The seagull population on B.C.'s south coast has been cut in half in the last 30 years due to a decline in the marine life that feeds them according to a new study out of the University of British Columbia.

Researchers looking at 110 years of data found the main reason for the decline was diet, said Louise Blight, the study's lead author.

“These birds are the ultimate generalist — they can eat whatever’s around,” Blight said. "If this extreme generalist is not doing well, it's a pretty strong message that the marine ecosystem has changed. Not for the better." 

The glaucous-winged gulls, as the common seagull is properly known, used to mainly eat small fish and shellfish, but they've started to eat garbage and earthworms found on land.

“They’re presumably turning to land-based prey sources because the things they prefer to eat are less available,” Blight said, adding there are fewer fish in coastal waters than there were prior to industrial fishing.

The study looked at the seagulls found in the Lower Mainland, Victoria, Nanaimo and elsewhere in the region.

A lack of sea food may also explain why the population of two other bird species in the region, Marbled Murrelets and Western Grebes, have dropped by 90 per cent since the 1950s and 1970s respectively, said the study's co-author Peter Arcese, FRBC chair of conservation biology in the Faculty of Forestry.

With files from Stephanie Mercier