LAUSR

Marine ecosystems are under increasing threat from warming waters. Winter warming is occurring at a faster rate than summer warming for ecosystems around the world, but most studies focus on the summer. Here, we show that winter warming could affect coastal fish community compositions in the Mediterranean Sea using a model that captures how biotic associations change with sea surface temperature to influence species’ distributions for 215 fish species. Species’ associations control how communities are formed, but the effect of winter warming on associations will be on average four times greater than that of summer warming. Projections using future climate scenarios show that 60% of coastal Mediterranean grid cells are expected to lose fish species by 2040. Heavily fished areas in the west will experience diversity losses that exacerbate regime shifts linked to overexploitation. Incorporating seasonal differences will therefore be critical for developing effective coastal fishery and marine ecosystem management. The rate of warming in many marine ecosystems is faster in winter than in summer. Winter warming will impact fish species’ associations in the Mediterranean more than summer warming, and this has implications for how communities form and for future biodiversity, particularly in heavily fished areas.