– Community Impact –

Adaptive Fisheries Management

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 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). Furthermore, 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 icing over of harbors 
becoming less and less common where previously freezing occurred each year.

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 research by the Downeast Institute and others (McClenachan et al. 2015) suggests 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 Soft-Shell Clam Fishery Has Declined

Over the past 40 years, during the same period of time the GoM has experienced warming, Maine’s commercial production of soft-shell clams, Mya arenaria, has 
decreased nearly 75%. 
Last year (2017) statewide clam landings were the lowest since the 1933.

The graph below shows the decline of statewide Maine soft-shell clam landings along with rising winter sea surface temperatures in the Gulf of Maine.

Graph courtesy of Dr. Brian Beal. It uses landings data from DMR and Boothbay Harbor Sea Water Temperature Record data.

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.

Informing Shellfish Management and Offering Techniques to Adapt

DEI’s large-scale applied marine research provides ecological information that can be used to inform shellfish management techniques and help the fishery adapt to warming oceans.

Update Shellfish Management Tools: Many of Maine’s shellfish management techniques have not been changed in over 55 years (Dow & Wallace 1961, Hanks 1963). The exceptions are the establishment of a minimum clam size limit of two-inches that was added in 1984. Results from DEI’s extensive repeated field experiments indicate that updating traditional shellfish management tools to reflect the changed environment would benefit clam survival. This includes:

  1. Install an upper size limit to protect breeding clams.  Larger clams produce exponentially more spawn than smaller clams. This is a biology-based management tool that can protect clam spawning stock, similar to how lobsters are managed.
  2. Adopt Adaptive Ecosystem–Based Fisheries Management: The ecology of our coastal systems has changed. Fisheries will need to adapt to and work around the high population densities of green crabs and other predators, and the longer feeding season these predators have available to them.
  3. Expand from a Passive Enforcement Focused Management Approach to a More Directed Active Management Approach. The role of paid staff of the municipal 
shellfish program – typically a Marine Resource Officer/ Warden – should  expand to a project management focus, integrating clam biology and marine ecology and implementing clam conservation projects on the mudflats.
  4. Swiftly Implement Large-scale Clam Predator Protection Projects. 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.
    • Utilize the existing intertidal leasing provision which allows up to 25% of a town’s intertidal area to be 
leased by individuals so that they can institute clam protection methods on their own.

Clam Protection Projects Should Apply Established Science to Protect Clams from Predators:

  1. Limiting the effects of predation on small clams clam 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 to raise the pH of the flats.
  3. Due to labor and maintenance costs, exclusionary fencing, such as those used by biologists in the 1950s, is not the best choice for predator exclusion.
  4. Flexible netting is a promising clam protection tool, but it will work best in areas where milky ribbon worm populations are low.
  5. Mud snails eggs can impede clam nets from operating correctly.
  6. Boxes that protect clams from both the top and bottom are a better method to protect planted clams from both milky ribbon worms and green crabs.

Utilize Updated Resource Assessment Tools:

  1. Recruitment boxes are an simple tool that can be used to increase information about the production of a particular flat. The use of these boxes has redefined the terms “productive,” and “dead mud.” Data (see this and this for examples) from the recruitment boxes indicates that areas with few live commercial or juvenile clams may actually be tremendously productive in 
terms of wild clam recruitment- it’s just that these animals aren’t surviving to commercial sizes due to predation.

This diagram depicts how a recruitment box works to understand how many juvenile clams are settling onto a mudflat.

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.

Our Community Impact:


DEI provides science-based leadership to adapt to a changing marine environment.


DEI has produced hundreds of millions of seed clams for coastal communities.


DEI has assisted fishermen and other entrepreneurs create new aquaculture opportunities.


DEI’s marine education program makes relevant scientific exploration part of pre-K to 12 grade education


Celebrate our community of marine innovation at our annual festival in Beals.

Start typing and press Enter to search