– 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 in helping shellfisheries adapt to a changing marine environment. Our immediate emphasis is on the crises caused by the predatory milky ribbon worm and invasive green crab.
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
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 the four years of field experiments in Freeport indicate that it is necessary to update the traditional shellfish management tools. This includes:
- Install an upper size limit of 3.5 or 4 inches. This is because larger clams produce exponentially more spawn than smaller clams. This is a biology-based management tool needed to protect clams spawning stock, similar to what is done for lobsters.
- Adopt Adaptive Ecosystem–Based Fisheries Management: The ecology of our coastal systems has changed. We 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.
- Expand from a Passive Enforcement-Only 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– needs to expand from a police focus to a project management focus with an understanding of clam biology and marine ecology and the ability to perform stock assessments and undertake large-scale conservation projects on the mudflats.
- Swiftly Implement Large-scale Clam 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 for clam farming.
Clam Protection Projects Should Use Science-Based Methods for Protecting Clams from Predators:
- Limiting the effects of predation on small clams clam using a variety of protection techniques is the best strategy that municipal shellfish programs could use to increase commercial clam abundance.
- At this time there is no reason to spend time and money on sediment buffering to raise the pH of the flats.
- 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.
- Flexible netting is a promising clam protection tool, but it will work best in areas where milky ribbon worm populations are low.
- Mud snails eggs can impede clam nets from operating correctly.
- Clam boxes (with mesh netting on both top and bottom) are a better method to protect planted clams from both milky ribbon worms and green crabs.
Utilize Updated Resource Assessment Tools:
- This research has redefined the terms “productive,” and “dead mud.” Data 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.
- Recruitment boxes could be used as a tool to increase information about the production of a particular flat that is generated from traditional clam surveys.
Our Community Impact:
ADAPTIVE FISHERIES MANAGEMENT
DEI provides science-based leadership to adapt to a changing marine environment.
FISHERIES STOCK ENHANCEMENT & RESTORATION
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
SHELLFISH FIELD DAYS
Join us August 11th for this annual event and our ribbon-cutting ceremony for our new facility.