Prey, not temperature, is the dominant driver of juvenile growth in North Sea sandeels

Declining body sizes are prevalent in marine fish and have been suggested to be a response to increasing temperatures. However, the evidence is mixed, and the underlying causes are often unknown. Here, we explore drivers of spatio-temporal patterns in size in juvenile lesser sandeel Ammodytes marinus, focusing on ongoing size declines in parts of the North Sea. We combine experimental and field data with theory to develop a biologically realistic dynamic energy budget model that explicitly models feeding, metabolism and energy allocation to produce daily predictions of sandeel length during the growth season from 1979 to 2016 in 4 North Sea sub-populations. When forced with daily temperature estimates and zooplankton data from the Continuous Plankton Recorder, model predictions largely match observed spatio-temporal patterns. Our results suggest that the most plausible driver of observed size declines in the western North Sea is declining prey densities. In contrast, the direct effect of temperature on sandeel size is small but interacts with local prey availability so that increasing temperatures may boost growth rates in areas with high food availability but reduce growth rates in areas with low food availability. Our results thus suggest that to understand the effects of climate change on fish size, we need to account for both direct physiological effects and changes in resource availability. Finally, we show that early-life phenology and turbidity (via its impact on intake rates in the visually foraging sandeel) may also impact sandeel size, highlighting the importance of broadening our view of potential drivers of size declines.