Warming Temperatures are Changing Ocean Dynamics:
- Gulf of Maine (GoM) Sea Surface Temperatures (SST) have been rising faster (0.03 °C/year) than the global mean rate of 0.01°C/year since the early 1980s, (Pershing et. al 2015).
- Since 2004, SSTs in the GoM have increased faster than 99% of the oceans around the world. Scientists predict seawater temperatures will continue to rise.
- Maine winters have become milder and shorter (Fernandez, et al. 2015), with annual icing over of harbors becoming less common.
- Increased temperatures have led to shifts in the distribution and abundance of marine species, with invasive predators such as green crabs, as well as some native predators, becoming more abundant. Red tide and other algal blooms also occur more frequently.
- Summertime temperatures are now lasting until mid-fall, elongating the time period in which predators are foraging and feeding at a high intensity.
A female green crab with a just-released egg mass (27 June 2013).
Increases of Temperatures Correspond with Increases in Green Crabs:
- In the early 1950s, Maine experienced warmer sea waters, which correlated with an explosion of green crab populations in northern New England (Glude 1955, Ropes 1968, Welch 1968, Grosholz & Ruiz 1996).
- Recent work in Maine suggests that green crab populations are strong, especially along the mid- and southwestern coasts where seawater temperatures tend to be highest (McClenachan et al. 2015).
Rising Ocean Temperatures are Associated with Higher Levels of Predation on Clams and other species:
Green crabs are biologically suited to warmer waters. Locally, their populations have increased with warming waters, and are expected to continue to rise as ocean temperatures continue to increase.
- Green crabs are invasive, which means that they did not evolve within our ecosystem and food web. Ducks, fish, and other crustaceans will eat them opportunistically but not at a rate that will keep their populations in check..
- Rising seawater temperatures increase the metabolic rates of most predators and they become more active and hungry from the spring through fall, further increasing predation rates during times when vulnerable larval clams and other shellfish are swimming and settling onto flats.
Maine Soft-Shell Clam Decline: 1980s to Now
- Since the 1980s, SSTs have increased while clam landings have fallen.
- 2017 clam landings were the lowest in the past 80 years.
- Maine’s commercial harvests of soft-shell clams, Mya arenaria, have decreased nearly 75% over the past 40 years,.
The graph shows the decline of statewide Maine soft-shell cla
m landings along with concurrent rising winter sea surface temperatures in the Gulf of Maine.
Green crabs have been found in southern Maine since 1905-, and should be considered permanent inhabitants of the Gulf of Maine ocean environment. Their numbers won’t decline significantly unless water temperature declines and winters get colder.
Habitat Impacts
Green crabs cause coastal erosion through their burrowing and foraging behavior.
Clammers discovered that the edge of some upper intertidal areas colonized by salt marsh grass, Spartina alterniflora, provides refuge for many green crabs. The crabs burrow into the roots and rhizome structures of the plants and carve out elaborate subterranean burrows and galleries.
A piece of salt marsh bank on a mudflat near Flying Point, Freeport, Maine (28 June 2013).
Sampling of several shorelines along the Harraseeket River, lower Maquoit Bay, Harpswell Cove, and Johnson Cove on Chebeague Island have shown that the burrows are colonized mostly by female green crabs along with a few large males. Sex ratios in the colonies are typically 8 females:1 male.
As crabs continue to create burrows that kill the underground roots and rhizomes of the marsh grass, seawater enters these burrows daily, and eventually large portions of the bank erode and slough onto the flats. Erosion caused by sea level rise is a major concern for adjacent landowners and municipal planners and will be exacerbated by green crab colonies.
DEI conducted the largest intertidal research project in Maine history in order to determine the cause of the soft-shell clam decline in Casco Bay. Our research found that green crabs and other predators are causing the soft-shell clam decline. Our five-year investigation yielded many discoveries about green crabs. Read more here: 2013-2017 Green Crab Research.
