256
Chapter 9: The Atlantic Ocean
Banks that had been left untrawled for 12 years recorded minimal disturbance, and it was
found that structural changes in the benthic community mimicked changes due to natural
causes: no distinctive trawling signature could be detected (Kenchington et al., 2001).
It is assumed that the pristine benthic fauna exhibited a parallel series of species
associations to those better explored off Europe in the early 20th century and before to
the start of heavy industrial trawling; indeed, some examples of isocommunities from
this region were reported by Thorson (1957). The recent JGOFS investigations in the
Gulf of St. Lawrence included studies of the benthic communities (Desrosiers, 2000) that
characterized trophic relations and microbial activity in relation to depth, slope, and
substrate: deep settling basins are dominated by surface deposit–feeding invertebrates,
whereas on sloping ground, subsurface deposit feeders either dominate or have equivalent
biomass to surface feeders. These two trophic groups comprise >60% of total benthic
biomass within the Gulf. Polychaetes (Capitella, Orbinia) and bivalves (Nucula) largely
dominate the subsurface feeding biota, with some seasonal replacement. Surface-feeding
polychaetes and bivalves are more diverse: Laonice, Paraonis, Terebellides and Chaetoderma, Nuculana, and Antalis, respectively. The presence of these species associations has
a very strong impact on the temporal composition and granulometry of the deposits and
of their characteristic microbial populations.
A strange form of benthic-pelagic coupling was uncovered during the JGOFS study
of Georges Bank (Concelman, et al,. 2001). Abundant individuals of the benthic hydroid
Clytia spp. were found in the mesozooplankton samples over the central parts of the
Bank; examination of archived plankton samples showed that these are present in some
years, absent in others. The source of these individuals is located on the NE peak of
the Bank, whence they are detached from the epibenthos by anomalously strong wind
mixing during the passage of major storm systems across the Bank, to be progressively
retained in very high concentrations in the central low-advective regions. Here, they can
form ∼90% by numbers of all zooplankton, with copepods, chaetognaths, and others
thus forming ∼10%. Growth of pelagic hydranths, which require four to five Artemia
nauplii daily in vitro, is enhanced by turbulence. When present, they form an important
item in the diet of some fish, especially winter flounder, and are a major predator of
other mesozooplankton.
It is difficult to know how to write about the demersal ecology of this province,
especially of the northern regions that dominate the whole: the shock of the collapse of
the cod stock of the Grand Banks and other areas of this province in 1992 is still with
us. That this also had severe impacts of local human demography in the coastal fishing
communities (Hamilton, 1998) is perhaps not so well understood except to those who
live in the Atlantic Provinces of Canada.
There are, of course, lessons for ecologists to be learned from the event. For instance,
one of the reasons that stock assessment data were somewhat misleading in the years
prior to the crash is that we now know that species distributions may be changed not
only by between-year environmental changes, but also by excessive fishing pressure:
as the northern cod stocks (Gadus morhua) declined, so the range occupied by the
remnant shrank progressively, and the area of highest concentration moved progressively
southward. Like other fish with associative behavior, local densities varied significantly
less than the overall reduction in biomass of the stock, which fell by about six orders
of magnitude during this period (Atkinson et al., 1997). Under such circumstances, it
is not surprising that assessment surveys at sea did not return accurate estimates of
population size.
However it happened, the remnants of the Grand Banks cod stock that had sustained
a fishery for 500 years was declared commercially extinct in 1992 and the fishery was
closed, as was that on the Scotian shelf shortly thereafter. Despite this moratorium, the
biomass of the northern cod stocks continued to decline even further, from 11,700 t in
Chapter 9: The Atlantic Ocean
Banks that had been left untrawled for 12 years recorded minimal disturbance, and it was
found that structural changes in the benthic community mimicked changes due to natural
causes: no distinctive trawling signature could be detected (Kenchington et al., 2001).
It is assumed that the pristine benthic fauna exhibited a parallel series of species
associations to those better explored off Europe in the early 20th century and before to
the start of heavy industrial trawling; indeed, some examples of isocommunities from
this region were reported by Thorson (1957). The recent JGOFS investigations in the
Gulf of St. Lawrence included studies of the benthic communities (Desrosiers, 2000) that
characterized trophic relations and microbial activity in relation to depth, slope, and
substrate: deep settling basins are dominated by surface deposit–feeding invertebrates,
whereas on sloping ground, subsurface deposit feeders either dominate or have equivalent
biomass to surface feeders. These two trophic groups comprise >60% of total benthic
biomass within the Gulf. Polychaetes (Capitella, Orbinia) and bivalves (Nucula) largely
dominate the subsurface feeding biota, with some seasonal replacement. Surface-feeding
polychaetes and bivalves are more diverse: Laonice, Paraonis, Terebellides and Chaetoderma, Nuculana, and Antalis, respectively. The presence of these species associations has
a very strong impact on the temporal composition and granulometry of the deposits and
of their characteristic microbial populations.
A strange form of benthic-pelagic coupling was uncovered during the JGOFS study
of Georges Bank (Concelman, et al,. 2001). Abundant individuals of the benthic hydroid
Clytia spp. were found in the mesozooplankton samples over the central parts of the
Bank; examination of archived plankton samples showed that these are present in some
years, absent in others. The source of these individuals is located on the NE peak of
the Bank, whence they are detached from the epibenthos by anomalously strong wind
mixing during the passage of major storm systems across the Bank, to be progressively
retained in very high concentrations in the central low-advective regions. Here, they can
form ∼90% by numbers of all zooplankton, with copepods, chaetognaths, and others
thus forming ∼10%. Growth of pelagic hydranths, which require four to five Artemia
nauplii daily in vitro, is enhanced by turbulence. When present, they form an important
item in the diet of some fish, especially winter flounder, and are a major predator of
other mesozooplankton.
It is difficult to know how to write about the demersal ecology of this province,
especially of the northern regions that dominate the whole: the shock of the collapse of
the cod stock of the Grand Banks and other areas of this province in 1992 is still with
us. That this also had severe impacts of local human demography in the coastal fishing
communities (Hamilton, 1998) is perhaps not so well understood except to those who
live in the Atlantic Provinces of Canada.
There are, of course, lessons for ecologists to be learned from the event. For instance,
one of the reasons that stock assessment data were somewhat misleading in the years
prior to the crash is that we now know that species distributions may be changed not
only by between-year environmental changes, but also by excessive fishing pressure:
as the northern cod stocks (Gadus morhua) declined, so the range occupied by the
remnant shrank progressively, and the area of highest concentration moved progressively
southward. Like other fish with associative behavior, local densities varied significantly
less than the overall reduction in biomass of the stock, which fell by about six orders
of magnitude during this period (Atkinson et al., 1997). Under such circumstances, it
is not surprising that assessment surveys at sea did not return accurate estimates of
population size.
However it happened, the remnants of the Grand Banks cod stock that had sustained
a fishery for 500 years was declared commercially extinct in 1992 and the fishery was
closed, as was that on the Scotian shelf shortly thereafter. Despite this moratorium, the
biomass of the northern cod stocks continued to decline even further, from 11,700 t in