Vacationing on Nantucket in early July, Terrence Boylan cast his line in to the strangely warm surf. The creature he reeled in left the veteran angler baffled. It was a skinny, 4-foot-long fish with a needlelike mouth and menacing teeth.

“He was completely surprised,” his wife, Jennifer, said. “He didn’t know what it was.”

The Boylans would soon learn Terrence had hooked a houndfish, a warm-water species that may never have been caught before along the Massachusetts shore.

By itself, the catch would be just a fluke. But it is one of a slew of unusual fish reports from the shores of New England in recent years. Scientists studying the warming waters in the region say it is part of a pattern and an ominous signal of climate change.

Ocean temperatures along the East Coast are near or above their warmest levels on record for this time of year, and they are not only drawing in unusual sea creatures but also helping to fuel the busiest Atlantic hurricane season on record to date.

Now, Hurricane Isaias is poised to draw energy from these abnormally toasty waters as it rides up the East Coast and, depending on its course and speed, the consequences could be severe from Florida to Maine.

A marine heat wave

Much of the Eastern Seaboard, from the Georgia coast to southern Maine, is in the midst of what scientists define as a marine heat wave. They occur when ocean temperatures are abnormally warm (in the 90th percentile of available data) for an extended period (at least five days).

Marine heat wave intensity is categorized from moderate to extreme. While the waters off the Southeast coast are mostly in a moderate heat wave, the intensity becomes strong along pockets of the Mid-Atlantic coast before swelling to strong to severe off the shores of Massachusetts and southeast Maine.

Temperatures off the Northeast coast are 5.4 to 7.2 degrees above normal, said Andrew Pershing, chief scientific officer at the Gulf of Maine Research Institute, in an email.

Due to human-caused buildup of greenhouse gases in the atmosphere, marine heat waves have increased dramatically in frequency, size and severity in recent decades. They’ve altered fisheries and killed seabirds in the North Pacific and Bering Sea, and damaged or killed parts of the Great Barrier Reef, a World Heritage site.

A study published in December warns that by late this century there may be “a permanent marine heat wave state” in many parts of the ocean because of continued warming. The oceans are absorbing the vast majority of the extra heat pumped into the climate by the highest levels of greenhouse gases in human history, and marine heat waves and altered ocean currents are just some of the consequences.

The heat waves have profound effects on marine ecosystems, by supporting some species and disrupting others. The website of the Marine Heatwaves International Working Group, operated by leading researchers studying the topic, notes “[r]ogue animals can also find their way well outside their normal range, following the warm waters of a marine heat wave.”

‘Weird’ creatures

According to an online database of observed sea life maintained by Duke University, houndfish, the species caught by Boylan, have been spotted only twice north of Cape Hatteras, N.C., and never in Massachusetts.

David Policansky, a retired scientist who worked at the National Academy of Sciences, sees the catch of a species outside its normal zone as not necessarily caused by the warm water, but made more likely.

The houndfish is just the latest example of a subtropical species finding its way to the Massachusetts coast.

A fisherman himself, Policansky recalls another angler hooking an adult tarpon, a warm water fish common to the Florida coast, on Nantucket last year. “He was shaking when he came in the shop to tell us about it,” Policansky wrote in an email. “[F]or sure nobody had reported hooking one off the beach on Nantucket previously.”

“These warm waters are concerning,” said Glen Gawarkiewicz, an oceanographer at Woods Hole Oceanographic Institution. “We’ve been seeing really weird things the last couple years.”

In an interview, he described Gulf Stream fish being caught in 30 meters of water off Block Island, R.I., in January 2017 and increases in the “rate and amount” of species like mahi-mahi passing through.

“A lot of these species you think of [as living] south of Cape Hatteras and now you’re seeing them in southern New England,” Gawarkiewicz said.

A deadly great white shark attack in Maine on Monday made headlines for its rarity so far north, but scientists disagree over whether warm waters played a role.

Gregory Skomal, a fisheries scientist and shark expert with the Massachusetts Division of Marine Fisheries, doesn’t think there is any connection between water temperature and the attack. Shark tagging data show a “well-established summer presence” of the species off the Maine coast, he said via email.

Bob Hueter, director of the Center for Shark Research at the Mote Marine Laboratory in Sarasota, Fla., said, “There are very likely more white sharks off the Maine coast now than people realize.” But Hueter also said water temperature is a factor in white shark distribution with a “sweet spot” in the low-to-mid 60s. This is around what the water temperatures were where the attack occurred in the Gulf of Maine.

Gawarkiewicz explained waters this warm enter the Gulf of Maine when “warm core rings,” which are small warm pools that have broken off from the Gulf Stream, flow toward the coast. “The sharks definitely use the rings as preferred habitat, so it is reasonable to expect that one might have gone into the Gulf of Maine from the ring,” he said.

His research has shown a doubling in the number of these warm core rings since 2000. This is being driven in part, he said, by a northward shift in the Gulf Stream.

In recent decades, researchers have observed a northward shift in the Gulf Stream, and this, along with changes in other ocean currents, may be causing dramatic warming in some areas, including the Gulf of Maine.

Record northbound shifts in warm water

Warm waters are oozing north farther than they have in modern records, says Kris Karnauskas, a professor of atmospheric and oceanic sciences at the University of Colorado.

In a tweet, he showed that water temperatures as high as 82 degrees (28 Celsius) were about as far north as they’ve been since such data began in 1982. This is just above the temperature threshold for fueling and sustaining hurricanes.

Next he showed the zone of 64-degree temperatures (18 Celsius), which great white sharks prefer, had also shifted farthest north on record.

“It’s pretty shocking actually,” he said in an interview. “The gap between this year and the next northernmost year is huge.”

Karnauskas says the Gulf of Maine is “warming excessively.”

Indeed, analyses have shown the Gulf of Maine has been warming seven times faster than the rest of the world’s oceans during the past 15 years.

Both short-term (back to the early 1980s) data drawn from weather satellites and longer-term (back to the late 1800s) indicators show significant warming in this region, but the warming has accelerated in recent decades.

Karnauskas said human-caused climate change is driving much of the warming, but natural fluctuations in this region are adding more heat compared to surrounding areas, where temperatures are not rising at the same rate.

Potentially dire implications with hurricanes

The marine heat wave doesn’t just have consequences for fish.

The warm waters, not only next to the East Coast but sprawling over much of the tropical Atlantic Ocean, are fueling what is the most active hurricane season on record to date.

Tropical storms and hurricanes derive their energy from warm ocean waters, which makes the presence of such a large area of unusually warm water so worrying to meteorologists. An ocean with high heat content values, with warm water extending through the water column, can increase the odds for storms to rapidly intensify, something we’re seeing more of as the planet warms.

Hurricane Isaias, which is taking aim at the entire East Coast, became the earliest ninth Atlantic-named storm on record Wednesday night. The fifth-named storm to form this month tied the 2005 record for having the most storms on record during July.

This year has also featured the earliest-forming “C,” “E,” “F” and “G” storms on record in the Atlantic — Cristobal, Edouard, Fay and Gonzalo.

Some of the storms which have formed since June have done so much farther north than usual, in areas where ocean temperatures were well above average. Dolly, in late June, flared up farther north than any tropical storm on record during the month, tweeted Sam Lillo, a postdoctoral researcher at the National Oceanic and Atmospheric Administration.

As a measure of how warm the water was in the northwest Atlantic when Dolly formed, a buoy at Georges Bank, 170 nautical miles east of Hyannis, Mass., recorded a sea surface temperature above 80 degrees on June 26. Data shows such a water temperature is unprecedented during the month at that location dating back to at least 1984.

Gawarkiewicz identified the warmer-than-normal water temperatures south of Cape Hatteras as an area of particular concern. “If we do a get a hurricane coming up the East Coast, that’s an area where we could get intensification,” he said Tuesday.

Isaias will pass through those waters Monday.