222
Chapter 9: The Atlantic Ocean
in atmospheric forcing of circulation, stratification, and mixed-layer temperatures over
the continental shelf here as elsewhere.
Although it must be an oversimplification, the timing and abundance of planktonic
copepods has become the paradigm for the factor most directly involved in the matchmismatch model of the survival of larval fish. This is thought to occur only when the
date of occlusion of fish eggs matches the date of sufficient copepod abundance in
their immediate surroundings (e.g., Cushing, 1990). By extension, it is supposed that if
copepod ecology is forced by a single environmental factor, perhaps the relative westerly
wind stress in winter indicated by the value of the NAO, then this factor should serve
as an indicator of future survival of young fish. Many people have now attempted to
hindcast the recruitment to fish stocks by reference to a simple environmental index.
Unfortunately for such predictions, the factors determining the timing of copepod
abundance are very complex and bear no simple and direct relationship with the timing
and strength of phytoplankton growth, itself rather directly forced by wind and sun. But
the timing of the appearance of the new generation of Calanus is dependent not only on
nutrition, but also on the fate each year of the deep, overwintering generation. Because
the winter environment of these copepods varies strongly, the reappearance of Calanus
in the surface waters is unpredictable and irregular.
If this is the general case, and if survival of fish larvae does depend importantly on
the availability of sufficient copepod food without delay after occlusion, then it would
be surprising that subsequent recruitment should be a simple function of potential egg
production of the adult stock. In fact, it was for many decades a mantra of stock
assessment methodology that recruitment was maximal at some intermediate stock size,
as in the conventional but mythical relationship for Arcto-Norwegian cod that you will
find in text books. The reality, usually concealed behind log-normal plots, is seen in the
subsequent year-class strengths of each stock: in the case of North Sea cod, 1965–1985,
this varied from 7 to 493 fish at unit age expressed as catch per unit effort (Garrod and
Schumacher, 1994). During the same period the total catch, a weak indicator of total
stock biomass, varied by little more than a factor of 2. Even more striking is the variability
of North Sea haddock, which in the period 1944–1971 had year-class strength at unit age
varying from <50 to 28,152 fish also expressed as catch per unit effort. A longer view
of recruitment to the same stock (1900–1971) shows how recruitment in most years is
modest, or effectively a failure. Interspersed among these, and at intervals ranging from
3 to 6 years, are the 10 or so “super-year-classes” that must have formed the basis of
the stock during this period. By far the three strongest year-classes were recruited in the
years of the “gadoid outburst” of the 1960’s.
This event itself illustrates very well the sequential effects of environmental forcing of
physical oceanography, through the planktonic ecosystem and on to higher trophic levels.
During the period 1960–1990 the seasonal cycle of abundance of Calanus finmarchicus
and C. helgolandicus in the northern North Sea underwent a progressive evolution: C.
finmarchicus abundance progressively decreased, and its period of maximum abundance
occurred later, and extended later, from 1960 to 1980 (Beare and McKenzie, 1999b).
Using preliminary data on copepod abundance, Cushing (1990) noted that a stepwise
multiple regression of cod recruitment against Calanus abundance and timing (“delay”)
and March water temperatures showed that recruitment was correlated with the index of
delay, and modified by Calanus abundance and March water temperatures.
This finding is probably typical, but not directly exportable to other species. Note
that each fish species has a characteristic relationship with its ecological environment
during the critical period of planktonic larval life. Thus, each has an individual period of
maximum occlusion rate from plankton eggs, ranging from February for plaice (Platessa)
to late April for whiting (Merlangius). We may presume that each is matched to the period
Chapter 9: The Atlantic Ocean
in atmospheric forcing of circulation, stratification, and mixed-layer temperatures over
the continental shelf here as elsewhere.
Although it must be an oversimplification, the timing and abundance of planktonic
copepods has become the paradigm for the factor most directly involved in the matchmismatch model of the survival of larval fish. This is thought to occur only when the
date of occlusion of fish eggs matches the date of sufficient copepod abundance in
their immediate surroundings (e.g., Cushing, 1990). By extension, it is supposed that if
copepod ecology is forced by a single environmental factor, perhaps the relative westerly
wind stress in winter indicated by the value of the NAO, then this factor should serve
as an indicator of future survival of young fish. Many people have now attempted to
hindcast the recruitment to fish stocks by reference to a simple environmental index.
Unfortunately for such predictions, the factors determining the timing of copepod
abundance are very complex and bear no simple and direct relationship with the timing
and strength of phytoplankton growth, itself rather directly forced by wind and sun. But
the timing of the appearance of the new generation of Calanus is dependent not only on
nutrition, but also on the fate each year of the deep, overwintering generation. Because
the winter environment of these copepods varies strongly, the reappearance of Calanus
in the surface waters is unpredictable and irregular.
If this is the general case, and if survival of fish larvae does depend importantly on
the availability of sufficient copepod food without delay after occlusion, then it would
be surprising that subsequent recruitment should be a simple function of potential egg
production of the adult stock. In fact, it was for many decades a mantra of stock
assessment methodology that recruitment was maximal at some intermediate stock size,
as in the conventional but mythical relationship for Arcto-Norwegian cod that you will
find in text books. The reality, usually concealed behind log-normal plots, is seen in the
subsequent year-class strengths of each stock: in the case of North Sea cod, 1965–1985,
this varied from 7 to 493 fish at unit age expressed as catch per unit effort (Garrod and
Schumacher, 1994). During the same period the total catch, a weak indicator of total
stock biomass, varied by little more than a factor of 2. Even more striking is the variability
of North Sea haddock, which in the period 1944–1971 had year-class strength at unit age
varying from <50 to 28,152 fish also expressed as catch per unit effort. A longer view
of recruitment to the same stock (1900–1971) shows how recruitment in most years is
modest, or effectively a failure. Interspersed among these, and at intervals ranging from
3 to 6 years, are the 10 or so “super-year-classes” that must have formed the basis of
the stock during this period. By far the three strongest year-classes were recruited in the
years of the “gadoid outburst” of the 1960’s.
This event itself illustrates very well the sequential effects of environmental forcing of
physical oceanography, through the planktonic ecosystem and on to higher trophic levels.
During the period 1960–1990 the seasonal cycle of abundance of Calanus finmarchicus
and C. helgolandicus in the northern North Sea underwent a progressive evolution: C.
finmarchicus abundance progressively decreased, and its period of maximum abundance
occurred later, and extended later, from 1960 to 1980 (Beare and McKenzie, 1999b).
Using preliminary data on copepod abundance, Cushing (1990) noted that a stepwise
multiple regression of cod recruitment against Calanus abundance and timing (“delay”)
and March water temperatures showed that recruitment was correlated with the index of
delay, and modified by Calanus abundance and March water temperatures.
This finding is probably typical, but not directly exportable to other species. Note
that each fish species has a characteristic relationship with its ecological environment
during the critical period of planktonic larval life. Thus, each has an individual period of
maximum occlusion rate from plankton eggs, ranging from February for plaice (Platessa)
to late April for whiting (Merlangius). We may presume that each is matched to the period