If you spent time at the beach this summer, you probably encountered seagulls screeching overhead and eating trash. You probably also encountered their poop. Seagull droppings can carry disease-causing microbes that can contaminate beaches and water. The more birds, the more microbes, and the more likely the beach will meet the guidelines for closing, as advised by the Environmental Protection Agency.

Now scientists have found a way to fight back: Release the hounds. In a new study, researchers show that unleashing dogs keeps the seagulls away – and the water at the beach free of microbes.

A team of researchers led by Reagan Reed Converse, an environmental microbiologist at EPA in Chapel Hill, examined Lake Michigan water quality at North Beach in Racine, Wis. During the summer, managers regularly “groom” the beach by turning over the sand, which buries any microbes left from bird poop. North Beach’s managers have also worked to remove other sources of pollution, such as sewage runoff, leaving the gulls as the primary source of contaminants in the water.

The team collected beach water samples for 11 days in August 2011 to get a baseline bacterial count. Then, they sent in the dogs. One hired dog posse – one or two trained border collies and their human handler – chased ring-billed and herring gulls away from the sand from sunrise to sunset. (The handlers make sure that dogs leave endangered species, such as piping plovers, alone.)

After a week of similar beach clearing, the researchers began sampling again – while the dogs kept patrolling – for a total of nine relatively gull-free days.

The dogs had a significant impact. When the team analyzed its samples in the lab, measuring the concentrations of E. coli, Enterococcus, and other contaminants, it detected the presence of potentially pathogenic bacteria like Salmonella and Campylobacter (a species that includes C. jejuni, a common cause of gastroenteritis) for seven of the 11 pre-dog sampling days.

But during the nine gull-free days, the team couldn’t detect the bacterial pathogens at all, and the levels of E. coli and Enterococcus species dropped dramatically and rapidly: A reduction of half the gull population decreased E. coli and Enterococcus species by 29 percent and 38 percent, respectively, the researchers report in Environmental Science & Technology.

Although technically “gull harassment,” Converse says the dogs provide a humane and effective deterrent method, albeit one that can be costly, if the dogs were brought in daily throughout the summer.

The “critical next step,” she adds, is to show that those pathogens are actually infective strains that could get humans sick.

Connecting gull poop to actual human illness is difficult, however, cautions Richard Whitman, a research ecologist and station chief of the U.S. Geological Survey’s Lake Michigan Ecological Research Station in Porter, Ind.: Other birds – including Canadian geese – and mammals could be sources of poop-borne bacteria.

Even so, he says the new study shows how effective controlling seagulls can be in improving water quality.