Salmon just can’t catch a break.

Not only have their numbers been reduced by sea lice, warmer waters and a loss of spawning habitat – they are also physically shrinking in size, according to new research published in the Proceedings of the National Academy of Sciences.

"A four-year-old fish is much smaller than it was 40 years ago," said Tom Reimchen, a University of Victoria professor who co-authored the study.

"This is the pattern you repeatedly see in the analysis of commercial fisheries."

Reimchem and Chris Darimont reviewed research on 29 species in 40 regions around the world. It showed that trophy hunting and commercial fisheries – what Reimchen called human "super-predator" behaviours -- are responsible for rapid evolutionary changes in the wild.

"If you're always taking the biggest individuals, you're removing from the population those genotypes that tend to be fast-growing. That's led to a gradual reduction in the size of animals at a particular age," explained Reimchen.

Reimchen said commercial fisheries have the greatest influence on diminishing fish size.

"Sport fishing, which we thought would have had a large effect, was not as much of a factor," he said. In the commercial fishery, he said, "we need a gradual reduction of quota…and a gradual shift to the younger fish" to address this problem.

Des Nobels, a former commercial fishermen who now works with the T. Buck Suzuki Environmental Foundation, said it's not necessarily wise to target a certain size of fish, large or small.

"You want a range of diversity of size, shape, everything," he said.

And a big fish doesn't necessarily bring the highest price, he pointed out. If it's too large – over the 40-pound range – it will likely be chopped up and frozen, and frozen fish is worth less than fresh.

"In many respects, it's the market that dictates what is wanted. On the recreation end of things, the market demands big trophy fish. In the commercial sector…it's the quantity and particular quality that's driving it."

Colleen Kimmett is a regular contributor to the Hook.