As part of DEI’s four year investigation into the soft-shell clam decline in Casco Bay, DEI examined the effectiveness of different methods to protect shellfish from green crabs and other predators. As part of this we conducted systematic green crab trapping in 2014 and 2015, and combined that [...]
– RESEARCH –
Invasive Green Crabs, Carcinus maenas, have been negatively affecting one of Maine’s most valuable fisheries, soft-shell clams, as seawater temperatures have risen over the past 40 years. DEI has been studying the impacts of green crabs on soft-shell clams for 30 years.
Warming Temperatures are Changing Ocean Dynamics:
- Since the early 1980s, 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 (Pershing et. al 2015).
- Since 2004, SSTs in the GoM have increased faster than 99% of the oceans around the world.
- Maine winters have become milder and shorter (Fernandez, et al. 2015), with icing over of harbors becoming less and less common where previously freezing occurred each year.
- Scientists predict seawater temperatures will continue to rise.
- The increased temperatures have led to shifts in distribution and abundance of marine species, increases in the abundance of invasive predators such as green crabs, and higher frequencies of red tide and other algal blooms, among other changes.
Increases of Temperatures Correspond with Increases in Green Crabs:
- In the early 1950’s, Maine experienced warmer seawaters which was 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 coast 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 adapted to warmer waters. Locally, their populations have increased with warming waters, and are expected to continue to rise along with ocean temperatures.
- Green crabs are invasive, which means that they did not evolve within our ecosystem and foodweb, and have no natural predators to keep their populations in check. Although ducks, fish, and other crustaceans will eat them opportunistically.
- In addition, rising seawater temperatures raise the metabolic rates of most predators and they become more active and hungry during the spring through fall, as their predation rates increase.
Maine Soft-Shell Clam Decline: 1980s to Now
- Concurrent with GoM was warming, Maine’s soft-shell clam landings have declined. Since 1980 SSTs have increased gradually and clam landings have fallen.
- Last year (2016) statewide clam landings were the lowest since the 1940s.
- Over the past 40 years, Maine’s commercial production of soft-shell clams, Mya arenaria, has decreased nearly 75%.
The graph shows the decline of statewide Maine soft-shell clam landings along with rising winter sea surface temperatures in the Gulf of Maine.
Green crabs have existed in southern Maine since 1905, and should be thought of as part of the permanent inhabitants of the ocean environment. They are not going away, and their numbers won’t decline significantly unless water temperature declines and winters get colder.
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.
Qualitative sampling from several shorelines along the Harraseeket, 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 typically are 8:1 female: male).
Watch this video from the Portland Press Herald about green crabs in Freeport.
Eventually, as crabs continue to create burrows that kill the underground roots and rhizomes of the marsh grass and seawater enters these burrows daily, large portions of the bank begin to erode. With time sloughing occurs onto the flats. Erosion of this type poses a major problem to adjacent landowners and municipal planners.
Recently DEI concluded the largest intertidal research project ever conducted in the history of the state of Maine to investigate the soft-shell clam decline in Casco Bay. Our research found that predation, by green crabs and other predators, is the cause of the soft-shell clam decline. Our four-year investigation yielded many discoveries about green crabs. Click here to read more: 2013-2017 Green Crab Research.