Spatial variation in growth, maturation schedules and reproductive investment of female sole Solea solea in the Northeast Atlantic

Latitudinal variation in life-history traits is often explained by phenotypically plastic responses or local adaptations to different thermal regimes. We compared growth, maturation schedules and reproductive investment of female sole Solea solea between 8 populations, covering much of the species' distribution in northern Europe, with respect to thermal gradients. An energy allocation model was fitted to size–age data, and probabilistic maturation reaction norms were estimated from size–age–maturity data. We found that northern populations from colder environments had higher rates of energy acquisition and reproductive investment, an intrinsic tendency to mature earlier, and had smaller asymptotic sizes than southern populations from warmer environments. Consequently, growth rate was higher before maturation but lower after maturation in the north compared to the south. This is opposite to Bergmann's rule according to which slower growth, delayed maturation and larger asymptotic sizes are usually observed at lower temperatures. The observed patterns could indicate strong countergradient thermal adaptation for rapid growth and development as well as sustained fecundity in the north, or indicate a response to other selection pressures correlated with the thermal gradient. Potentially higher mortality in northern populations during cold winters might be one of the key drivers of the observed geographical variation in growth and maturation of sole.

► Fishery data from three renown research institutions have been combined. ► A novel methodology is used to simultaneously estimate multiple life history traits. ► Plasticity and local adaptation are evaluated as response to the temperature gradient. ► Findings are opposite to Bergmann's rule indicating counter-gradient variation. ► Differences in natural mortality seems the main cause for life history adaptation.