Problems in Intertidal Fisheries

Intertidal ecosystems and the fisheries that depend on them are facing many problems. At the same time, people who work in shellfish industries, conduct scientific studies of them, enjoy looking at coastal ecosystems, or draw meaning from them as part of their communities also have different perceptions of these problems.

Here we draw from multiple forms of knowledge including clammer and Indigenous knowledge, ecosystem studies, and social science to identify and describe perceptions about problems in northeastern coastal regions in different ways. We started by focusing on water quality and predation and we will continue to develop this site to describe perceptions of problems related to ocean warming and climate change, acidification, economic uncertainty, social bias, and related issues. We also describe these problems in greater detail in this paper.

We have also added an additional notes section which incorporates some of the follow up resources we have provided to people who have contacted us about some of the ongoing issues in the intertidal. Currently, this includes discussions on biodiversity and changes to the composition of mudflats.

Water Quality

Water quality is any compounding pollution problem that has affected the quality of coastal waters, which is an issue that impacts many communities in Maine. Usually, this pollution is an input of fecal coliform bacteria, or a bacteria found in the waste of humans, birds, cows, deer, and other small mammals. This bacteria when consumed by humans, can cause a number of health problems. The National Shellfish Sanitation Program (NSSP) and the Maine Department of Marine Resources (DMR) run a water quality monitoring program. This program takes water samples across the coast, testing for the presence of fecal coliform bacteria, and assigns closure designations based on the concentration of bacteria. For more information on this program, please click, here.

This system designates four categories, conditionally approved meaning you can harvest there except after storm events, prohibited meaning you cannot harvest there ever, restricted meaning you need a special license to dig there, and finally open, meaning you can harvest there year round. If water quality continues to degrade, closures can cover larger areas and shift from conditionally approved to prohibited, meaning harvesters would no longer be able to dig from those areas.

Many communities are working on resolving their water quality problems. One example would be Waldoboro, ME. The Waldoboro Shellfish Committee helped lead the Medomak Taskforce, a collaborative team made up of DMR representatives, Department of Environmental Protection (DEP) representatives, and members of various conservation groups. To read more about them, click here. For communities interested in resolving their own water quality issues, please visit our Water Quality decision support tool.


Since the 1980s, the Gulf of Maine has become one of the fastest warming oceans on the earth, with winters becoming milder and shorter. 11,15  Warming seawater has the effect of increasing predation on clams, mussels, scallops and other bivalves and enables invasive green crabs, a major predator, to thrive.  Over the same time period that ocean waters have warmed (1980-2019), soft-shell clam landings have declined by 72%.4,13 Recent research discovered a significant negative relationship between Maine clam landings and average winter (January–March) sea surface temperature.13

As climate change continues to increase the temperature of the Gulf of Maine, predation rates on soft-shell clams will continue to rise, decreasing the chance for clams to survive to reach a harvestable size. During this time of warming ocean temperatures, there are two major predators of particular concern: Green crabs (Carcinus maenas) and the Milky ribbon worm (Cerebratulus lacteus).

Green Crabs

Green crabs are an invasive species that arrived in the U.S. in the early 1800s and were first spotted in Casco Bay in 1900.2,4 In the early 1950s there was a unique period of warm  seawater temperatures in the Gulf of Maine and during this time an explosion of green crab populations occurred.9,10,17,21 By 1951 green crabs had been spotted in Lubec. By the mid 1950s Maine’s seawater temperatures cooled back down for the next few decades. However, since, over the last 40 years temperatures in the Gulf of Maine have continually trended upward. Seawater temperature, especially winter severity, seems to be the key mechanism for keeping populations of green crabs down.1-4 Especially since 2012’s “ocean heat wave” green crabs have increased dramatically, decimated clam flats across the coast, and disrupted lobster traps offshore.2-4,12 A single adult green crab can eat up to 50 mollusks a day.3,9 Juvenile crabs are increasingly eating juvenile clams. This is a key factor causing devastation to shellfish populations.1-4,16

Their impacts are explained in this Portland Press Herald video. More information green can be found on the Maine Department of Marine Resource website, here, as well as from the Proceedings from the Maine Green Crab Summit on the Maine Sea Grant website.

Green crabs, pictured above, are considered a major threat to the shellfish industry. Picture by Jeff Nichols, Department of Marine Resources

There are many people who are studying green crabs, creating new predator protection devices, and working to develop a new fishery based on green crabs in order to mitigate the impacts the green crab is having on the shellfish industry. Predator boxes, designed and developed by Dr. Brian Beal of the University of Maine Machias, have been used for localized studies on predator prevention across Maine.1-4,6,9 At the end of the clam growing season, accompanying surveys of the unprotected intertidal flats adjacent to the boxes allow for an understanding of the amount of clams that survive the intense summer and fall predation (i.e. “true survival rate”) allowing for clam growth rates to be assessed.1-4,6 Marissa McMahan and others have been working to develop a commercial fishery for the green crab to support selling it in restaurants across Maine and New England. For more information, please read this article by Kathryn Miles in DownEast. 

Milky Ribbon Worm

Milky ribbon worms, Cerebratulus lacteus, are another predator that eats soft-shell clams and other shellfish. These worms are long, cream colored, flat, and can grow up to 4 ft in size.3,8,16 They are observed commonly in southern and midcoast Maine near clam beds but can be found anywhere on the Atlantic coast, from Florida to Atlantic Canada. Milky ribbon worms may prefer clams over other shellfish and appear to favor slightly larger clams.5,7 Like green crabs they can survive without feeding for extended periods of time.19,22  Milky ribbon worms are regenerative and split apart when under stress. Less is known about the life history of Milky ribbon worms compared to green crabs. It is believed that observations regarding increased densities may be the result of worms congregating on remaining clam beds to feed. Baseline sampling conducted in Gouldsboro, Bremen and Harpswell found milky ribbon worms in each cove in both May and November of 2019.2

Communities have tried combating these worms by removing them while clamming or using predator protection to the extent possible. Hand removal from the flats may not be effective since it encourages the worm to split into pieces. To read more about milky ribbon worms and efforts being made to develop effective predator protection, please visit the Downeast Institute website, here. To read more about milky ribbon worms, please read a guidance document developed by the Casco Bay Regional Shellfish Working Group, here


Based on follow up conversations through our contact page with shellfish harvesters, managers, and other members of the public, we have added an additional notes section which describes some of the questions we were asked as well as our response. The questions have been adapted from original emails and are presented italicized with our response below.

Do you have additional information about why some intertidal organisms are disappearing?

There are a number of compounding reasons why many marine biologists and other coastal scientists believe biodiversity is declining in coastal ecosystems, particularly within the Gulf of Maine. To start, one of the most cited issues is the shifts in planktonic populations that are impacting food webs throughout the Gulf of Maine. This is due to shifts in temperature due to climate change and our geographic location. For a long time, the Gulf of Maine has been home to many species on their southernmost extent, meaning that the Gulf of Maine was as far south as they go. As waters continue to warm, most especially in coastal tidal areas, these species have begun retreating north, shifting to cooler temperatures. In addition, shifts in the food web, where zooplankton and other small critters are shifting both in diversity and population are also impacting some of the larger fauna you may recognize in tide pools. Declines in zooplankton have been seen in estuaries across Maine, usually due to low salinities (more fresh water) and more variable temperature conditions. This is shown in a pretty comprehensive but highly statistical paper Johnson, et al., (2011) that you can download by clicking the link below, that describes this phenomenon in a lot of technical detail. However, that paper also shows that offshore zooplankton populations remained at the time still high and diverse.

More recent news, posted in the link below highlights that this trend has continued over the decade since the paper was published, and is now impacting puffins, whales, and cod. 

A Big Climate Warning from One of the Gulf of Maine’s Smallest Marine Creatures – Inside Climate News

Another impact that is often discussed in the Gulf of Maine is the introduction of non-native species and the human-driven regional changes in the gene pool. The Gulf of Maine Association describes this trend briefly, here, but more actively discusses this in their report Marine Invasive Species that you can download in the link below.

From this report developed in 2010, they highlight those changes such as increased presence of aquaculture, habitat modifications such as dredging, and climate change will continue to shape how invasive species are able to survive and, in some cases, thrive in coastal waters. The competition between invasive species and native species may continue to contribute to shifts in biodiversity.

Beyond both of these Maine-centric sources, the Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services published a report in 2019 that summarizes broad trends in biodiversity. It is extremely large and in-depth, so below is a subsection that describes the summary findings for policy-makers.

One of the major findings, discussed in the article linked below, is the impact of marine pollution and coastal development. These issues among many others are seen on the upswing here in Maine as well and will impact biodiversity into the future.   

World is ‘on notice’ as major UN report shows one million species face extinction | | UN News

Overnight, the mud near me has changed from looking smooth to looking very aerated or textured, why?

There could be many reasons the mudflat looks textured, but most likely, based on your description and how quickly it came about, is likely due to the waves and tides. Generally, mudflats can be very smooth and almost glasslike where the tides come in smoothly, and there aren’t large waves. This happens because there are shallow, large expanses of mud. Near a channel or deeper area however, waves can get larger, taking air from the surface and driving it towards the bottom. This creates a kind of bubbly texture, where soft mud has been aerated. A few days ago, there would have been large gusts of wind that corresponded with a larger tide near you, most likely creating more waves and shaping the mud. 

Alternatively, an aerated texture in mud can happen when there are moving populations of animals, such as green crabs, which generally stay towards deeper areas near mudflats, and can cause pocket like indentations in mud as they move and feed. However, there would most likely be pockets across the flat, not just near the deeper part of a channel, it is a possibility. Another possibility is the warmer weather impacting bacteria that live in the mud. Bacteria such as cyanobacteria, which create oxygen and other gases, may simply be making more gases because the water is warming with changing weather.


  1. Beal, B. F., Coffin., C.R., Randall, S.,F., Goodenow, C., Pepperman, K., & Ellis, B. 2020a. Interactive effects of shell hash and predator exclusion on 0-year class recruits of two infaunal intertidal bivalve species in Maine, USA. Journal of Experimental Marine Biology & Ecology 530-531, 151441. Jembe.2020.151441.
  2. Beal, B. F., Randall, S.R., & K. Pepperman. 2020b. Comparative Field Trials to Examine the Efficacy of a  Traditional Management Tool—Brushing—To Enhance Local Densities of 0-Y Class Recruits in the Soft-Shell Clam Mya Arenaria L. Fishery in Maine, USA. Journal of Shellfish Research 39 (3), 1-17.
  3. Beal, B. F., Coffin, C. R., Randall, S. F., Goodenow, C. A., Pepperman, K. E., Ellis, B. W…Protopopescu, G. C. (2018). Spatial Variability in Recruitment of an Infaunal Bivalve:Experimental Effects of Predator Exclusion on the Softshell Clam (Mya arenaria L.) along Three Tidal Estuaries in Southern Maine, USA. Journal of Shellfish Research, 37(1), 1–27. doi: 10.2983/035.037.0101.
  4. Beal, B. (2016) Green crabs: ecology, and their effects on soft-shell clams. Green CrabSummit. Orono, Maine. December 16. 2016. 2013MGCS/ Beal%20MGCS%202013.pdf
  5. Beal, B. F., Nault, D.-M., Annis, H., Thayer, P., Leighton, H., & Ellis, B. (2016). Comparative, large-scale field trials along the Maine Coast to assess management options to enhance populations of the commercially important softshell clam, mya arenaria L. Journal of Shellfish Research, 35(4), 711+.
  6. Beal, B. (2015) Interactive effects of tidal height and predator exclusion on growth and survival of wild and cultured juveniles of the soft-shell clam, Mya arenaria L., at two intertidal flats in southern Maine. Maine Sea Grant Publications. 40
  7. Bourque, D., Miron, G., Landry, T., & MacNair, N. G. (1999). Endobenthic predation by the nemertean cerebratulus lacteus in soft-shell clam (mya arenaria) populations in prince edward island. (). Retrieved from ASFA: Aquatic Sciences and Fisheries Abstracts; Natural Science Collection Retrieved from
  8. Coombs, M. (2017). The elephant in the flats. National Fisherman, 98(2), 10.
  9. Glude, John B.  1955. The Effects of Temperature and Predators on the Abundance of the Soft-Shell Clam, Mya Arenaria, in New England. Transactions of the American Fisheries Society 84:1, 13-26, DOI: 
  10. Grosholz, E. D. & G. M. Ruiz. 1996. Predicting the impact of introduced marine species: lessons from the multiple invasions of the European green crab Carcinus maenas. Biology Conservation 78: 59–66. 
  11. Fernandez, I. J., C. V. Schmitt, S. D. Birkel, E. Stancioff, A. J. Pershing, J. T. Kelley, J. A. Runge, G. L. Jacobson & P. A. Mayewski. 2015. Maine’s climate future: 2015 update. Available at: http://cci.siteturbine. com/uploaded_files/ Future_2015_Update2.pdf.
  12. Floyd, T. and J. Williams. (2004) “Impact of green crab (Carcinus maenas L.) predation on a population of soft-shell clams (Mya arenaria L.) in the Southern Gulf of St. Lawrence.” Journal of Shellfish Research 23(2), 457+.
  13. Maine Department of Marine Resources, 2019. Maine Commercial Landings. https:// 
  14. McClenachan, L., G. O’Connor & T. Reynolds. 2015. Adaptive capacity of co-management systems in the face of environmental change: the softshell clam fishery and invasive green crabs in Maine. Marine Policy 52:26–32. 
  15. Pershing, A.J., Alexander, M.A., Hernandez, C.M., Kerr, L.A., Le Bris, A., Mills, K.E., Nye, J.A., Record, N.R., Scannell, H.A., Scott, J.D., Sherwood, G.D., Thomas, A.C., 2015. Slow adaptation in the face of rapid warming leads to collapse of the Gulf of Maine cod fishery. Science 350, 809–812. 
  16. Ritger, R. K. (2004). The biology and systematics of a new species of ribbon worm, genus 0RW1S34RfeSDcfkexd09rT2tubulanus1RW1S34RfeSDcfkexd09rT2 (phylum nemertea) (Order No. 1421360). Available from ProQuest Dissertations & Theses Global. (305214358). 
  17. Ropes, J.W. (1968) The Feeding Habits of the Green Crab Carcinus maenas (L.) Fishery Bulletin 67(2), 183-203.
  18. Tan, P., E. Bryan B. Beal (2015) Interactions between the invasive European green crab,Carcinus maenas (L.) and juveniles of the soft-shell clam, Mya arenaria L. in eastern Maine, USA. Journal of Experimental Marine Biology and Ecology 462, 62-73.
  19. Thiel, M., & I. Kruse (2001) Status of the Nemertea as predators in marine ecosystems. Hydrobiologia 456. P. 21-32
  20.  Whitlow, W.L. (2010) Changes in survivorship, behavior, and morphology in native soft-shell clams induced by invasive green crab predators. Marine Ecology 31(3), 418-430.
  21. Welch, W. R. 1968. Changes in abundance of the green crab, Carcinus maenas (L.), in relation to recent temperature changes. Fisheries Bulletin 67: 337–345.
  22. Wilson, C. B. (1900). Habits and Early Development of Cerebratulus lacteus Verrill.