The National Park Service said it will limit cruise ship speeds in the lower part of Alaska's Glacier Bay to 10 knots to safeguard whales in the bay.
Previously, cruise ships could operate at 13 knots.
It said "extremely high humpback whale densities" have made the change necessary until further notice.
Cruise ships operate under permits in the bay, a popular part of itineraries in southeast Alaska that bring passengers close to several large glaciers.
The service said reduced speed is the best way to avoid vessel-whale collisions.Follow @tstravelweekly on Twitter.