Wild Atlantic salmon Salmo salar L. strains have greater growth potential than a domesticated strain selected for fast growth

A study was undertaken to examine the responses of three Atlantic salmon Salmo salar strains to growth hormone (GH) treatment. A positive growth response to sustained‐release GH implants was found in two wild strains (Namsen and Imsa) as well as one domesticated strain (AquaGen). The data revealed that the growth‐selected AquaGen strain has further growth potential, however, a stronger growth response was observed in the wild strains which outgrew the domesticated strain after GH treatment. These observations suggest that some growth potential may have been lost during the selection for rapid growth in the AquaGen strain. In September, the parr were GH implanted and in December sampled for plasma GH and insulin‐like growth factor I (IGF‐I) levels, liver, muscle and gill GH receptor, IGF‐I mRNA levels, gill Na+,K+‐ATPase activity, muscle and liver lipid content and body silvering. Low temperature and seasonal growth cessation probably explains the relatively limited GH effects found. Body silvering in all strains was positively correlated to size. GH increased IGF‐I plasma levels in the Namsen strain inspite of liver IGF‐I mRNA levels being lower in GH‐treated fish.