Abstract
Growth hormone transgenic coho salmon Oncorhynchus kisutch fed at the same ration level as non‐transgenic controls (Tc) had the same growth rate as non‐transgenic controls (Nt). In contrast, growth hormone transgenic coho salmon (Tf) fed ad libitum ate about twice as much and had much higher growth rates than the other two groups. The most obvious result was the significantly larger caeca in the Tf group relative to both Nt or pair‐fed Tc. The Tf fish had more caeca that were longer. The results suggested that the effect was indirect and the enlarged caeca required both the GHtransgene and hyperphagia to cause enlarged caecal capacity. A small part of the results, however, also suggested that there was a direct effect of the GHtransgene on some gut tissues, particularly the intestine.