The best control of swimmer's itch at a bathing beach is to

The best control method for swimmer's itch at a bathing beach is to use fine mesh nets. This is effective because swimmer's itch is caused by cercariae, the larval stage of certain parasitic worms. These cercariae are released from infected snails and can penetrate the skin of bathers, leading to irritation and discomfort.

Using fine mesh nets can help physically block these cercariae from reaching swimmers, reducing the incidence of swimmer's itch significantly. The nets act as a barrier, preventing the parasites from entering the swimming area while still allowing clean water to flow through.

In contrast, adding copper sulfate is primarily used for controlling algae in water bodies and does not have a direct effect on swimmer's itch. Chlorine, while important for disinfecting recreational water, is effective at preventing pathogens in water rather than controlling parasites like those responsible for swimmer's itch. Adjusting chlorine levels, whether increasing or decreasing, does not address the root cause of swimmer's itch, which is related to the presence of infected snails and their larvae in the water. Therefore, using fine mesh nets provides a direct and practical solution to mitigate the risk of swimmer's itch at bathing beaches.