BOSTON – The unusually large concentration of rare right whales that gathered last month to feed in Cape Cod Bay has moved on.

The state reported Wednesday that the North Atlantic right whales have left, citing aerial surveys and acoustic buoys that listen for the whale.

Boaters in the area had been warned to slow down to avoid the animal and stay 500 yards away if they spotted one.

About 25 to 50 whales annually migrate to the bay in April, but unusually rich plankton slicks this year drew a bigger crowd.

Scientists said 75 to 100 animals were in the bay. That’s roughly a quarter of their population, estimated at 350 to 400.

Now they likely have moved north and east of the bay to seek greater densities of plankton.