PROVIDENCE, R.I. (AP) _ An unusual number of whales in the waters off Rhode Island this year may simply be following their favorite fish, scientists said.

Humpback whales, which for 10 years have clustered around Steelwagen Bank 13 miles north of Cape Cod, are being sighted off Block Island in increasing numbers.

''Now, it's pretty tough to find a humpback north of Chatham (Mass.),'' said Michael Payne, research biologist with the Manomet Bird Observatory in Plymouth, Mass., which is studying the distribution of whales for the National Marine Fisheries Service.

One reason may be the changing distribution of the sand lance, a fish which is the humpbacks' favorite food, said Howard Winn, oceanography professor at the University of Rhode Island.

There may not have been enough food off Cape Cod to support the burgeoning sand lance population, so the fish migrated to the waters southeast of Rhode Island, Winn said.

Aerial photographs of the area from Block Island to Shinnecock Inlet on Long Island show an increase in whales, according to the Okeanos Ocean Research Foundation of Long Island, N.Y.

Last year, there were 200 to 300 animals; this summer, there are more than 800, said Judy Raab, the foundation's administrative director.

The number of humpbacks in the Gulf of Maine has also increased, from roughly 200 in 1981 to 325 in 1986, said Steven Katona, biology professor at the college of the Atlantic in Bar Harbor, Maine.

Research on the numbers, distribution and behavior of whales is somewhat sketchy. The animals swim great distances underwater and are believed to live up to 60 years.