Geoscience Reference
In-Depth Information
In recent years, European green crabs have been invading northern eelgrass beds, which they also utilize as
a nursery. Molecular evidence indicates that these crabs originated from the cold waters of northern Europe
and probably arrived in the southern gulf in ships' ballast. The highly adaptable green crab is a voracious pred-
ator and has had a major impact on native species. Declines in eelgrass beds in the gulf region have been
strongly associated with abundant and increasing numbers of green crabs, which disturb the sediment and
thereby uproot eelgrass plants.
Between 1930 and 1933, a wasting disease decimated eelgrass beds on both sides of the Atlantic, causing
the loss of 90 percent of beds along the eastern seaboard. The culprit proved to be a pathogenic slime mold, al-
though other stress factors were probably involved. The loss of eelgrass caused widespread erosion, but it had
its most devastating impact on brant, which relied heavily on eelgrass as a food source. Their numbers nose-
dived, and to survive they switched their diet to widgeon grass and sea lettuce. Many of the eelgrass beds have
recovered, having been seeded by plants that survived in the fresher parts of the estuaries, where the pathogen
could not tolerate the low-salinity environment.
DENYS ACCURATELY DESCRIBED
the topography of the coast as “great flats of sand which advance far out into
the sea.” One of the places he explored was the estuary of the mighty Miramichi River. Here again, he gives us
an insight into the great fertility of these waters: “If the Pigeons [Passenger pigeons] plagued us by their
abundance, the salmon gave us even more trouble. So large a quantity of them enters into this river that at
night one is unable to sleep, so great is the noise they make falling upon the water after having thrown or dar-
ted themselves into the air.”
Although Atlantic salmon numbers have been drastically reduced since Denys's time—not only on the
Miramichi but throughout the world—by a variety of human-induced factors, the Miramichi remains the
greatest producer of Atlantic salmon in North America. In recent years, between 40,000 and 160,000 salmon
have returned to its waters after spending one to two years feeding at sea. As recently as the 1930s, that num-
ber was close to a million spawning salmon. Great damage was done to the stocks by the budworm spraying
programs, which began in the 1950s with the drenching of New Brunswick's affected forests with ddt and con-
tinued until the 1990s with various other organochlorine pesticides.
The four main branches of the Miramichi system, along with some twenty-six tributaries, drain one-third of
New Brunswick and provide 1,000 square kilometers (386 square miles) of spawning habitat. This vast water-
shed is sparsely populated and has not been dammed, as many other salmon rivers have been in the species'
traditional range from Connecticut to Labrador. Even so, the system produces far fewer salmon than the optim-
um number—estimated at between 300,000 and 400,000—because of limiting factors both on land and in the
oceans, including poor forestry practices and possibly climate change.
In boreal estuaries like the Miramichi, the smelts and salmons are the dominant fish families. Along with the
sturgeons, basses and sea lampreys, they have evolved the anadromous habit of moving from salt water to
freshwater to spawn. Anadromy is the rule rather than the exception among estuarine fishes in northern waters,
but this behavior declines in tropical waters. This trend may relate to the principle of competitive exclusion, in
which interactions with other fauna are more important than the physiological adaptability of individual spe-
cies in making the transition from salt to freshwaters for spawning. At high latitudes there are fewer freshwater
fish species than in the tropics. Two factors may, therefore, discourage anadromy in tropical waters: predation
pressure and the increased demand for spawning space.
In northern latitudes, anadromous species utilize the brackish estuarine waters in different ways, depending
on the phase of their life cycle. Adults use the estuary as a resting place where they can acclimate themselves
to the freshwater before making their spawning run. The estuary performs an even more critical function for
the young. Shads, smelts, and bass quickly drop back into estuarine waters, where the rich production of small
life-forms yields ample food for growth. The young of Atlantic salmon may spend several years in their
