Equatorward shifts of Marine Fishes under Climate Change: A Rule Rather than Exception?

A ubiquitous response to climate change is distributional shifts of marine and terrestrial species. The most frequently observed and predicted movements are to higher latitudes and altitudes, and towards greater depths in aquatic systems. This pattern is commonly explained by species tracking their climatic niches. But climate is not the only factor that restricts species’ distributions. Here we show that the conventional thinking of expecting poleward range shifts is caricatured and incomplete. Using models, we illustrate that poleward shifts of visual foragers may be constrained by direct effects of temperature and increasing seasonality in light with latitude on energy acquisition and loss, and by indirect effects of predation risk. We identified three mechanisms that could act as constraints: 1) increased winter temperature, leading to a mismatch between a higher energy demand and the energy available due to seasonality in the light regime, 2) faster digestion associated with warmer temperatures during the feeding season, leading to higher energy acquisition in situ for digestion-limited individuals, and 3) higher predation- and starvation-related mortality at higher than lower latitudes, caused by the interaction between a higher metabolic cost and increasing seasonality in light with latitude. Fewer dark hours with increasing latitude in summer leads to higher risk taking, and a less opportunity to accumulate energy reserves for the dark winter period when access to food is restricted by light. Furthermore, our findings suggest that equatorward shifts, to less seasonal latitudes, may be optimal for high latitude populations under ocean warming. Our study highlights the need to account for direct and indirect effects of increasing seasonality in light with latitude on individual and population performance when predicting distributional shifts under climate change.