One-of-a-kind orca Old Thom holds unique place in New England's marine culture

Sharks aren't the only big animals you might spot in the waters off the Cape.

Last week, an orca named Old Thom showed up off the coast of Cape Cod. He's been seen off Massachusetts and up in Canada's Bay of Fundy for two decades and is one of the few orcas regularly seen in the area.

Here's what to know about our orca visitor and what his presence could mean for local marine life:

Who is Old Thom? 

Old Thom is a large adult orca, also known as a killer whale, said John Chisholm, a scientist who mainly studies sharks at the New England Aquarium, in an appearance on WBUR's Radio Boston. Scientists estimate that he's about 30 feet long, which is quite large for a male orca.

Old Thom has been spotted around the Bay of Fundy, Grand Manan and off the coast of Cape Cod on several occasions. He was already an adult when he was first seen off the New England coast 16 years ago, Chisholm said, so they don't know exactly how old he is.

What makes Old Thom so unique? 

Orcas are social animals, but Old Thom seems to like his alone time.

"Orcas are usually seen in packs and he's pretty much solitary, except he sometimes is seen with a pack of white-sided dolphins," Chisolm said.

Chisholm added that the dolphins could also be "on his menu," but numerous videos online show Old Thom traveling with other white-sided dolphins.

How do we know that the orca that has been coming here for so many years is the same one? 

Chisholm said Old Thom has a unique mark: About two-thirds up the back of his dorsal fin is a "very distinct notch." The dorsal fin is "the part you can see sticking out of the water sometimes, kind of like a shark fin," Chisholm said.

Are there tensions between Old Thom and white sharks? 

No. While orcas do eat white sharks in other parts of the world, Chisholm said Old Thom doesn't seem to be interested in them at all.

"The white sharks don't seem to be bothered by his presence," Chisholm said. "If they had been hunted, if they were aware, they probably would leave the area."

Why don't see see more orcas off of Massachusetts? 

Orcas are rare in the Northwest Atlantic, Chisholm said, but they are here. Over the last few years, there were numerous sightings of orcas off of the Bay of Fundy in Canada.

"The only orcas I've ever seen were in the 1980s, so it's really rare," he said.

In fact, officials with the National Oceanic and Atmospheric Administration spotted two orcas south of Martha's Vineyard on May 25.

And while ocean waters are warming quickly, especially in the Gulf of Maine, Chisholm said he doesn't think that is causing the orcas to show up.

"I don't necessarily think the appearances of the orcas are related to climate change. I think they're traveling the old areas they've always traveled," Chisholm said.

There's also no indication the number of orcas is increasing, Chisholm added.