Green Crabs of the Bras d'Or Lakes
A SUMMARY OF THE PHYSICAL OCEANOGRAPHY OF THE
BRAS D’OR LAKE SYSTEM
David P. Krauel
The Bras d’Or Lake System is an enclosed body of water located in Cape Breton Island, Nova Scotia. It has
limited exchange with the Atlantic through three channels, but only one of these, the Great Bras d’Or
Channel, provides significant interchange. The Great Bras d’Or Channel opens into the Atlantic to the northeast
with a sill depth of 9 metres and a minimum width of 500 metres. The Little Bras d’Or is a meandering
channel of 8 km length, which also opens into the Atlantic in a northerly direction and has a sill depth of
5 metres and a mean width of 100 metres. St. Peters Canal opens to the south into St. Peters Bay and the
Atlantic Ocean through a pair of locks so that it does not provide continuous exchange.
The Lake System consists of the Bras d’Or in the south, which is separated from a series of interconnected
channels to the north~ by the Bara Straits of minimum width 500 metres and (depth 22 metres. The bathymetry
is highly variable with a number of major basins of extreme depth. St. Andrews Channel has the deepest basin
with an observed depth of 280 metres. The Great Bras d’Or Channel and Lake contain a number of basins of
depths 50 to 200 metres. The bay at the southwest end of St. Patrick’s Channel has a basin of 38-metre
depth and one of 46-metre depth. This bay is separated from the rest of the Lakes by Little Narrows of
sill depth 13 metres. Bras d’Or Lake also has an irregular bottom with basins from 50 to 150 metres in
The bedrock of the area is primarily sedimentary - shale, sandstone and conglomerates. Some volcanic rock
also occurs. The area was glaciated during the Pleistocene and the superficial layers are largely composed
of glacial till. The channels forming the Lakes have a northeast-southwest trend and are generally
surrounded by crystalline uplands of similar trends. Only St. Patrick’s Channel is surrounded by
Carboniferous lowlands (Johnson, 1925).
This paper is a descriptive summary of an intensive circulation and mixing study by the Marine Ecology
Laboratory at Bedford Institute, which began in the summer of 1973 and continued in 1974. Continuous
water level and water current data were recorded at locations throughout the Bras d’Or Lake System and
wind and barometric pressure were recorded at two sites (Fig. 2). Temperature-salinity data at a grid
of stations covering the Lakes (Fig. 3) were observed repeatedly throughout the study period. These data
are available elsewhere (Krauel, 1975) and will not be repeated in the present report.
There have been no previous investigations into the physical circulation and mixing in the Bras d’Or Lakes.
Some studies, primarily interested in some other facet of the Lakes, have observed temperature and
salinity in localized areas and made some qualitative comment concerning tides and currents but no one has
previously attempted to examine the physical processes within the System as a whole.
The following papers, while mainly concerned with the biology of the Lakes, contain some
observations of the physical characteristics of the Bras d’Or Lakes. Geen (1965) and
Hargrave (1966) reported on the primary and secondary productivity of the Lakes.
Through the years many investigations have been concerned with oysters e.g. Medcof
(1938, 1939, 1940, 1955) and Smith (1936, 1937). Scott and Black (1960) studied the life cycle
Bras d'Or Lakes Watershed: Green Crab
The following are some observations made over the past four years. The trends in abundance, although not a
scientific survey, are based on weekly and in some cases daily observations in a very familiar area. Green
Crab appears to have displaced Rock Crab in our area of the Bras d'Or. Green Crabs of all sizes are common
on shore with larger animals in deeper water. Rock Crab are becoming quite rare. Crab Larvae (unspecified)
are more common in daily plankton tows (summer). Smaller mussels (1-3 cm) are increasingly difficult to find,
especially in the last two years. Another anomaly is the occurrence of full-grown Cyanea medusae in April.
These jellyfish have been common in herring nets in recent years, but were smaller animals. Periwinkles
have been very prolific in '97 and '98. Larger vessels, from who knows where, have been able to enter the
Bras d'Or in recent years. These vessels have to load water as they unload their cargoes. In the case of
the gypsum bulk carriers, this water is usually from some industrial port in the southern United States.
After their return to Cape Breton in ballast, this water ballast is dumped into the Bras d'Or, as gypsum
is loaded for another trip south. A local pilot of these vessels recently confirmed this practice. The main
concern is that the changes, described above, could be indicators of some of the affects of thousands of
tonnes of ballast waters regularly inoculating this pristine sea. The Green Crab might be the best indicator,
given that it was first introduced to southeastern U.S. in ballast of European vessels. A simple comparative
survey, outside versus inside the Bras d'Or watershed, would be useful. In addition, a survey of mussels
would establish predatory preferences and effects. The occurrence of full-grown medusae in late winter and
again in late summer may indicate two stocks. This could also mean earlier planktonic forms arriving in
ballast and over wintering as growing medusae. This begs further investigation. There is intense, organized,
local interest in the above concerns and it might be in our best interest to at least do some background
research. This is a unique, relatively closed marine ecosystem where groundbreaking species interaction
work should be done. Little ecosystem work has been done to date. Science Branch should be leading the
work before the system undergoes further changes. The Ecosystem Approach begs to be applied here.
Fisheries Research Board of Canada
Watch For The Green Crab? A New Clam Enemy
By J. C. Medcof and L. M. Dickie
Atlantic 26 Biological Station. St. Andrews, N. B. General Series Circular No. 6, July, 1955.
The green crab is a serious, clam enemy. In the last tcn years it has worked its way up the coast of Maine
and in 1951 it appeared in Canada. Now it occupies both the Nova Scotia and New Brunswick side of the Bay
of Fundy and last year there was evidence from Wedgeport that it is rounding the southwestern tip of Nova
Scotia and spreading eastward along the outer coast. It is increasing rapidly in this area and young
molluscan shellfish are its favourite food. Wherever it has appeared it has attacked clam stocks and
in some cases practically wiped them out. We know little about its food habits or what would happen if it
pushed its way into other clam areas or into oyster areas like the Bras d'Or Lakes and the southern Gulf
of St. Lawrence. While searching for some means of control the Fisheries Research Board is trying to keep
up to date in knowing how far the animal has extended its range. We need all the information we can get
about this new menace to our shellfish stocks.
The green crab usually measures two to three inches across the shell and on first sight it is often
mistaken for one of the two species of rock crabs commonly seen along our shores and taken in lobster
traps. The rock crabs ordinarily grow as large as five to six inches across the shell and their greater
size alone may often distinguish them from green crabs. As the name suggests, green crabs are greenish in
general appearance but many of them, especially older ones, are so mottled with black as to scarcely
justify their name. Ordinarily, rock crabs are brown or reddish-brown and can thereby be distinguished
from green crabs. Green Crab Carciuldes macnas has five large, sharp, teeth on each side counting outward
along the shell. The habits of green and rock crabs differ, too. Green crabs live from high-water mark
down to a few fathoms below low-water mark on rocky, sandy or muddy shores. They are quick walkers, able
swimmers and expert burrowers in sand. When the tide is low, they stay on the beaches burying themselves
in sand or hiding under weed or stones. If cornered, they fight fiercely. By contrast, rock crabs tend to
spend their time below water and except when small, are not common on beaches at low ride. They are clumsy
walkers and cowardly. The surest and best basis for id identification is a tooth-count of the shell edge.
The green crab has five large, sharp, teeth like the teeth of a rip saw, on both sides, counting outward
from the eye along the shell edge. In contrast, the two species of rock crabs, which resemble one another
very closely, have nine or eleven such teeth, and many of them are small and rounded. Watch for green crabs
and report discoveries.
Because this invasion of green crabs is such a menace to our molluscan shellfish industries we urge
everyone, especially those in southwestern Nova Scotia to keep a sharp lookout for them and report any
discoveries. If you find them tell your Fisheries Officer and give him samples, if you can. He/she will
send them to us.
If you prefer. write and send a sample directly to the
Director, Atlantic biological
Station, St. Andrews, N. B.
From the Vancouver Sun, 03 Jan 1996: byline Stephen Hume
Waiting for the invasion: The green crab from Europe has come to our shores, threatening B.C.'s
lucrative shellfish industry. The unwelcome organism has some scientists sounding the alarm about how
modem transportation poses a growing threat to several forms of life, including humans. Seldom larger
than the palm of a woman's hand, its mottled carapace ranging from green to reddish hues, the European
green crab is an astonishingly attractive creature. But don't be fooled by its looks. The $200-million-a-
year shellfish industry in Washington and British Columbia are now considered at serious risk from an
ecological invasion by this small, non-native swimming crab. Native to European shores from Morocco to
Norway, the crab grows to about 7.5 centimetres in width and is easily identified by five spines at the
front of its carapace and the flat back legs, which enable it to swim. Carcinus maenus is of particular
interest because it is unusually smart for a crustacean.
More importantly, this pretty little European colonist is a fierce competitor of the big, meaty, and
red-shelled rock crab, that are sold live in up-market seafood restaurants. In addition, the green crab
hunts and devours a wide range of native shellfish species, some of which can command up to $400-a-kilo
in the fresh carriage trade market. Ocean scientists are tracking the green crab's invasion of the Pacific
coast with a mixture of awe and alarm. "This could affect the way in which we do all our intertidal
aquaculture," warns Glen Jamieson, a crab specialist at the Pacific Biological Station in Nanaimo. "Its
main prey seems to be bivalves, which include commercially valuable clams, oysters, scallops and geoducks
and there's nothing you can do to get rid of it." The study of the large-scale movement of species and
their potentially profound impacts upon evolution and biogeography is a hot new field in the life sciences,
and it has a name - invasion ecology, Proponents of the new discipline are particularly interested in what
appears to be a worldwide alteration of naturally evolved ecosystems.
Some have political overtones - a decision to introduce Atlantic salmon to the Pacific was made despite
objections from the British Columbia Transplant Committee. Commercial value was deemed to outweigh
potential risks to existing ecosystems. And some changes occur by unwitting error or by accident.
Whatever the reasons, many modifications are now occurring on a continental scale and at a pace that
seems unprecedented in the planet's evolutionary record. Ecosystems are dynamic, of course, and have
always changed as different species evolve, migrate and seize advantage in the competition for niches
in the complicated hierarchies of life. Without such change, we humans wouldn't be here to think about it.
"Indeed, British Columbia, itself, is largely occupied by plant and animal invaders that came by both land
and sea," point out Verena Tunnicliffe and Melissa McQuoid, two highly respected scientists at the
University of Victoria. The large mechanisms by which new species are introduced from one region to
another are relatively simple. Continental drift either isolates populations or brings them together.
Climatic change alters range availability for differently adapted species. Finally, there are what
scientists call, " unusual transport opportunities", hypodermic syringes exchanged by drug addicts, for
example, or the fouled bottoms of super tankers. When the Fraser ice sheet advanced and retreated from
British Columbia, between 30,000 and 10,000 years ago, shorelines changed radically; land corridors and
new climatic regimes emerged. These factors all facilitated species interchange between Asia arid the
Americas and back and forth between North and South America. "Today, scientists have determined that
while less than 10 per cent of North American mammals have southern origins, over 60 per cent of South
American mammals have northern origins," Tunnicliffe and McQuoid write in a recent discussion paper
entitled Invasion Dynamics and Exotic Species Introductions. All these changes occurred relatively
slowly, taking place over millennia. What's at issue is the speed with which human technologies of
commerce and industry appear to be transforming large and complex ecosystems, which took millions of
years to develop. These changes can have economic as well as ecological consequences. When zebra mussels
were imported unintentionally to the Great Lakes their population suddenly exploded, colonizing and
eventually choking the expensive plumbing of industrial plants. A species of jellyfish has recently
invaded the Black Sea, displacing more than 80 per cent of the indigenous species in the process and
ruining commercial fisheries. Some of these invasions have had a high media profile. The introduction of
sea lampreys into the Great Lakes via the St Lawrence Seaway caused millions of dollars damage to valuable
sports fish stocks and galvanized public attention. And hybrid Africanised honeybees that escaped from a
Brazilian lab 40 years ago were sensationalized as "killer bees" because they are more prone to sting
aggressively. Their migration north from South America has been steady. By displacing more easily handled
European bees as they advance, the Africanised bees threaten to add costs to North America's domestic
Some invasions have been decidedly low profile.
A mud snail introduced to California from the Atlantic coast with the importation of eastern oysters
between 1901 and 1907 has successfully displaced the indigenous species. And a predatory north Pacific
starfish that now infests Australian estuaries has pushed a native species - the spotted handfish - onto
the endangered list by devouring its eggs. Here in the Lower Mainland, a non-native species called purple
loosestrife, which generates a million seeds per plant and invades fish restoration areas, is now present
in 20 out of 50 sites surveyed in the Fraser River estuary. To be nondescript is not to be ineffective.
Purple loosestrife is responsible for the conversion of more natural wetlands in B.C. than all human
development. On the Atlantic coast, bottom-smothering Japanese algae now choke scallop ponds.
On the East Coast, the green crab invaded the once-lucrative scallop fishery off Martha's Vineyard. Where
once 100 boats would set out to fish for scallops that wholesale for about $22 a kilo, last year only 20
bothered to fish depleted stocks. Behind the immediate financial implications for B.C.'s economy that are
posed by the advance of the green crab into West Coast Canadian waters loom much bigger issues. The
European green crab's destructive beachhead in the commercially valuable shellfish beds of the Pacific
appears to be one more signal that the rate of change to large natural ecosystems is accelerating. For
example, the green crab was first identified on the West Coast seven years ago. Already its range has
extended half the distance from San Francisco Bay - where it was first reported - to the rich and
potentially vulnerable waters of Puget Sound and B.C.'s south coast. Considering that it took three years
for the invader to establish a self-sustaining breeding mass in California, the advance toward B.C. has
been swift. On a global scale, developments like this might be compared to a shuffling of the genetic
cards that comprise the world's complex inventory of biodiversity. And nobody knows what the large-scale
consequences of these altered and combined ecosystems will be. "We know that introduced species can have
devastating effects," says Jamieson. He points to the Black Sea.
There, invading jellyfish disrupted the natural food chain, consuming the microscopic creatures that
sustained more highly evolved animals during their immature stages. Populations higher up the food chain
were starved out and replaced by a jellyfish population that then exploded to fill the vacant ecological
niche. In effect, the genetic cards in the Black Sea deck were fundamentally rewritten, probably
irreversibly. "It didn't really destroy the existing biomass," Jamieson says, " it just convened it (from
many species to jellies." Driving these changes is the rapid growth of trans-world shipping as a global
free trade zone expands. The Port of Vancouver, for example, averaged seven foreign ships a day during
1996. Greg Ruiz, a researcher at the Smithsonian Institute's environmental research centre on Chesapeake
Bay - another invasion point for the green crab - warns in a recent research publication that the
inevitable expansion of this ocean trade means the disruption of continental ecosystems will continue. At
any one time there may be tens of thousands of vessels moving around the world carrying ballast water. The
effect has really been to open up a conduit for the transfer of species from one part of the world to
another part of the world. Last year, for example, ocean scientists from the University of Victoria and
the federal government met to discuss the problem of the Introduction of exotic species to B.C.'s marine
ecosystems. Minutes from their meeting note concerns that there is no regulation of the discharge of
ballast water from freighters that may have taken it on in foreign ports and that there are a multitude of
risks inherent in the practice. Ballast water samples from 50 boardings at Vancouver, Nanaimo and Prince
Rupert confirmed the presence of 20 different invertebrates originating in Japan, China, Korea and the mid-
Some of this invasion biology can be lethal to humans:
Virulence during plankton blooms infects humans who eat contaminated fish and systems with explosive speed.
The microorganism responsible for cholera is believed to have hitchhiked to Latin America in the ballast
water of ships originating in China and there is evidence that natural plankton populations enhance the
growth of the bacterium. Following an epidemic in South America, studies found the cholera bacterium in
the ballast of three of 14 ships sampled. Since then a new and more virulent strain has appeared in Indian
and Bangladesh while a strain resistant to chlorine - the standard disinfectant - has appeared in Peru.
"These events resulted in the death of thousands of people and with more virulent strains involved there
is potential to harm many more," Tunnicliffe and McQuoid warn. British Columbia could similarly receive
this organism from ship transport and our coast does support blooms of some of the organisms previously
found to enhance the growth of Vibrio cholera. Although our high standards of hygiene might protect us
from the epidemics observed in Latin America, our sewage disposal methods would recycle the pathogen back
into its optimal environment and this could result in repeated contamination of coastal fish species.
Another high profile example of invasion biology is the human immuno-deficiency virus. One of at least 30
previously unknown viruses to emerge from the tropical jungles in the last 20 years, it hitchhiked along a
new highway through equatorial Africa, then rode its hosts aboard ships and jet planes. More than 30
million people from the Canadian Arctic to the tropics of Asia are now infected and the virus expands its
range at the rate of 6,000 new hosts per day. Viewing biological change as a function of commercial trade
is a new way of conceptualising evolutionary principles. All of which places scientists like Tunnicliffe,
McQuoid, Jamieson and his equally concerned colleagues at the University of Washington. the Pacific
Wildlife Research Centre in Delta and the department of zoology at the University of New Hampshire right
at the cutting edge of invasion ecology studies. Jamieson and his colleagues will soon publish their
research on the potential ecological implications of the introduction of the European green crab to B.C.
and Washington in the European Journal of Natural History. Meanwhile, here on North America's West Coast,
these scientists are looking at the opportunity of embarking on a complete study of the invasion of a
large ecosystem from the very first arrival of the alien species.
While all this provides for exciting science, it has its sombre side in the practical world....
The green crab clearly has the potential to simultaneously devastate commercial stocks while sharply
increasing input costs for a crucial fishery. In 1995, B.C. waters produced 30,000 tonnes of shellfish
for domestic and export markets. After salmon and roe herring, shellfish harvesting is the most important
and commercially valuable fishery in the province. It is worth more than all the remaining fisheries -
including cod, hake, red snapper and halibut - combined. Meanwhile, scientists have discovered the green
crab capable of learning and improving its prey handling techniques even while foraging. It is also
quicker, more dexterous and has more ways of opening shells than other crustaceans, which is where the
commercial threat lies. One green crab is capable of eating three oysters and up to three dozen mussels
per day. It can dig out clams buried up to 15 centimetres deep in mud or sand. It is a prolific breeder.
The imminent arrival of the green crab means shellfish growers in B.C. and Puget Sound might soon be
forced to move from low-cost seabed production to a much more costly suspended system. And if experience
elsewhere holds true, they will face an expensive installation of small-meshed nets to protect immature
clam and oyster colonies from the voracious predator. At equal risk is the $25-million-a-year commercial
crab fishery, since the green crab is an aggressive predator of juvenile Dungeness crabs, killing and
eating them before they have an opportunity to grow to commercial size. There is also concern about the
invader's potential impact upon the many migratory birds, which pass through the Lower Mainland and feed
on a wide range of the intertidal species, which inhabit B.C.'s rich coastal estuaries. "These birds feed
heavily on small crustaceans," Jamieson points out. "If they decline sharply (because of prior predation
by green crabs) there could be an impact on shore birds. It's a change in the whole ecosystem." To make
things worse, the habitat in which the green crab thrives most prolifically - protected coastal ponds,
lagoons, embayments and estuaries - is prime habitat for both commercial shellfish production and for
migrating shore birds. First identified in San Francisco harbour in 1989, the European green crab probably
established itself when a visiting ship pumped out ballast water, which contained sufficient larvae to
establish a viable breeding population. Since native species have not evolved in the presence of the
invading crab, many - including most commercial species - don't have defensive strategies. And lacking a
normal range of natural predators of its own, the crab brings the potential for a population explosion. On
the east Coast, a single scallop pond yielded seven tonnes of green crabs over five months. From its
first sighting, the crab has advanced steadily, its larvae drifting northward on currents that carry it
up to eight kilometres a day. By 1997, the species had reached Coos Bay, Oregon, about 300 kilometres
north of its last known colony at Humboldt Bay, California. Jamieson says the prevailing ocean flow
patterns mean that it is now only a matter of time,- a few years at the most,- before the introduced
species invades the lucrative shellfish beds of Puget Sound and the B.C. coast. "This is the first time
that a major predator has been introduced to our waters," says Jamieson. "It means a change in the whole
coastal eco-system." "Green crabs have not had a noticeable effect along the California and sourthern
Oregon coast because there are no estuaries with shellfish," he said. "The big impact will come when they
hit the Washington and B.C. coast," he said.
(Victoria Times Colonist)