There is a great need in within Maine’s marine economy and coastal communities to adapt to the changing (warming) marine environment.

DEI provides science-based leadership to help shellfisheries adapt to the changing marine environment. Our immediate emphasis is on the crises caused by the predatory milky ribbon worm and invasive green crab. Our applied marine research provides valuable insights in how our communities can adapt to the warming ecosystem.

The Problem: Warming Temperatures are Changing Ocean Ecology

Since the early 1980’s, 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 in 99% of the world’s oceans, and scientists predict seawater temperatures will continue to rise. Maine winters have become milder and shorter (Fernandez, et al. 2015), with winter icing becoming less and less common.

The increased temperatures have led to many negative consequences for fisheries, as explained below.

Higher Temperatures Correspond with Increases in Green Crab Abundance:

In the early 1950’s, Maine experienced warmer seawater, which correlated with an explosion of green crab populations in northern New England (Glude 1955, Ropes 1968, Welch 1968, Grosholz & Ruiz 1996). Recent research by Downeast Institute (Beal et al. 2020a, Beal et al. 2020b, Beal et al. 2018, Beal et al. 2016)  and others (McClenachan et al. 2015) shows that Maine green crab populations are strong, especially along the mid-and southwestern coast where seawater temperatures tend to be highest.

Since the 1980’s Maine’s Soft-Shell Clam Fishery Has Declined

Over the past 40 years, during the same period of time the GoM has been warming, Maine’s commercial production of soft-shell clams, Mya arenaria, has 
decreased nearly 75%. Since then clam landings continue to set new low records. In 2017 statewide clam landings were the lowest since 1933 and in 2020 they declined even further, with only 6,542,197 pounds landed. 

The graph below shows the statewide decline in Maine soft-shell clam landings, as well as rising winter sea surface temperatures in the Gulf of Maine from 1940 to 2018.

Rising Ocean Temperatures are Associated with Higher Levels of Predation on Clams (and other species):

Graph showing 1940- 2021 Maine clam landings and the average minimum winter temperature (January through March). Average minimum temperature from ME DMR Boothbay Harbor records.

Green crabs are biologically adapted to warmer seawater. Locally, their populations have increased with warming waters and are expected to continue to rise with additional ocean warming.

Green crabs are invasive, which means 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 Europe, green crab populations are regulated by both biotic and abiotic factors; hence, their overall effect on creating crashes in shellfish populations is moderated, compared to their effect on similar populations here in North America.

Rising seawater temperatures cause an increase in the metabolic rates of green crabs and most other predators, resulting in hungrier, more active predators during the spring through fall.

Shellfish Management and Techniques to Adapt

DEI’s large-scale applied marine research provides ecological information intended to inform shellfish management policies and suggest techniques to help the fishery adapt to warming oceans.

Update Shellfish Management Tools: Many of Maine’s shellfish management techniques have remained the same for over 55 years (Dow & Wallace 1961, Hanks 1963). The exception was the establishment of a minimum clam size limit of two-inches in 1984. Results from DEI’s extensive and repeated field experiments indicate that updating traditional shellfish management tools to reflect the changing environment would benefit clam survival.  For example:

1. A statewide upper size limit to protect breeding clams. Larger clams produce exponentially more spawn than smaller clams. Instituting a biology-based upper size limit can protect clam- spawning stock, potentially outpacing predation with an increase in clam biomass.  Similar principles are used in managing lobsters. Learn more about DEI’s research into the fecundity of clams.
   
2. Localized Ecosystem-Based Fisheries Management can account for ecological variations along the coast. The ecology of our coastal systems has changed. Fishing communities, through the co-management system, can devise adaptive strategies unique to their regions that take into consideration the high population densities of green crabs and other predators and the longer feeding season these predators now enjoy.
3. Municipal enforcement staff can learn and help implement proactive measures to help the local fishery adapt. The role of paid staff of the municipal shellfish program, typically a Marine Resource Officer/Warden, should expand to include a project management focus, integrating clam biology and marine ecology into clam conservation projects on the mudflats.
4. Large-scale clam predator protection projects can help mitigate against predation and ensure the viability of the local fishery. These projects can take either or both of these forms:
   • Through the town’s Shellfish Program, utilizing the Marine Resource Officer or other paid staff and clammer conservation time.
   • Utilizing the existing intertidal leasing provision, which allows up to 25% of a town’s intertidal area to be leased by individuals, allowing them to institute clam protection methods on their own.

Clam Conservation Efforts Can Apply Lessons DEI Has Learned From Over 30 Years of Research:

1. Limiting the effects of predation on small clams using a variety of protection techniques appears to be the best strategy that municipal shellfish programs can use to increase commercial clam abundance.
2. At this time there is no reason to spend time and money on sediment buffering (spreading shells on the mudflats) to raise the pH of the flats, since our experiments show this practice does not increase survival of juvenile clams (Beal et al. 2020).
3. Due to labor and maintenance costs, green crab exclusionary fencing, such as that used by biologists in the 1950s, is not the best choice for predator protection as fencing is not more effective than netting in protecting clams (see results of our fencing trials from 2013 and 2014).
4. The tradition of brushing flats to increase clam settlement does not work while seawater temperatures are high, because the brush shelters green crabs, who eat the settling clams (Beal et al. 2020b).
5. Flexible netting is a promising clam protection tool, however, it will work best in areas where milky ribbon worm populations are low (see how Milky ribbon worms affect clams here).
6. Mud snail eggs can impede clam nets from operating correctly (see “Reducing Mud Snail Eggs on Predator Protective Netting”)
7. Boxes that protect clams from both the top and bottom are a better method of protecting planted clams in areas where there are milky ribbon worms (see here for information on protecting clams from Milky Ribbon worms).
   

Some of the techniques are explained in this editorial by DEI’s Director of Research Dr. Brian Beal and Maine Clammers Association President Chad Coffin.