And how do researchers know that?
From a University of Chicago media release:
Sockeye salmon are evolving through natural selection to deal with a warming climate, according to a study by researchers at the National Oceanic and Atmospheric Administration. In recent decades, scientists have observed that salmon in the Columbia River are starting their migration earlier in the year. The fish now migrate upstream an average of 10 days earlier than they did in the 1940s. Researchers knew that the change was associated with warming water temperatures, but an important question remained. Is this a behavioral response in reaction to warmer water, or are the fish evolving modified behavior through mortality and natural selection? Using 60 years of water temperature records and data on salmon mortality during migration, researchers led by Dr. Lisa Crozier have determined that the latter has played an important role. According to their model, up to two thirds of the 10-day advance in spawning is explained by natural selection, with a behavioral response to changes in river flow explaining the rest. “Evidence of an evolutionary response in Columbia River sockeye salmon is good news, because it appeared to reduce their exposure to potentially lethal river temperatures in recent years,” said Dr. Crozier. “This study gives managers insight into the multiple processes that help salmon persist in the face of a changing environment, and augments our toolbox for predicting how other species might respond to similar changes.”
Sorry, but models aren’t science; they are assumptions. And it would be so difficult to show in a scientific manner that natural selection played any role as to be impossible.