The link between shellfish mortality and predators

Brian F. Beal, Professor of Marine Ecology (University of Maine at Machias)

Director of Research (Downeast Institute)

How important are natural predators such as green crabs, bottom-feeding fish, ducks, gulls, marine worms, horseshoe crabs, moon snails, and other consumers of soft-shell clams on clam populations? That question has provided much of the focus of my fieldwork along the coast of Maine for the past twenty-five years. Here, I distill some of the published and unpublished research that I and others have discovered about soft-shell clam predators along the Maine coast.

Although some would argue with my opinion, it appears to me that predation on soft-shell clams by the consumers mentioned above is one of the largest contributors to clam mortality in Maine, and, therefore, is one of the most costly in terms of clam production from our intertidal flats. The percent of clam mortality due to predators changes with the size of clams. Generally, smaller clams are preyed on more heavily than larger clams. If we think about the size of clams when they first settle from the plankton onto the intertidal flats (about 180 microns, or 0.007 inches), we begin to see the picture. These animals are smaller than grains of sand! These animals are under enormous risks from predators just a bit larger, and too numerous to recount in this brief article. But, as clams grow from these tiny, microscopic specks into “larger” clams about the size of your thumbnail (which occurs between the time they settle to the flats in the summer and early fall to the following spring and early summer), they are now “the perfect size” for many predators that we commonly see when we are clamming.

Perhaps the biggest menace to Maine’s soft-shell clam population comes from the European green crab, Carcinus maenas. This is an invasive species, and does not belong in our waters. Introduced accidentally from the British Isles to Long Island Sound during the period of the Civil War, presumably from the ballast water of ships or clinging to seaweed that fouled the bottoms of these ships, the animals quickly colonized harbors, coves, and embayments both north and south of that region. According to Leslie Scattergood’s account of green crab migration in Maine waters that was first published in a Maine Department of Sea and Shore Fisheries Circular in 1952, these animals first appeared in Casco Bay around 1900. By 1930, the recorded range was from “Thomaston, Maine to New Jersey.” Scattergood reported green crabs as far east as Winter Harbor in 1939, but none east of that location as late as 1942. By 1950, green crabs were reported in large numbers from what used to be the most productive flat in Jonesport, the Great Bar. In 1951, Scattergood found the first green crab in Lubec, which was the northern record of its range at that time. It did not take long for green crabs to cross the international border into Canada. The first report of green crabs in Passamaquoddy Bay in the Digdeguash River occurred in July, 1951. Today, green crabs can be found throughout the Bay of Fundy, Nova Scotia, and into Prince Edward Island.

Scattergood has an excellent recounting of how green crabs migrated “against the tide” from Portland north towards Lubec and Eastport.

“Undoubtedly, man’s activities are partially responsible

for the remarkable spread of Carcinides (the genus name

in 1952, which was changed to the more familiar Carcinus).

The lobster and sardine fisheries probably provide the

principal means by which crabs may be transported from

one area to another. Since the crabs can live for several

days out of water, it is relatively easy for the crabs to be

carried in lobster smacks, lobster-carrying trucks, lobster-

fishing boats, sardine carriers, and sardine-fishing boats. I

have seen live crabs in crates of live lobsters and have noticed

them aboard sardine carriers and fishing boats…For many

years, lobsters have been carried about from fishing ground

to lobster pound to market, and, in these moves, often covering

hundreds of miles, there were many opportunities to spread

live green crab over wide areas.”

The following account is so remarkable that it is almost unbelievable, but gives an excellent idea of what clammers in eastern Maine where up against during the 1950’s when clam populations decreased by nearly 80% from 1950, when 6.9 million pounds of clams were landed, to 1959, when 1.5 million pounds were landed. In 1953 and 1954, the Department of Sea and Shore Fisheries (now the DMR) fished “three standard lobster traps” two days in the first week of each month, May through October, at a site in Jonesport known as Cumming’s Beach. The traps were actually set below mean low tide off of Cumming’s Beach in Moosabec Reach. The account below comes from a presentation made by Walter Welch at the Fifth Annual Clam Conference in Boothbay Harbor in March 1955.

“The only major difference between 1953 and 1954 catches

was found at Jonesport, where the 1954 catches were consistently

lower by one to two hundred crabs per trap per day.”

Since that time, we have learned just how devastating green crabs are on clam populations by looking specifically at areas that have been seeded with small clams and comparing plots protecting clams with netting vs. adjacent plots where no netting is used. For example, studies conducted in Stockton Springs and Searsport with my students at UMM in this decade showed that netted plots enhance the survival of seeded clams by a factor of seven. Similarly, my colleague, Gayle Kraus, and I showed that protective netting increased survival of hatchery-reared seed clams by 20% at a site in Jonesboro along the Chandler River. In addition, the netting resulted in an increase of wild seed by three-fold compared to areas without netting. Using predator netting does not always result in greater clam survival or enhance wild spat abundance. In that same study at a different location (Holmes Bay, Cutler), we found relatively high survival rates (ca. 75%) in both netted and unnetted plots. Similarly, in Georgetown in the early years of this decade, my colleague, Stephen Fegley, and I found no real differences in numbers of small clams sampled in netted plots vs. areas outside of the netted plots. Typically, areas with large populations of green crabs are associated with lots of seaweed attached to rocks along the upper shore. It has been my observation over the years that many of the large, expansive flats that do not have lots of rocks and seaweed have few green crabs. Recently, I visited a flat in Sargentville along the shores of Eggemoggin Reach. Twenty years ago, the flat was considered very productive, with lots of commercial size clams. What I observed was the lack of any commercial clam populations at the flat, with many small pits in the sediments created by green crabs at high tide. The shore of that flat is surrounded by ledge with a large abundance of seaweed that contained hundreds of small green crabs that use it for cover at low tide.

Although there can be a separate story about all clam predators, the one that causes severe damage to clam populations in Cobscook and Passamaquoddy Bay needs to be reported here. This is the moon snail, of which there are two species: the northern moon snail, Euspira heros, and the spotted moon snail, Euspira triseriata. My mentor at UMM when I was a student here in the late 1970’s was John Commito. He conducted a series of investigations at a flat in North Lubec and discovered that moon snail predation, particularly on the young juveniles (1-2 year old clams) was so intense, that only 3-4% survived to reach the next year. He found that once clams reach about an 1.25-inches, that moon snail predation drops off to practically nothing. Last summer and fall, I conducted an experimental investigation of moon snail predation on soft-shell clam juveniles with some Lubec clammers on a flat not too far from Commito’s study site. We placed small, hatchery-reared clams (about a half-inch in length) in plots with and without netting at two different tidal heights (low tide and mid tide) in May, and let the experiment proceed until November. Of the 360 clams we planted at the low tide, we recovered 4 live clams, and nearly 80% were found dead with a countersunk hole in one valve, typical of moon snail predation. Of the same number planted near the mid tide, we found 12 live clams, and nearly 75% that had been preyed on by snails. This is an incredibly efficient and destructive predator, and we were not able to keep it from preying on clams in netted plots as it can bury underneath the netting because it moves through the mud.

Predators take a huge toll on our Maine clams each year. Their effectiveness depends on where you are along the coast, what kind of flats you have in your area, the size of most of your seed in your flats, and, most importantly, the number and kinds of predators in your area. What controls predator populations is a great question that will have to wait for another issue.

Lastly, I should acknowledge those marine biologists who have spent many hours working the mud, sand, and gravel of our beautiful coastline sampling clam populations, devising field experiments, and collecting data to try and better understand the relationships between clams and their predators. Although it is likely that some clam research in Maine occurred prior to the 1940’s, most of the work I have read about seemed to begin in that decade, with an explosion of activity in the 1950’s associated with an annual “Clam Conference” hosted in Boothbay Harbor by state and federal scientists. Although this should not be considered an exhaustive list, the following is my best attempt to name those individuals (and their affiliations at the time of their work) who have contributed to our knowledge of clam predators and their effect on clam populations in Maine.

Maine Department of Sea and Shore Fisheries (DMR):

Ron Aho, Hannah Annis, Don Card, David Clifford, Ted Creaser, Robert Dow, Walter Foster, Phil Goggins, Donald Harriman, John Hurst, Denis-Marc Nault, Malcolm Richards, Leslie Scattergood, Brad Sterl, Louis Taxiarchis, Dana Wallace, Walter Welch, Hal Winters.

U.S. Department of the Interior, Fish and Wildlife Service:

Gareth Coffin, William Brown, John Glude, Robert Hanks, Harlan Spear, Alden Stickney.

Maine Universities and Colleges

Will Ambrose, Jane Arbuckle, Carolyn Baier-Anderson, Robert Bayer, Brian Beal, Sam Chapman, John Commito, William Congleton, Laurie Connell, David Dean, Stephen Fegley, Mark Green, Scott Hamilton, Herb Hidu, M. Gayle Kraus, Carter Newell, Matthew Parker, Chris Petersen, Robert Vadas, Tracy Vassiliev, Kenneth Vencile, Les Watling,

Universities outside Maine:

Monica Bricelj, Lindsay Whitlow, Kenneth LaValley

Other Businesses and Agencies

Christopher Heinig