A small hammerhead shark mere feet from the shore of a Nantucket beach Sunday is just the latest sighting that’s captured the attention of Massachusetts beachgoers and shark enthusiasts.
The surprise visitor — so close that it looked as though it might come ashore — is one of a slew of recent sightings that have some beachgoers wondering if sharks are edging closer to shore along the Cape and the Islands.
According to some experts, these close encounters are signs of a slowly unfolding climate crisis. They say there’s been a steady increase in recent years in the number of baitfish moving toward warmer waters, followed closely by predators, amplified by people capturing the remarkable events on camera and sharing them online.
“I don’t think there’s any evidence that this is something new or different that’s happening,” said Nick Whitney, a senior scientist at the New England Aquarium’s Anderson Cabot Center for Ocean Life.
“There have been a large number of sightings in recent days and weeks, but this is also obviously peak summer beach season … which means a lot of eyes looking out in the water,” he said.
Some scientists believe the white shark activity in the area is a result of that species specifically recolonizing its previous range as it rebounds and as populations of seals, one of its favorite foods, rebound.
Since the 1980s, the northern edge of the high-catch density line for tigers sharks — the area where these sharks can be caught in abundance — has moved northward by 250 miles. This year, fishermen have been catching yellowfin tuna 25 miles offshore — that’s about 75 miles closer than in previous years.
Jon Dodd, executive director of the Atlantic Shark Institute, says that it’s likely indicative of higher than normal water temperatures in many areas in southern New England this summer.
“The yellowfin tuna fishery is the best inshore fishery in 25 years,” Dodd said. “You’re doing what is normally overnight fishing for yellowfin, in one day,” Dodd says. Yellowfins favor water temperatures in the 70s and 80s.
The presence of an abundance of sharks (and whales) near the New England coastline should not be seen as a resurgence in populations but as an indication of closer than normal baitfish congregations likely based on warmer than normal offshore water temperatures, Dodd said.
“Water temperatures are a big catalyst,” he said. “The temperature will cool and when it does this will all go away. ... My hope is that people don’t misinterpret this. We are seeing higher concentrations of bait and predators and prey in an area — this isn’t everywhere, it’s here in pockets.”
Sharks are creatures designed to migrate to waters they prefer. Higher numbers of sharks the Atlantic Shark Institute is tagging are tropical fish.
“These tropical region sharks (tiger, hammerhead, blacktip, spinner…) are moving north at a fairly rapid rate,” Dodd says. “The temperatures allow them to. When they run into water that’s too cold for them and they don’t function well, they stop.”
Waters in the mid-50s are preferred by porbeagles, blue sharks, and even some makos. Tropical sharks like hammerheads, tiger sharks, blacktips, and spinners enjoy the warmer waters along the shoreline more than our native makos, blue sharks, white sharks, and porbeagles.
Thresher sharks, which have long, distinguished tails, are known to move close to shore to hunt schooling fish. They slap the school with their tail and pick up the stunned fish along the beach.
When the mackerel and schooling fish come in, the sharks will find them,” Dodd says. “That’s why I suspect we will see a greater increase in sightings of tropical region sharks, because of those species.”
A new NOAA Fisheries study published in January in the journal Global Change Biology, shows “tiger sharks are migrating into northern latitudes earlier and expanding their movements further north due to ocean warming.”
The large-scale northward expansion is driven by climate change, specifically the overall warming of the U.S. Northeast Continental Shelf Large Marine Ecosystem, NOAA researchers found.
“Our tagging and tournament sampling data show that tiger sharks have always spent time in northern latitudes at least going back to the 1960s and 1970s,” said Cami McCandless, study co-author and lead for the NOAA Fisheries’ Apex Predators Program, in a report published by NOAA Fisheries in January. “But now they are not only arriving earlier but spending more time in northern latitudes due to ocean warming.”
Dodd’s summer research confirms NOAA’s findings.
“This summer’s been a little bit unique in that there have been a lot more tropical sharks showing up in our research than in the past,” he said. “We are seeing that trend continue each and every year. It’s equal parts fascinating and disturbing. These sharks are finding themselves more and more comfortable being up here in these northern latitudes because water temperatures are comfortable.”
The Globe reported in May that more white sharks were detected in Massachusetts and Rhode Island waters in 2021 than in prior years.
“What we’re seeing right now is the species recolonizing its historic range as it rebounds and as populations of seals, one of their favorite foods, rebound,” said Megan Winton, staff scientist at the Chatham-based Atlantic White Shark Conservancy, in an email Wednesday to the Globe.
“A lot of folks think white sharks in the Gulf of Maine are a new phenomenon, but there are some great accounts from fishermen in the early to mid 1900s (and earlier!) in that region, before shark populations coast-wide took a nosedive,” Winton said.
“Based on trends in historical catch and sightings records, a few scientists have estimated that the white shark population declined by almost 80 percent between then and the early 1990s, when a shark fishery management plan was put into place and white sharks were listed as a prohibited species in the US Atlantic,” she said.
Another consideration: the prevalence of devices that detect and amplify their presence here.
Dodd notes that the number of receivers deployed to detect sharks has spiked in recent years.
Sunday’s viral hammerhead, Whitney explained, was one of many that have been captured doing “normal sharky things” near the shore. In this case, it appeared to be chasing a fish.
“The biggest trend is just that there seem to be more sightings that go viral because of the fact that now, everyone at the beach has a smartphone and a social media account,” Whitney said.
There were at least 20 shark fin sightings at Nantucket beaches last weekend, according to the Nantucket Current, which reported that the beach sightings “prompted lifeguards to close many of them to swimming for hours.”
While Whitney said Sunday’s several-foot-long hammerhead was no danger to the public, he cautioned beachgoers to remain vigilant about sharks and other ocean creatures when spending time in the water.
“People should be alert, and that’s one of the good things about the sightings: it’s raising people’s awareness that there are sharks in the ocean and they do come close to shore,” Whitney said.
“The ocean is not a swimming pool when you go there,” he added. “You need to be aware of your surroundings and look out for the wildlife.”
The Atlantic Shark Institute has opened two new research projects because of the presence of tropical fish in the region: one on blacktip sharks and one on spinner sharks.” It will investigate where the northern edge of the high-catch density is located and what species they could displace.
The most common shark in New England is the blue shark, which enjoys open ocean waters, and the endangered shortfin makos are now totally prohibited from recreational or commercial fishing.
Dodd said he recently ran into two great white sharks close to the shore of Long Island, New York, that were chasing menhaden, or bunker fish, and mackerel.
“When they move to the shore and get pushed along beaches these sharks will follow them,” he said. “That’s when you have these human and shark interactions, which happen intermittently.”
Dodd said the area was jammed with predators — sharks, tuna, whale, porpoises — converging on baitfish.
“When I say everything, we were out three weeks ago and you could see when bait would break the surface,” he said. We had whales flying toward the boat at high speed. Everything converges on these bait balls. It’s quick and explosive.”
“Where the bait goes, everything else goes.”
This story has been updated with comments from Megan Winton of the Atlantic White Shark Conservancy, and additional information from Jon Dodd, executive director of the Atlantic Shark Institute.
Shannon Larson of the Boston Globe contributed to this report. She can be reached at email@example.com.