Selection programs for fish frequently target growth rate as a breeding goal, yet surprisingly little is known about which mechanisms underlying the growth process are being targeted. The aim of this study was thus to examine whether the process of artificial selection of Atlantic salmon (Salmo salar) that has resulted in higher growth rate resulted in underlying changes in the growth hormone (GH) – insulin-like growth factor I (IGF-I) axis of endocrine growth regulation. This was tested by comparing similarly reared seventh-generation farm salmon with wild salmon from the principal founder population of the farm strain at three life stages. Not unexpectedly, the domesticated fish outgrew their wild counterparts; this was most evident in salt water, where they averaged three times the weight by the end. Pituitary GH content was positively correlated with growth rate and correspondingly was significantly higher in the faster growing domesticated fish than in the wild fish. Plasma GH levels were also significantly higher in the domesticated fish, whereas IGF-I levels did not differ. These findings provide some of the first direct evidence indicating a link between domestication selection for growth and its endocrine regulation, whereby individuals with more active endocrine growth regulatory components are targeted.

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