Bergmann Patterns in Planktivorous Fishes: A Light‐size or Zooplankton Community‐size Rule is Just As Valid Explanation As the Temperature‐size Rule

Aim: Bergman patterns, the tendency of organisms to be larger at higher latitudes and lower temperatures, are a well-studied biogeographic pattern. Yet, there is no consensus on the driver or underlying mechanisms. We aim to scrutinize the influence of several key proposed drivers of Bergmann patterns (temperature, seasonal light availability, prey size and seasonal abundance) on optimal body size in planktivorous fishes across high latitudes in the Northeast Atlantic.Location: Northeast Atlantic between 55 and 75 degrees N, with implications for high-latitude oceans globally.Time period: Present day.Major taxa studied: Pelagic planktivorous fishes, with Atlantic herring (Clupea harengus) as model organism.Methods: We use a model that incorporates explicit mechanisms for vision-based feeding and temperature-dependent physiology of a planktivorous fish to explore how intrinsic and extrinsic constraints affect energy budgeting and thereby expected optimal body size based on bioenergetics. We run the model at latitudes with increasing seasonality and test the individual and joint effects of relevant drivers.Results: A Bergmann pattern emerges from the interaction between visual feeding opportunities and temperature-dependent physiology. Small individuals profit from faster energy processing at higher temperatures in the south, whereas large individuals benefit from a lower metabolic cost at colder temperatures and more daylight hours for feeding in the north. In isolation temperature, daylight hours, and prey size each produced Bergmann patterns, but the most pronounced pattern arose from all drivers combined.Main conclusions: Studying biogeographic body size patterns requires a holistic view, accounting for interactions between drivers and both intrinsic and extrinsic constraints on energy budgeting. Across latitudes, temperature effects on digestion and metabolism interact with effects of light availability, prey size and abundance on food accessibility, and thereby shape the optimal size. Our study highlights how details of ecological mechanisms and lifestyles are important for improving predictive ability.