Three-dimensional maturation reaction norms for North Sea plaice

Probabilistic maturation reaction norms (PMRNs) with up to 3 explanatory dimensions were estimated for female North Sea plaice Pleuronectes platessa. The 3-dimensional PMRNs reported here (1) are the first to be obtained for any organism, (2) reveal the differential capacity of alternative life-history state variables to predict the onset of reproduction, (3) document consistent temporal trends in maturation, and (4) help disentangle the contributions of genetic and plastic effects to these trends. We first show that PMRNs based on age and weight provide slightly more accurate approximations of maturation probabilities than PMRNs based on age and length. At the same time, weight-based PMRNs imply a much wider spread of maturation probabilities than length-based PMRNs. We then demonstrate that including condition as a third explanatory variable improves predictions of maturation probability, The resultant 3-dimensional PMRNs for age-length-condition or age-weight-condition not only show how, at given size and age, maturation probability increases with condition, but also expose how this impact of condition decreases with age and has changed over time. Our analysis reveals several interesting temporal trends. First, it is demonstrated that even after removing plastic effects on maturation captured by age, length, weight and condition, residual trends towards maturation at younger ages and smaller lengths remain. Second, we find that the width of both length- and weight-based PMRNs decreased significantly over time. Third, age and condition are nowadays affecting maturation probabilities less than they did decades ago. We conclude that plaice are currently maturing at a very low age, size and body condition, and that the narrow and steep reaction norms do not allow a strong continuation of the observed trends. The findings obtained are in good agreement with predictions from life-history theory based on the hypothesis of evolutionary change caused by heavy exploitation.