Salmon are shrinking and this has genetic basis
Salmon are shrinking and mature faster - life short science news

By studying scales, collected over a 40 years period, biologists have found that male salmon are becoming smaller. It appeared that the reduced body size has a genetic basis, such that the genotype for larger body size is now less common than the genotype for smaller body size. This change in the genes shows that the change in body size reflects “evolution in action”. It seems that salmon mature faster, i.e. at a younger age, and therefore don’t grow as big as they used to do. It is not clear as to what the evolutionary benefit might be of smaller body size, but biologists hypothesize that salmon are more likely to die during their time at sea, and would thus be better off returning to the rivers to spawn earlier.

Read the full story: University of Helsinki
Scientific publication: Nature Ecology & Evolution