Atlantic Coastal Biome
257
1994 to only 900 t in 1996; a corresponding decline in incoming year-class strength was
attributed principally to the reduction in offshore spawners, but also to increasing natural
mortality in the planktonic phase.
There was almost certainly an environmental component in the stock decline, associated with the extreme variations in temperature and salinity recorded in the early 1990s
after a long period of generally declining temperatures over the Grand Banks. But this is
not to say that political pressure to maintain catch levels, and widespread avoidance of
regulation at sea, together with application of inappropriate models of stock dynamics
were not the primary cause of the crash. From this experience, a fundamental reevaluation
of fish stock management science is in order.
One lesson learned from this event is that it is not sufficient simply to stop fishing in
order to reestablish the status ante quo, because not only did the stock continue to decline
after the moratorium, but a decade later it still shows only very slight recovery: had a
federal election not been called in 2005, it seems doubtful if the quotas introduced for
some stocks would have been permitted. Associated with the lack of population regrowth,
there is evidence that major shifts have been induced in the entire structure of the benthic
ecosystem. As the large fish stocks declined, and the area they occupied shrank on the
Grand Banks, so the local population of northern shrimp (Pandalus borealis) increased
both its biomass and area of distribution; this was attributed initially to environmental
change but is now acknowledged to have been an ecosystem response to altered predation
pressure. Metanalysis demonstrates that the reciprocal abundance exhibited by cod and
northern shrimp over long periods and large areas is a top-down, predation effect (Worm
and Myers, 2003). That ecosystem structure is unstable seems an inescapable conclusion:
consider the evidence for a major shift in trophic links in the Gulf of St. Lawrence in the
period 1959–2000. During the first 30 of these years, in stomachs of 30- to 60-cm cod,
6–70% of prey mass was composed of euphausiids: but only trace amounts were found
after 1990. During the same period, herring consumption by cod shifted from 1.3% of
prey mass to almost 20%, matching changes in the relative abundance of herring over
the period. It would be a brave ecologist who would confidently predict future ecosystem
structure on these shelves, yet such is the stated objective of a new thrust of fishery
science toward ecosystem-based stock management.
Synopsis
Case 2—Nutrient- limited spring production peak but modified by shallow-water processes.
The mixed-layer Z m undergoes significant boreal winter excursion from 10–20 m (June–
September) to 50–100 m (February–March), while Z eu lies consistently at 20–30 m. Winter
mixing carries the pycnocline to the bottom along the midshelf, and to moderate depths
seaward of this line. The thermocline is therefore illuminated during boreal summer
from May to September. The rate of P begins to increase in late winter, matched rather
closely by chlorophyll accumulation, with some indication that in autumn chlorophyll
accumulates by reduction of herbivore pressure (Fig. 9.26). Alternatively, we cannot
exclude the possibility that the significant chlorophyll accumulation in some years during
boreal winter (October–March) in satellite imagery may be partly the effects of river
discharge and winter resuspension.
Guianas Coastal Province (GUIA)
Extent of the Province
The Guianas Coastal Province (GUIA) extends from Cape de Sao Roque (5
S) in Brazil
to Trinidad (10
N) within the offshore limits of the North Brazil (or Guiana) Current
257
1994 to only 900 t in 1996; a corresponding decline in incoming year-class strength was
attributed principally to the reduction in offshore spawners, but also to increasing natural
mortality in the planktonic phase.
There was almost certainly an environmental component in the stock decline, associated with the extreme variations in temperature and salinity recorded in the early 1990s
after a long period of generally declining temperatures over the Grand Banks. But this is
not to say that political pressure to maintain catch levels, and widespread avoidance of
regulation at sea, together with application of inappropriate models of stock dynamics
were not the primary cause of the crash. From this experience, a fundamental reevaluation
of fish stock management science is in order.
One lesson learned from this event is that it is not sufficient simply to stop fishing in
order to reestablish the status ante quo, because not only did the stock continue to decline
after the moratorium, but a decade later it still shows only very slight recovery: had a
federal election not been called in 2005, it seems doubtful if the quotas introduced for
some stocks would have been permitted. Associated with the lack of population regrowth,
there is evidence that major shifts have been induced in the entire structure of the benthic
ecosystem. As the large fish stocks declined, and the area they occupied shrank on the
Grand Banks, so the local population of northern shrimp (Pandalus borealis) increased
both its biomass and area of distribution; this was attributed initially to environmental
change but is now acknowledged to have been an ecosystem response to altered predation
pressure. Metanalysis demonstrates that the reciprocal abundance exhibited by cod and
northern shrimp over long periods and large areas is a top-down, predation effect (Worm
and Myers, 2003). That ecosystem structure is unstable seems an inescapable conclusion:
consider the evidence for a major shift in trophic links in the Gulf of St. Lawrence in the
period 1959–2000. During the first 30 of these years, in stomachs of 30- to 60-cm cod,
6–70% of prey mass was composed of euphausiids: but only trace amounts were found
after 1990. During the same period, herring consumption by cod shifted from 1.3% of
prey mass to almost 20%, matching changes in the relative abundance of herring over
the period. It would be a brave ecologist who would confidently predict future ecosystem
structure on these shelves, yet such is the stated objective of a new thrust of fishery
science toward ecosystem-based stock management.
Synopsis
Case 2—Nutrient- limited spring production peak but modified by shallow-water processes.
The mixed-layer Z m undergoes significant boreal winter excursion from 10–20 m (June–
September) to 50–100 m (February–March), while Z eu lies consistently at 20–30 m. Winter
mixing carries the pycnocline to the bottom along the midshelf, and to moderate depths
seaward of this line. The thermocline is therefore illuminated during boreal summer
from May to September. The rate of P begins to increase in late winter, matched rather
closely by chlorophyll accumulation, with some indication that in autumn chlorophyll
accumulates by reduction of herbivore pressure (Fig. 9.26). Alternatively, we cannot
exclude the possibility that the significant chlorophyll accumulation in some years during
boreal winter (October–March) in satellite imagery may be partly the effects of river
discharge and winter resuspension.
Guianas Coastal Province (GUIA)
Extent of the Province
The Guianas Coastal Province (GUIA) extends from Cape de Sao Roque (5
S) in Brazil
to Trinidad (10
N) within the offshore limits of the North Brazil (or Guiana) Current