DNA source tracking highlights seagull contamination as key health risk at Canadian beaches

A new study led by Toronto Metropolitan University (TMU) has found that while water quality at Canada’s publicly monitored beaches is generally safe for swimming, seagull contamination is a significant, previously under-recognized contributor to real-world health risks.

Led by TMU professor Ian Young, the comprehensive study was conducted between 2023 and 2025 at popular beaches across the country, including those in Ontario (Toronto and Niagara regions), Manitoba, Halifax, and Vancouver. The research tracked beach water quality alongside the activities and subsequent health outcomes of swimmers.

Advanced source tracking

Across all monitored sites, researchers found that overall rates of reported gastrointestinal illness—such as vomiting and diarrhea—were low, affecting approximately 2.6 per cent of participants after a beach visit.

However, when water contamination spiked, the risk of illness increased for individuals who swam, particularly those who reported swallowing water. To pinpoint the exact cause of these spikes, the research team used advanced DNA “source tracking” techniques to identify the origin of the pollution.

While markers of human sewage were detected at low levels at some sites, researchers discovered that elevated levels of E. coli—the standard indicator for fecal pollution—were predominantly tied to seagull-related contamination. Notably, this seagull contamination was directly associated with an increased risk of gastrointestinal illness.

“We found that while beaches are generally quite clean and safe, there are still occasional spikes in contamination,” Young said. “What stood out in this study was the consistent role of seagulls, and the fact that higher levels of E. coli—often linked to seagull activity—were associated with increased risk of illness, especially when people swallowed water.”

Validating current guidelines

This correlation between avian wildlife and swimmer health is an emerging finding in recreational water safety. Previous beach water quality research and monitoring programs have traditionally focused on human sewage sources, often overlooking the impact of local wildlife.

The TMU study also reinforces the validity of Canada’s existing public health guidelines for recreational water use. The researchers found that the current bacterial thresholds used by health authorities to issue swimming advisories are accurate and effective indicators of increased health risks.

“When those guideline levels were exceeded, we did see higher risk,” added Young. “Overall, the guidelines are a useful and protective tool for the public to make informed decisions about when to swim at public beaches.”

Ultimately, the study highlights how environmental factors and wildlife activity can dynamically influence water quality, providing water managers and public health officials with critical insight into managing beach safety and mitigating wildlife impacts.

Quick Facts: TMU Beach Water Quality Study

• Overall Safety: Canadian beaches were found to be generally safe for swimming, with low overall rates of reported illness (2.6 per cent of participants).

• The Seagull Factor: During temporary spikes in water contamination, seagulls played a consistent role in elevating E. coli levels, which correlated directly with higher gastrointestinal risks for swimmers who swallowed water.

• Methodology: The findings are based on field research from 2023–25 combining rigorous beach water sampling with comprehensive surveys of swimmers’ activities and health outcomes.

• Guideline Effectiveness: Current public health thresholds for issuing swimming advisories were validated as effective tools for protecting public health.