• alternative reproductive phenotypes;
  • Atlantic salmon;
  • genetically modified organisms;
  • growth hormone;
  • mature parr;
  • proximate mechanisms;
  • Salmo salar;
  • transgenesis


1. Understanding the proximate and ultimate mechanisms shaping the expression of alternative reproductive phenotypes is a fundamental question in life-history evolution. Precocial maturation in fishes, one such alternative phenotype, has been thought to reflect rapid growth and/or energy accumulation; however, mechanistically linking these specific traits to discrete life-history patterns is complex and poorly understood.

2. Here, we use growth hormone (GH) transgenic Atlantic salmon to elucidate the effects of intrinsically fast growth on precocial male maturation as parr (freshwater life stage). Despite facilitating growth to sizes typical of mature wild-type parr, transgenesis did not influence maturation in the first year of life. In the second year, the number of maturing transgenic parr was only half that of non-transgenic individuals.

3. By manipulating intrinsic growth and controlling for both environment and genetic background, this study provides direct empirical evidence suggesting that the physiological mechanisms promoting growth do not play a causative role in precocial male maturation in fishes.

4. In addition, this study provides the first empirical data on the relative incidence of precocial male maturation in GH transgenic and non-transgenic Atlantic salmon and, therefore, provides valuable information for the ecological risk assessment process.