Authorities on Tuesday say they have referred 33 people to U.S. law enforcement after the group allegedly harassed a pod of wild dolphins in waters off the Big Island.

It’s against federal law to swim within 50 yards of spinner dolphins in Hawaii’s nearshore waters. The prohibition went into effect in 2021 amid concerns that so many tourists were swimming with dolphins that the nocturnal animals weren’t getting the rest they need during the day to be able to forage for food at night.

The rule applies to areas within 2 nautical miles (3.7 kilometers) of the Hawaiian Islands and in designated waters surrounded by the islands of Lanai, Maui and Kahoolawe.

The state Department of Land and Natural Resources said in a news release that its enforcement officers came upon the 33 swimmers in Honaunau Bay on Sunday during a routine patrol.

Aerial footage shot by drone shows snorkelers following dolphins as they swim away. The department said its video and photos showed swimmers “who appear to be aggressively pursuing, corralling and harassing the pod.”

Enforcement officers contacted the group while they were in the water, and told them about the violation. Uniformed officers met the swimmers on land where state and federal officials launched a joint investigation.

Hawaii’s spinner dolphins feast on fish and small crustaceans that surface from the ocean’s depths at night. When the sun rises, they head for shallow bays to hide from tiger sharks and other predators.

To the untrained eye, the dolphins appear to be awake during the day because they’re swimming.

But because they sleep by resting half of their brains and keeping the other half awake to surface and breathe, they may be sleeping even when they’re maneuvering through the water.