About a dozen visitors were seen removing small crabs and shells from a tombolo on Hong Kong’s Sharp Island on Friday, the first day of mainland China’s five-day Labour Day “golden week” break, prompting conservation officers and volunteers to confront them and issue warnings.

At around 3.30pm on Friday, a South China Morning Post reporter observed the visitors, often accompanied by their families, collecting crabs and shells along the narrow sandbar connecting the island’s two landmasses.

“We...