VANCOUVER - A new study by University of British Columbia researchers suggests there are half as many seagulls in the Strait of Georgia as there were in the 1980s.

They say the marine food supply the gulls rely on has dropped during the same period, and that's a contributing reason the gull population has shrunk.

The study tracks a century of data, showing a rapid increase in the number of seagulls in the early 1900s, with the drop beginning in the mid-1980s.

Lead author Louise Bright says gulls have adapted to changes in their habitat by altering their diet and eating garbage and earthworms.

She says fish populations need to be restored to address the loss of diversity and abundance in the marine ecosystem.

The study also says the numbers of two other bird species -- Marbled Murrelets and Western Grebes -- has dropped by 90 per cent for similar reasons.