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Image by Ed Roworth and Rich Signell of the U.S. Geological Survey - USGS

Why Georges Bank Seawater May Be Warming Faster Than Gulf of Maine

Yale E360 recently published an excellent article by Rebecca Kessler on how fish and other marine animals are responding to changing conditions in the Gulf of Maine.  Particularly good news are reports of black sea bass moving into waters north of Cape Cod.

Marissa McMahan, a doctoral student at Northeastern University in Boston. . . grew up crewing on her father’s lobster boat, but didn’t see her first black sea bass until 2012. . . “It skyrocketed, the amount of fish that they were catching as bycatch. I mean, it went through the roof.”

http://e360.yale.edu/author/Rebecca_Kessler/123/

The article is misleading to claim “Scientists report that the gulf’s waters are warming faster than 99.85 percent of the rest of the world’s oceans.” I suspect that this claim is the result of looking at what scientists are reporting for water temperatures in the Gulf of Maine for the last year or three, looking for a trend, and comparing that snapshot in time to “the rest of the world’s oceans.” However, there are oceanographic reasons for why the Gulf of Maine is infamous for greater than the Atlantic Ocean annual variations in seawater temperatures as fishermen know all too well, structural reasons separate from climate change that is also impacts the region.

The Gulf of Maine is a sea beside the Atlantic Ocean. Despite the surface distance, as the crow flies, between Cape Cod and Nova Scotia, the Gulf of Maine is separated from the Atlantic Ocean by George’s Bank east of Cape Cod and Browns Bank south of Nova Scotia, leaving only 30 miles of deep water entrance (exchange) between Gulf and Ocean.  The mighty St. Johns River, the Kennebec, Merrimack and other rivers empty into the Gulf of Maine.  Seawater in the Atlantic Ocean is 36 parts per thousand. In the Gulf of Maine seawater is only 34 ppt.  This might qualify the Gulf of Maine as an estuary, it’s not open ocean.  As we discussed aboard Gannet, it is the shape of the Gulf of Maine like a big wash tub that creates enormous rises and fall of the tides at the northern end and only 6-8 feet on the opposite shore (Boston).  The Atlantic Ocean is too big to slosh like that.

It gets even more complicated.  Of the 232 commercially valuable fish in American waters two fish stocks are cod.  There is the cod of the Gulf of Maine fish stock and the cod of Georges Bank and south of Cape Cod, another fish stock.  These are the southern most populations of Atlantic cod.  Cold water from the Labrador Current enters the Gulf of Maine through a less than 40 mile wide channel.  As a result the cod on Georges Bank are suffering more because the waters there are warmer than in the Gulf.  Climate change factors mean there is no hope for a cod fishery of the fish stock on Georges because that place is becoming less habitable for cod.

Apparently the annual influx of cold Labrador Current water into the Gulf of Maine is fickle.  It is very difficult to observe because the saltier cold ocean water flows in beneath the warmer fresher outgoing water.   Scientists refer to this changing phenomenon as “barn door open,” “barn door closed” and “barn door ajar.”  The recent year when lobsters molted 2-3 weeks early was a “barn door closed” year. There was little cooling in the Gulf of Maine by the Labrador Current.  Scientists have yet to find a pattern in the annual sequencing of barn door open, closed or ajar.  It is as chaotic as mapping the cooling rates of liquid coffee portions within a cup.  You know the surface will cool first but how that cooler water sinks into and exchanges heat with the rest of the fluids is random.

The Yale E360 article correctly explains parts of how the Gulf of Maine ecosystem is reorganizing itself.  Marine life must do so every year due to changing temperature regimes caused by amounts of Labrador Current water entering the gulf.  For cod and other groundfish scientists have observed changes in how much breeding occurs at identified sites coincidental with the three states of Labrador Current input (barn door openings).  Years when the cold water does not enter the Gulf of Maine there is significantly less reproduction of ground fish and years when “barn door is open” significantly more breeding success and more fish of that year class. In the Gulf of Maine, cod reproductive success is more tied to water temperature, than it is effected by the fishermen’s size of catch.  This further complicates the work of the fishery management council.

From the cod’s perspective Georges Bank is warming faster than is the Gulf of Maine because Georges Bank is never cooled by the Labrador Current and it is located further south.  Even with no fishing, expect to see the fish stock of cod on Georges Bank decline faster than the fish stock of cod in the Gulf of Maine.  For the latter cod stock it is likely that one year soon the Labrador Current barn door will open giving this long-lived fish cool water and a good reproductive year.

2 responses on “Why Georges Bank Seawater May Be Warming Faster Than Gulf of Maine

  1. Rob Moir, Ph.D. Post author

    Unfortunately The Working Waterfront continues the myth that a portion of the Atlantic Ocean warming faster than other parts in Ben Martens’ excellent report on the Atlantic States Marine Fisheries Commission’s decision to close northern shrimp fishery:
    “The Gulf of Maine is warming faster than 99 percent of the world’s oceans, and warmer waters are making it harder for some species to live here. Maine fishermen have lost shrimp for the second year in a row and new science suggests that the spawning biomass of cod (another cold water species) is at 3 percent of its target.”
    Whenever, whatever year, Labrador Current water cools the Gulf of Maine, northern shrimp and cod first year class will increase regardless of what fishermen do. Good to reduce these fisheries given abysmal fish stocks; bad to blame fishermen for loss of marine life due to the amount of cold Labrador Current water entering the Gulf of Maine.

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