WASHINGTON: Fewer big predatory fish are swimming in the oceans because of overfishing, leaving smaller species to thrive and double in force over the past 100 years, scientists say.

Big fish such as cod, tuna, and grouper have declined worldwide by two-thirds while numbers of anchovies, sardines and capelin have surged in their absence, University of British Columbia researchers said.

People around the world are fishing more and coming up with the same or fewer numbers in their catch, indicating that humans may have reached the limit of the oceans' capacity to provide food.

''Overfishing has absolutely had a 'when cats are away, the mice will play' effect on our oceans,'' said Villy Christensen, a professor in the university's fisheries centre.

''By removing the large, predatory species from the ocean, small forage fish have been left to thrive.''