The Gulf of Maine is warming three times faster than the average global ocean, driving some cold-water species like Atlantic herring — the preferred lobster bait — farther away from its shoreline spawning habitat earlier than usual and attracting species from warmer southern waters, including blue crab and black sea bass, a new survey found.
The warmth is stressing some of Maine’s keystone fisheries, according to the Gulf of Maine Research Institute’s report on the Casco Bay ecosystem released recently. The report is the first time the institute took a longer, 10-year look at the pace of environmental changes and their effects on ecosystems close to shore. It found that warming waters related to climate change, along with human activities, ocean acidification and harmful algal blooms, are causing different behaviors in species that could hinder their ability to reproduce and thrive.
The average sea surface temperature in the Gulf of Maine is rising almost 1 degree Fahrenheit per decade, driven by climate change. The Atlantic herring juveniles are likely moving into deeper waters near the shore earlier in the season because they cannot withstand the warming waters, institute researchers believe. The research was conducted close to the shore in Casco Bay, a 230-square-mile area in the Gulf of Maine containing Portland and 13 other coastal communities in southern Maine.
“This could have implications for mortality as the juveniles may not be able to take advantage of important cover habitat for as long as they should,” said Graham Sherwood, a senior scientist at the institute who also heads its Fisheries Ecology Lab. “The nursery areas they have relied on for probably millennia may no longer be as available.”
Atlantic herring use inshore areas, including docks, as nursery habitats in their first year of life. They also use inshore waters to grow when it is cold enough, and then move to deeper, offshore waters as they grow.
Atlantic herring have been the preferred bait of the lobster industry, but a collapse of the stock and fishing quotas in the last several years have caused a switch to other bait, including menhaden and redfish. Atlantic herring have also been an important food for groundfish, tuna, whales and seabirds.
“Atlantic herring are the centerpiece of the entire Gulf of Maine food web,” Sherwood said. “They’re an incredibly important species.”
Atlantic herring made up 7 percent of the 204.7 million pounds of commercial fish landings in Maine 2023, according to preliminary data from the Maine Department of Marine Resources. Lobster led the list at 46 percent of the landed catch. Atlantic herring catches weighed in at a little more than 15 million pounds in 2023 as fishing quotas eased a bit. That is up from the 11.5 million pounds in 2020, when scientists said it was overfished and regulators initiated strict fishing quotas, with 2022 landings at a five-year low of less than 4 million pounds.
A clear sign that warming is affecting their population is that while they normally grow when the water warms, they can only grow to a certain point called the “thermal maximum.” If the waters are warmer, they do not grow as much. The study showed that Atlantic herring grew less in the warmest years in Casco Bay, which means they exceeded their thermal maximum and could not grow further, Sherwood said.
“If we continue to keep going beyond that thermal maximum, it could have implications for their growth and reproduction and, ultimately, their survival,” he said.
At the same time, other species from southern waters, including blue crab and black sea bass, have moved into Casco Bay in larger numbers. They have a better chance than species that prefer cold water of surviving Maine’s milder winters, Sherwood said. That could slowly change the fisheries in the area, he said, because very aggressive blue crab can win in a fight against native lobster and other species. On the other hand, blue crab is very sought after as a fishery.
“It’s alarming that a lot of the species that we have relied upon for many decades are looking like they’re not doing as well,” he said. “Industries have been built around those species for centuries, and you can’t shift on a dime to another species.”
In its 10 years of monitoring, the institute has caught 50 unique species in its beach seine that is operated from the shore. The eight most common catches are green crab, Atlantic silverside, winter flounder, mummichog, alewife, Atlantic herring, Atlantic tomcod and American sand lance. Warmer water species from the south have expanded the range in which they live recently and are becoming more abundant, including permit, crevalle jack, white mullet and summer flounder.
“Other than green crabs, which were introduced a little over 100 years ago, we haven’t seen the types of range expansions in recent memory that are taking place right now,” he said.
Lori Valigra is an investigative environment reporter for the BDN’s Maine Focus team. She may be reached at [email protected]. Support for this reporting is provided by the Unity Foundation and donations by BDN readers.
