Climate effects on size‐at‐age: growth in warming waters compensates for earlier maturity in an exploited marine fish

Over the past 3 decades, North Sea Atlantic cod (Gadus morhua) have exhibited variable length‐at‐age along with declines in spawning stock biomass and timing of maturity. Multiple factors affecting growth and development in fish acted on this economically important stock over the same period including warming waters and an intensive fishery. Here, we employ North Sea cod as a model population, exploring how a physiologically relevant temperature metric (the growing degree‐day, GDD; °C day) can be used to compare year‐classes on a physiologically relevant time‐scale, disentangling influences of climate (thermal history) on observed length‐at‐age trends. We conclude that the trends in North Sea cod length‐at‐age observed during the last three decades can be explained by a combination of temperature‐dependent growth increases and a trend toward earlier maturation, the latter likely induced by the intensive fishing pressure, and possibly evidence of fisheries‐induced evolution.