Researchers at Bangor University say they found that over-harvesting larger fish leads to a population of smaller fish that are less fertile.
The research suggest that the change happens within a few generations.
The scientists say the findings could have a massive impact for the future of global fishing policies.
"Our findings have major implications for the sustainability of harvested populations," said Prof Gary Carvalho, of Bangor University's School of Biological Sciences.
He said a "shift in the genetic make-up of harvested fish to smaller less fertile individuals" would be "serious global consequences for the environment and for global fishing industry".
"We would urge the scientific community, policy makers and managers to consider the capacity of harvested stocks to adapt to, and recover from, harvesting and predation."