Humpback whales have been showing up in Boston Harbor this week, breaching and splashing right in view of the shore. Tony LaCasse of the New England Aquarium spoke with WGBH All Things Considered anchor Barbara Howard about the whale sightings. The following transcript has been edited for clarity.

Barbara Howard: I don't recall whales in the harbor before. How rare is this?

Tony LaCasse: Humpback whales in the harbor probably used to be about a once in a decade, once every 15-year event. But recently, they've been coming into the harbor probably more like every other year. Right now, we have a juvenile humpback whale working the outer harbor, and he is really attracted by some large schools of menhaden, which is a small schooling fish.

Howard: They've been really breaching and leaping out of the water. They're juveniles. Are they like puppies — more active because of that?

LaCasse: We really don't know what the breaching behavior is. Often, people will associate it with two or three different things. One is that it probably is fun. Humpback whales are the acrobats of the whale world. Number two, it actually might give some relief in terms of any kind of skin, crustaceans, that kind of thing. And third, some people think it might be sort of a communication as well.

Howard: How long can we expect these whales to be around?

LaCasse: This particular young male, he'll probably stay around as long as the schooling fish are there, which probably could be a few more days. We are concerned because he's in a busy shipping lane area and there's a lot of recreational boaters. The Coast Guard is announcing warnings to mariners to be on the lookout, because the last thing that we want to do is to have a whale that's struck by boat. And also, those whales are big enough that they're hazards to boats. Somebody striking a whale could easily end up in the water.

Howard: It’s not just this one juvenile. There are multiple whales, right?

LaCasse: We've had a number of different whales come in and out. One young male has been really consistent in terms of working the outer harbor — he's been there since Sunday. He actually, at one point, went almost all the way into downtown Boston. But in the last couple days, he's generally stayed around the area of Deer Island, the northern peninsula at the mouth of Boston Harbor.

Howard: What does this surge in whales say about the health of Boston Harbor?

LaCasse: Boston Harbor's health has improved dramatically since the days of the dirty water, back in the 60s and right up through the mid-1990s, as the cleanup was going along. I actually worked on Thompson Island, and the cleanup had begun then. We used to take a ferry to work each day, and I can remember the first time that the captain on board the ferry saw a harbor porpoise in mid-winter. It created tremendous excitement with the people who worked on the waterfront, because they knew it was a harbinger of good things to come.

Howard: I recall the aquarium once had an exhibit showing school kids just how dirty the harbor was — a big tank with some rust-colored water showing litter that you'd see if you went scuba diving in the harbor, like beer cans and old tires. I remember a grocery store shopping cart.

LaCasse: That exhibit showed people what the need was in terms of the cleanup. Part of the exhibit was showing how flounder, a flat fish on the bottom of the harbor, were actually affected with an abnormal number of tumors. The aquarium actually at that time didn't have an orientation to Boston Harbor. The harbor was so dirty that it was the kind of thing where we were oriented toward the city. Boston, being an older city, its sewage system combined with its street runoff. So when you would get rain of more than two inches, you could have millions of gallons of untreated sewage that would pour directly into the harbor from multiple points.

Howard: So what was put in place to remedy this?

LaCasse: The sewage treatment work was required by a federal court order. That was what began the whole effort to really clean up the harbor, and it took many years and several billion dollars. But I think most people would agree that it's been worth it.

Howard: Thanks, Tony.

LaCasse: Thanks, Barbara.

Howard: That's Tony LaCasse of the New England Aquarium, talking about the sightings of juvenile humpback whales this week in Boston Harbor.