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The Sound School Inter-District Marine Education Program Newsletter

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Habitat Information for Fishers and Fishery Area Managers

Understanding Science Through History

The Sound School – December 2013

Historic Eelgrass and Green Crab Habitats

Did Green Crabs Called the World’s Worst Invasive Species
Come to America Also Wrapped in Eelgrass?

Tim Visel – The Sound School

Vol. 1 #5

Historic Eelgrass and Green Crabs Habitats
“The green crab – this crab which is one of the most common species on the coast of Great Britain, also abounds upon our Atlantic Coast from Cape Cod to New Jersey and perhaps farther south. It is very abundant in Vineyard Sound, Buzzards Bay, and Long Island Sound. The green crab is an article of food in some parts of Europe, where it occurs abundantly, pg 774, Section I, Natural History of Aquatic Animals by Richard Rathbun, United States Fish Commission 1887.”

This historical passage over a century ago speaks to one of the largest recent questions regarding what has been called the world’s worst invasive species – green crabs, and introduces a potential bias in the current scientific literature regarding it. Much has been written about green crabs but very rarely has it been mentioned as an edible species. This fact alone should warrant a full review of its projected timeline of introduction here. It speaks to how much habitat knowledge has been lost in the last century and how our cultural perspective influences recent science.

The green crab today remains a valuable edible species along the English and Dutch Coasts, and shares a similar habitat profile with eelgrass Zostera marina. Green crab catch statistics about 2,000 Tons per year in the North Sea mostly Dutch, English and French coastal communities, and common names include “edible crab” or “shore crab.” Many recipes are on the Internet – large specimens only – as large as or slightly larger than our rock crab specimens. A habitat suitability model (Jodi Harney, NOAA July 2008) has been developed for NOAA – CORI project 2008-11 found than analysis of oceanographic and biotic factors indicated that the presence of eelgrass to be highly significant to green crab habitat (pg 5).

It is known that both green crabs and eelgrass were both prevalent along the North Sea Coast in the 1500s and 1600s (Sabrina, et al, US EPA National Center of Environmental Economics, Washington DC 2007) (Trask, et al, Recent Marine Sediments National Academic of Science 1967). Ocean transit at the time may have resulted in both species’ introduction: the green crab as an edible ship board staple and eelgrass as a packing life support vector for it. We need to review the possibility that centuries ago a North Sea strain of eelgrass may have been introduced into North America with green crabs as food, a perspective found in historical fisheries literature but missing today from current studies (George Goode 1887).

The long term habitat histories of eelgrass and green crabs may coincide in terms of habitat ecological services. Shellfishers in the 1950s had linked the spread of green crabs to the return of eelgrass on Cape Cod in the 1950s (John Hammond, personal communication 1981-82). This habitat association needs additional review.

One of the most curious discoveries of George Nichols 1920’s Vegetation Investigations was a series of extensive marine cores (vertical borings or plugs) in New England coastal coves. The bottom of such cores and salt marsh had no eelgrass remains as he surmised should be contained in these samples. It was a New Haven core that was most noteworthy. It was the most perplexing to him, and on pages 543-544 is found this section:

“Assuming the vertical or historic order of succession during the development of the marsh to have been coordinate with the present day lateral sequence of zones, as set forth in the second paragraph above, the peaty and mucky deposits underlying a salt marsh should show approximately the following sequence of layers, from below upward: (1) a layer of silt, with remains of eelgrass, extending from a variable depth to low tide level; (2) a layer of silt, with but few vegetable remains, extending from low tide level up to the level at which the salt marsh grass becomes established; (3) a layer of muddy peat with more or less abundant remains of salt marsh grass, extending upward nearly to mean high tide level; (4) a layer of peat made up largely of the remains of salt marsh peat along the New England coast has revealed a very different state of affairs. Bartlett (’09), for example, describes a salt marsh near Woods Hole in which the salt marsh peat near the surface is underlain the remains of a former Chamaecyparis bog, the stumps of large numbers of trees being preserved in situ. C.A. Davis (’10) reports that in the vicinity of Boston, the peat deposits underlying the salt marshes likewise consist, in many cases, of the remains of fresh water vegetation; in other cases peat deposits composed largely of the remains of salt meadow grasses extend from the surface downward to a depth below that of mean low tide level- in other words, to a depth many feet lower than that at which the plants which formed peat could possibly have grown. In no case, Davis emphatically states, does the peat show the hypothetical arrangement of layers specified above. The peat underlying a brackish meadow near New Haven, and sectioned during operations for brick clay, shows similar conditions: just beneath the surface (2) a layer of Distichlis peat and (3) a layer made up largely of cat-tail and fern remains, with (4) numerous scattered stumps resting in place on the underlying gravelly substratum, about five feet below the present mean high tide level.

From the foregoing observations it is clear that any assumed agreement between the present-day zonation of salt marsh associations in relations to tide levels and the succession of plant associations which has ensued during the development of the marshes, along the New England coast, is not in harmony with the facts as recorded by the underlying peat deposits, in so far as these records have been made available.”
The absence of eelgrass from these cores nearly a century ago is today the subject of habitat questions today is our dominant eelgrass strain present today actually native to our shores?

The first time I heard this theory I was meeting with John C. Hammond, a retired oyster “planter” on Cape Cod. He had been studying the impact of Eelgrass and a more recent seaweed invader, Codium, upon cultured oysters for decades, they both don’t belong here, I can recall him saying the Codium (also called dead man’s fingers) invasion was easy for me to believe; I had never seen anything like it growing up in Madison, CT- eelgrass was different, I had seen it, after storms the Hammonasset Beach wrack line was full of it, and it would fill menhaden gill nets set for lobster bait. It was 1982 and shortly after arriving on Cape Cod I would meet Mr. Hammond in Chatham many times. In reality I was sent by an organization of shell fishermen on Cape Cod to seek him out about shell fish habitat conditions and nitrogen concerns. It would be the first of many meetings.

To put it bluntly, Mr. Hammond had no use for either eelgrass or Codium, both in his opinion had damaged shellfish habitats; Mr. Hammond blamed eelgrass for ruining hard clam quahog bottoms (habitats) and Codium had caused much additional cleaning of harvested cultured oysters. At the time Codium (slightly buoyant) had grown so dense as to lift oyster pond oysters off the bottom, or at least had added so much resistance they were carried out on ebb tides to sea. The issue regarding eelgrass and hard clam habitats raised by Mr. Hammond came to an open conflict with shell fisheries on Cape Cod. Mr. Hammond reported to me that growths of eelgrass had overrun hard clam habitats in Pleasant Bay in the 1960s. Pleasant Bay efforts to control the spread of eelgrass even then included the use of powerful herbicides.

It is at one of our discussions about shellfishing that green crab predation in soft shell clams (Mya) and that Mr. Hammond connected these two species – Eelgrass and green crabs. According to Mr. Hammond, green crabs needed eelgrass as a “habitat cover” to spread. Not fully realizing the significance at the time of that habitat connection, I was however well acquainted with green crabs. I had trapped them and seined them in Tom’s Creek in Madison, CT for decades and I knew they liked shellfish. Our favorite early catching method was very simple—a strand of beach grass (now frowned upon, string works just as good) with a cracked mussel tied at the end. Even fishing in daylight among rocks off Webster Point, Madison, a cracked mussel was hard to resist. There seemed no end of the green crabs and it was Mr. Hammond who introduced the concept of night movements. I also had first knowledge of green crabs predatory aspects on shellfish, such as Quahogs and steamers (soft shell clams, Mya) also in Tom’s Creek, but eelgrass was scarce. Despite trapping and seining there seemed to be no end of the green crabs. On that topic, Mr. Hammond did agree with me.

In a book The Canadian Oyster by Joseph Stafford, (Ottawa, The Maritimes Company Ltd, 1913) gives an excellent turn of the century oyster industry – green crabs had yet to reach the Northern Maritimes (1912), but by the late 1950s, they had.

During a term on the Madison Shellfish Commission (1981) I had discovered correspondence with Commission members from Victor Loosonoff about the decline of soft shells and the increase in green crabs (1953). Green crabs in the late 1950s and 1960s were now spreading north into Maine ruining soft shell clam beds in the process – so was eelgrass.

After reaching enormous densities during The Great Heat (1880-1920), Eelgrass had declined after the 1938 Hurricane and the colder and stormier 1940s and 1950s in Southern New England. In the 1960s and 1970s eelgrass was back and so were the green crabs. Mr. Hammond had decades of shellfish habitat observations and mentioned that the green crab and eelgrass habitat histories to him were closely linked – some three decades later; it looks without any doubt he was correct. A direct and positive habitat relationship appears to exist (now frequently termed habitat services); eelgrass and green crabs do seem to have a habitat relationship – one that did not begin here but perhaps on a different continent in Europe, the English Channel and along the Dutch coasts.

At first, Mr. Hammond was a bit reluctant to show me his Eelgrass research; he had collected eelgrass blade samples and carefully preserved them on that heavy biological paper. I had seen other seaweed examples at the University of Rhode Island in the basement offices of Woodward Hall. Here one day, I discovered the remains of a collection of seaweed prints, dozens of pressed preserved seaweed mounts on similar heavy “biological” paper. I was later to learn that this was a popular activity- hobby actually after World War I. The URI collection was in a box and looking back most likely on the way out to discard. But, I had seen it so he continued: He had collected blade samples from the North Sea and compared it to those now on the Cape, he had concluded it didn’t belong here, it being he felt a North Sea strain. This was the first time I had ever come across this theory, but the most amazing piece of the puzzle was yet to come, people in this section of Europe apparently ate green crabs. As Mr. Hammond continued to tell me about this, I immediately thought of these green crabs I had long used for black fish bait, hardly the size of something I would at that time consider edible. The transplanting aspect however seemed very real to me, having oystered and seen oyster drills laying their egg capsules on living oysters and crabs moved in burlap oyster bushel bags. The concept of moving species across the Atlantic Ocean with shellfish was plausible, but eating green crabs- that was something I had not seen, not until 2011. Mr. Hammond felt that green crabs came to our coasts wrapped in eelgrass to keep them fresh and moist on the voyage here. Now it appears Mr. Hammond was correct again.

Eelgrass and Green Crab Relationship Now Needs a Review

After 1998, the habitat quality for blue crabs tremendously increased in Connecticut and the 2010 blue crab season was the best in decades. In 2011, I started a newsletter for this population surge – The Search for Megalops – The Rise of the Blue Crab. As such, I would talk to Connecticut blue crabbers about crabbing and catches. My trip to Niantic Bay in August, 2011 would forever changes my views towards Mr. Hammond’s earlier research. I heard that early morning coast time crabbing in Niantic Bay had been good recently, so one morning I set off early and arrived at Niantic Bay Cini Park about dawn. Parking and walking to a finger shaped point, I saw several crabbers (This is now a great DEEP East Lyme public fishing area.) I don’t like to barge in, so I watched a small family crabbing – handlines and bait and the usual scoop nets, and after a few crab returns (I recognized as blue crabs, I approached a man who appeared to be the father and asked about the crabbing- he said “Very good”. At that point a family member scooped a large blue crab and then flipped the net returning it to the water (without a measure) so I asked if it was short. He replied, “We don’t keep them,” so at this point I asked what he did keep. He motioned to a five gallon bucket and pulling off some rockweed, the bucket was nearly full of large green crabs; these were by far the largest green crabs I had ever seen, they were huge—I would say some were about the size of softballs; he seemed pleased with my reaction and my next question surprised me more, I said “Do they hide in rocks?” He shook his head, “no” and reaching down he picked up a blade of eelgrass- too big for rocks, they live in weed. I said, “Eelgrass.” He nodded, “Yes, they live in eelgrass but can catch only at night, the best fishing today was from 1 am to 4 am;” they were hoping to catch a couple more. I asked about taste; they apparently were great and far superior to the blue crab which they discarded. Well, it was more than one light bulb going off, how about a football stadium light? This one visit brought back my conversations 30 years ago with Mr. Hammond; eelgrass was carried here with edible size green crabs, the strain here was introduced and eelgrass restoration efforts could be possibly helping what now is termed the worst invasive species in the world—green crabs.

A few hours later at home I looked at English Channel crab fisheries and there it was, a giant green crab; it’s still an edible crab species in the North Sea; there are many green crab recipes on the Internet. Researching this further, I found that an aggressive coastal storm habitat species of eelgrass spread along the Dutch Coasts and that in the Middle Ages, 1500 to 1700 when green crabs were a favored food of the coast. Combined with the fact wrapping lobsters and crabs with seaweed for long voyages was a practice dating back to Viking exploration the scenario that Mr. Hammond described was falling into place.

1. A strong eelgrass/green crab habitat association;

2. The possible introduction of both species as edible food and storage wrapping together;

3. Our strain – aggressive to shellfish habitats following storms is not native, but perhaps developed genetic strains that could live in these storm prevalent habitats.

From the information I have been able to find, the storms in the North Sea resemble periods in the 1940s, 1950s and 1960s with brutal winter storms equivalent to our Nor’easters. In the southern section of the North Sea Basic where depths are less than 60 meters, (C.K. Cuders, sediments of the North Sea March, 1938) found storms did influence coastal sediments especially along the Dutch coastline (pg 336). This reworked material can be overlain by fine deposits and absent the fine grained terrigenous detritus. Large scale movement was also mentioned, currents of two to four miles per hour able to move vast quantities of sand along the Dutch coast for instance, a 1871 cable laid on the flat bottom in 15 years was buried by sand 5 to 11 meters deep (pg 337). Eelgrass living in this storm filled active zone would need to be an aggressive quick growing strain with enhanced soil holding abilities to live there. And Jensen (1926) reports that eelgrass growing in the waters of Denmark (pg 436 Trask) produced 100 grams of organic matter per square meter, making it one of the fastest growing strains. It would be natural for such a strain became prevalent owing to the habitat conditions in which it lived.

The association of green crabs to eelgrass and the aspect of green crabs as an edible food species should cause review of the introduction of both here back several hundred years, perhaps even back to the first Dutch explorers themselves. Green crabs the food of coastal communities would have been brought on long voyages as a familiar staple, preserved by the common practice of seaweed packing. There is no direct evidence to the contrary and perhaps explains the fact that “our” eelgrass spreads so quickly after storms. Several Canadian studies are underway currently looking at eelgrass as a vector in the spread of green crabs, I believe that when concluded they will show a strong positive relationship between them.

On November 1, 2013, I asked the Connecticut Invasive Plant Council about the process of investigating or declaring that our strain or strains of eelgrass are those of the North Sea. In recognition of his early work in this area I have asked if our strain is found to be non-native, its name should be Zostera-marina-hammond. As far as green crabs as food, it remains a significant fishery in landings in England as an edible crab. Several green crab recipes are on the Internet. This summer I plan to fish for them and near sea grass beds in Niantic Bay and give them a try. As for the time for its introductions to America (1805) (I think we need to review that, I believe some of the New World explorers brought some of the Old World with them.

The eelgrass “situation” has profound impacts for near shore aquaculture, that green crab known as voracious shellfish predator is helped by it; seed scallops that set on it are subject to blade attack from a relentless green crab population. Its holding capacity and slowing organic matter in doing so influences a clam set failure in acidic marine soils deserves another look also.

It was these same characteristics quickly spreading aggressive habitat changing Eelgrass (destructive to Quahog beds) that rang the alarm bell for Mr. Hammond for shellfish here, especially clamming (quahog) and oyster beds a half century ago. (Rate of spread green crabs can spread rapidly. In a little over a decade, green crabs spread from Oregon to British Columbia, a distance of about 400 miles in a decade (40 miles / year).

And it wasn’t just Mr. Hammond or Cape Cod shellfishers that had noticed this Eelgrass and green crab habitat connection. As eelgrass spread north into the Canadian Maritimes, so did green crabs. Oyster beds were eliminated by heavy stands of eelgrass between 1890 to 1920 (A Review of Oyster Culture and The Oyster Industry in North American George C. Matthiessen, PhD, Woods Hole Oceanographic Institution (1970).
After Eelgrass became established, green crabs soon followed; several researchers are presently examining the green crab/eelgrass habitat connection, perhaps a potential Capstone project?

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