A school of baitfish swims off the coast of Biddeford in  2018. Robert F. Bukaty/Associated Press, file

Last year was the fifth hottest year on record in the Gulf of Maine, continuing a trend that makes it one of the fastest-warming bodies of ocean on Earth, according to the latest annual report from the Gulf of Maine Research Institute.

Late winter and spring 2023 saw record-setting sea surface temperatures about five degrees above the climatological average. The second half of the year was relatively cool, thanks partly to the mixing effect of a passing storm, and much closer to long-term normals.

The outcome was an average temperature of 52.6 degrees, or 1.9 degrees above the long-term norm, a result that the Gulf of Maine Research Institute, or GMRI, found consistent with the long-term trend of warming conditions driven primarily by anthropogenic climate change.

According to GMRI’s warming report, eight of the last 10 years have been among the Gulf’s top 10 warmest.

“The complete picture of conditions for the Gulf of Maine reveal a region experiencing profound changes,” the report concluded. “Since 2010, the Gulf of Maine has experienced a disproportionate number of anomalously hot sea surface temperatures and prolonged marine heat wave events.”

Here are some other key ocean warming takeaways for the Gulf of Maine in 2023:

• February, March and April set records for the highest monthly average temperature.

• Record daily highs were reported in over half of 2023’s first quarter, including 25 days in February.

• Despite 2023’s hot spring, summer is still warming fastest overall – four times the global average.

• The longest of three marine heat waves lasted 113 days, from January through May.

The 36,000 sprawling square miles of the Gulf of Maine that stretch northeast from Cape Cod to Nova Scotia are warming by a little under 1 degree every decade, according to GMRI. That’s three times the rate of the global ocean, which is warming by about a third of a degree per decade.

The Gulf of Maine is a huge draw for Maine’s $9.1 billion tourist industry and home to a bevy of beloved wildlife, including rare whales and seabirds like the Atlantic puffin, iconic fish stocks like cod and haddock, and the $1.5 billion U.S. lobster industry – all of which are impacted by warming waters.

To see how the gulf’s year compares, 2023 was the hottest year on record for the gulf’s larger North Atlantic home, according to the GMRI report. Average North Atlantic ocean temperatures were 1.5°F above normal, the report said, and set record daily highs on 304 out of 365 days.

The Gulf of Maine is most influenced by the Labrador Current, which brings colder water from the north. The oscillating Gulf Stream is warming faster than the global oceans and is shifting closer to shore, with its warm-water breakaways increasing Gulf of Maine temperatures for months at a time.

The balance between Maine’s two dominant ocean currents is shifting, according to GMRI. The Arctic-infused Labrador Current isn’t as cold as it used to be and the Gulf Stream is warmer and wider than ever, a shift that is contributing to the region’s long-term ocean warming trends.

Even small changes in temperature can have a significant impact on some marine wildlife, including cold-water species like herring, which is in decline, and warm-water species like squid and butterfish, which are both increasing. It’s not just fish, either – puffins are changing what they feed to their chicks.

But it’s not all bad news. In a recently completed 10-year study of Casco Bay, GMRI noted a spike in the population of warm-water species, like blue crab, able to survive the increasingly mild winters to spawn in the spring and develop into a burgeoning fishery.

A report by the institute found 2023 consistent with the long-term warming trend driven primarily by human-caused climate change, although the authors noted individual years could be influenced by large-scale patterns of natural variability, especially on a regional level.

The state’s climate action plan, Maine Won’t Wait, has a whole section dedicated to the study of how a warming, rising Gulf of Maine is impacting the state’s coastal and marine resources and communities, ranging from the fishing industry to residential flood insurance.