swimmers

Shark lab researchers say they have a mountain of tracking data that shows juvenile great whites, some as long as nine feet, routinely cruise among Southern California swimmers and surfers with no apparent interest.

Some reservoirs contain automatic machinery operating below the surface and have strong currents which can pull under the strongest swimmers.

Powered vessels could be banned from small Lake District waters to avoid conflicts between sailors and swimmers.

While many people may be aware of the dangers of entering the Thames, other waterways can also be dangerous: strong currents, sudden drops, hidden hazards and cold-water shock can affect even the strongest swimmers.

Researchers will test swimmers' saliva to track cortisol levels - a stress-related hormone - before and after cold water immersion.