Our study therefore shows how local, long-term, high-resolution time
series of fish catches are essential to understand the general
phenological responses of marine fishes to climate warming and to define
ecological indicators of system-level changes.