ICES mar. Sei. Symp., 198: 520-528. 1994
Physical effects on recruitment of Faroe Plateau cod
Bogi H ansen, Eilif G aard, and Jâkup Reinert
Hansen, B., Gaard, E ., and R einert, J. 1994. Physical effects on recruitment of Faroe
Plateau cod. - ICES mar. Sei. Symp., 198: 520-528.
The cod stock on the Faroe Plateau has traditionally been considered as relatively
stable, with fairly constant recruitment levels. In the late 1980s this situation changed
dramatically when recruitment levels dropped by one or two orders of magnitude
below what appears to have been the norm during most of this century. Much of the
change seems to have been due to processes operating during the first three months of
the life history. Combining the available biological information on this stock with
results from investigations on zooplankton and hydrography, it is argued that survival
during these first months is critically dependent upon the advection of spawning
products from the spawning grounds to the shallow parts of the shelf, and upon the
production and advection of Calanus finm archicus from the Faroe Bank Channel to
the shelf. Evidence is presented that during the period of recruitment collapse spring
winds were abnormally strong from the southwest, increasing the probability of the
transport of cod eggs and larvae and copepods off the shelf and possibly also affecting
primary production and hence production of copepod nauplii. In addition, the Faroe
Plateau seems to have been less dom inated by Atlantic waters with weaker copepod
import and less stable advection paths as conceivable consequences.
Bogi Hansen, E ilif Gaard, and Jâkup Reinert: Fisheries Laboratory o f the Faroes,
Nôatûn, PO Box 3051, FR-110, Tôrshavn, Faroe Islands.
Introduction
The Faroe Plateau is separated from other shallow areas
by depths exceeding 500 m in all directions and is
dominated by Atlantic waters from the North Atlantic
Current (Fig. la). This area supports the Faroe Plateau
cod stock, which appears to be a fairly self-sustained
stock unit (see Jâkupsstovu and Reinert (1994) for
details on stock status, biology, assessments, and fish­
ery). Despite very high fisheries intensities during most
of this century, annual landings have been fairly stable
throughout most of the period, indicating that recruit­
ment levels must have been fairly stable in the period as
well. This is confirmed in Figure 2, where year-class
strengths, calculated from three different data sets (Jâk­
upsstovu and Reinert, 1994), are depicted for the period
1961-1992.
Figure 2 shows fairly stable fluctuations for most of
the period, but it also shows exceptionally small year
classes at the end of the period. This development has
North Allantic
Current
Slope Current
—♦
Cold current
s— >
[cel.-Faroe« Front
***
Deeper than 500m
□
200m - 500m depth
EM
Shallower than 200m ■
Standard lection
—
Figure 1. Bottom topography and main features of the flow field around the Faroes in the upper layers (a) and deeper than about
500 m (b). T he three standard sections regularly operated by the Faroese Fisheries Laboratory are shown in (b).
Physical effects on recruitment o f Faroe Plateau cod
IC ES mar. Sei. Sym p.. 198 (1994)
much too scarce for any indication to be derived of long­
term changes. A discussion on flow variations therefore
has to be based on the more indirect evidence of hydrographic parameters, especially tem perature and salinity.
VPA age 2
—b — Survey age 2
—
521
O-Group
E
»
Material and methods
I
E
3
Wind data
z
0 » ...............
1961
1966
i l l ) ..................................................
1971
! 9 76
1981
1986
1991
Figure 2. Estimates of year-class strength of the Faroe Plateau
cod in the period 1961-1992, based on VPA, on groundfish
surveys, and 0-group surveys (Jâkupsstovu and R einert, 1994).
The vertical scale refers to the VPA estimates. The other two
indices are in relative scales.
been so pronounced that it indicates a major change
rather than random variation from the norm, and some
factor or combination of factors must have caused it.
These factors may be strictly biotic or they may be
related to the high fisheries intensity, but a change in the
abiotic environment is also likely and it is the aim of this
article to explore that possibility. A more com prehen­
sive discussion, including the effect of other factors, can
be found in Jâkupsstovu and Reinert (1994).
The Faroe Plateau cod spawns in the second half of
March, while the 0-group index is derived from surveys
carried out some 3 months after this. In Figure 2 it can be
seen that despite discrepancies in occasional years, for
most years there is good agreement between the 0-group
index and the year-class sizes of two-year-old fish, calcu­
lated from groundfish surveys and from VPA. This
indicates that processes during the first 3 months in most
years determine year-class strength; we have chosen to
focus on this period.
Many physical parameters may, a priori, affect cod
recruitment. In other regions tem perature has been
considered important, but with the available data we
have not been able to establish any direct links between
temperature and recruitment. We have instead focused
on winds and advection by currents, since these are the
parameters considered most likely to influence the con­
ditions of the cod eggs and larvae in the first 3 months.
On the shallow parts of the shelf, advection seems to
be controlled largely by tidal effects (Hansen, 1992), but
although the tides have variations on annual and decadal
scales, an evaluation of these indicates that they are too
small to explain the dramatic changes in cod recruit­
ment. In deeper water the advection is dominated by the
large-scale flow past the Faroe Plateau (Fig. la). Unfor­
tunately, direct measurements of flow off the shelf are
Wind observations are often seriously biased in some
way. Therefore three different data sets were originally
investigated: synoptic data from two sites in the Faroe
Islands (Törshavn and Akrabyrgi) and data from a grid
point due northeast of the Faroes (62°20'N 5°49'W) in
the Norwegian Meteorological Institute Hindcast data
set. Intercomparisons between these three series
showed fairly large discrepancies between the two
synoptic series and analysis indicated that relocation of
the wind recorder in Törshavn to a slightly different site
in 1979 was the main reason for the difference. The
Akrabyrgi and the Hindcast data compared very favour­
ably, however, and as the Akrabyrgi series extends two
years longer, this was the one used.
Four wind indices were derived from the wind obser­
vations from Akrabyrgi for the period 1962-1992: storm
frequency, defined as the frequency of observations with
windspeeds above certain values (15 m s-1 and 20 m
s“ 1); mean wind energy input to the sea, defined as mean
windspeed cubed; and two components of the windstress, computed as the windspeed squared and decom­
posed along the east and north axes using the wind
direction. All four indices were computed as averages
for the period March to May during cod spawning and
the planktonic stages.
Hydrographic data
The main hydrographic data set used is a collection of
about 4000 CTD profiles obtained by the Faroese Fish­
eries Laboratory from 1976 to August 1993 (Hansen and
Kristiansen, in prep.). The CTD data have been aug­
mented by the inclusion of data from Nansen casts
obtained from the ICES Oceanographic D ata Centre.
The salinities from these stations are of variable quality,
but the constancy of the salinity in the deep water of the
Norwegian Sea has been used to screen the data. Three
time series were constructed from this data set.
To describe the dominance of Atlantic water off the
spawning grounds we defined an index, termed the width
o f Atlantic water north o f the Faroes. It was based on
temperature and salinity observations from all available
occupations of standard section N (Fig. lb ). For each
station the vertically averaged content of Atlantic water
was computed from 25 m depth down to the bottom at
the shallow stations and down to 500 m depth at the
B. Hansen, E. Gaard, andJ. Reinert
522
deeper ones, assuming the water to be a mixture of
Atlantic water, water from the East Icelandic Current
(t = 4°C, s = 34.80), and from the deep Norwegian Sea
(t = —0.5°C, s = 34.91). Atlantic water was defined as
the average between 25 and 100 m depth for the station
on the section, which had the highest average salinity in
this depth range. To exclude small cumulative errors,
observations with less than 20% Atlantic water were
assumed to have no Atlantic water and observations
with more than 100% to have pure Atlantic water. For
each occupation of the section, the width of Atlantic
water was then defined as the distance (in nautical miles)
from latitude 62°30'N (outer edge of shelf water) to the
latitude at which the average content of Atlantic water
fell below 50%.
The Faroe Bank Channel salinity was defined as the
vertically averaged salinity of the 100-300 m depth
interval for stations, located between 61°00'N and
61°30'N, 7°00'W and 9°00'W for which the bottom depth
was at least 700 m and the differences between 100 and
300 m level was less than 1°C in tem perature and 0.1 psu
in salinity.
The shelf water salinity was defined as the vertically
averaged salinity for stations, located between 61°30'N
and 63°00'N, 5°00'W and 9°00'W excluding the fjord
region between 62°01'N and 62°20'N, 6°40'W and
7°10'W, for which the bottom depth was between 50 and
IC ES m ar. Sei. Sym p., 198 (1994)
100 m and the difference between uppermost and deep­
est level was less than 0.1°C in tem perature and 0.03 psu
in salinity.
Results
Variations in winds
Figure 3 shows the time history of the four wind indices
defined above. It appears that since the early 1980s the
frequency of winds in spring above certain speeds has
increased as has the wind energy input to the sea. The
windstress component towards the north shows a some­
what similar, although less dramatic, tendency, while
the windstress towards the east showed exceptionally
high values in 1989 to 1991.
Variations in Atlantic water dominance north of
the Faroes
The time series of the width of Atlantic water north of
the Faroes is shown in Figure 4. Figure 4a indicates a
decrease in the width since the late 1980s. In Figures 4b
and 4c the seasonal variation of the width is shown
separately for the period before 1 January 1988 and after
that time. The irregular timing of the older surveys
makes the analysis difficult, but for the later part of the
3000
30
Frequency of winds (%)
above 15m/s
m l\f
2500 • .
Wind energy input (m /s)3
Ä
2000 ■
1500 •
10 . T
/
71
\/
T
I*
1
1000 •
abo ve 20ntfe
0
1962
1970
1980
1990
-
T
t
■ ♦
1962
4 ♦
♦ 11H
1970
- H
- W
M
♦
1980
♦
t H
4
t
t ♦ M
1990
40
100
80
A r\
500 ■
Wind stress towards E (m/s)
20
60
40
20
0
-20
-20
Wind stress towards N (m/s)
-40
-40
1962
1970
1980
1990
1962
1970
1980
1990
Figure 3. Time series showing four wind indices (defined in text) for the M arch-M ay period, based on synoptic meteorological
observations from Akrabyrgi.
Physical effects on recruitment o f Faroe Plateau cod
ICES mar. Sei. Sym p., 198 (1994)
523
80
E
.e .
60
o
■a
40
<
<H>
O
&
■a
% 20
0 —
1930
1940
1950
1960
1970
1980
1990
Before 1987
Jan
Mar
May
Jul
Sep
After 1988
Nov
Jan
Mar
May
Jul
Sep
Nov
Figure 4. Width of Atlantic W ater north of the Faroes (defined in text) plotted against time of observation (a) and against m onth
of observation separately for the period before (and including) 1987 (b) and after (and including) 1988. The range indicated by the
vertical lines shows distance to the stations between which the 50% Atlantic W ater limit was found. The most probable distance
indicated by filled circles was estimated by interpolation between these stations.
period there is at least an indication of seasonal variation
in the influences of Atlantic water north of the Faroes,
with a minimum in spring and maximum in late summer
or autumn. A comparison of Figures 4b and 4c supports
the indication in Figure 4a, namely, that the Atlantic
water influence has been weaker since 1988 compared to
previous years, especially for the spring period.
The influences of Atlantic water on the shelf
Figure 5 shows the salinity variation on the shelf and in
the Faroe Bank Channel. It can be seen that the Shelfwater has decreased in salinity since the mid-1980s. To
some extent this decrease reflects only the salinity de­
crease of the incoming Atlantic water, represented by
the upper waters of the Faroe Bank Channel. The
35.35
35.25
Faroe Bank
Channel
£ 35.15
35.05
1980
1985
1990
Figure 5. Average salinity of the 100-300 m layer in the Faroe
Bank Channel and average salinity of the Shelfwater plotted
against the time of observation.
524
IC ES m ar. Sei. Sym p., 198 (1994)
B. Hansen , E. Gaard , and J. Reinert
0.2b
0.25
n is •
u
è
0.10
Before 1987: ■
After 1988: n
□
0.20
□
□
B
0.10 •
■
yy 0.05
0.05
0.00
0 00
1980
1985
1990
0
□
B
b
Jan
Mar
May
Jul
Sep
Nov
Figure 6. Salinity difference between the upper water in the Faroe Bank Channel and the Shelfwater plotted against time of
observation (a) and against m onth of observation (b). In (b), observations before (and including) 1987 have been distinguished
from observations after (and including) 1988.
difference between Faroe Bank Channel salinity and the
shelfwater salinity, however, should be an indicator of
Atlantic water influence on the shelf and Figure 6 shows
the variation of this param eter plotted against time and
against the month of observation. H ere only CTD obser­
vations were used and only if they had been obtained
from the Faroe Bank Channel and the shelf simul­
taneously (within one month).
The number of observations before 1987 is too small
to warrant statistical analyses, but the figure indicates a
change sometime in the late 1980s. Before that time, the
Shelfwater consistently seems to have had a salinity 0.05
to 0.1 below the salinity in the Faroe Bank Channel.
A fter 1987 the difference was much more variable and
typically larger. Figure 6b indicates that the change is
not due to changed sampling strategy coupled to a
possible seasonal variation.
of the Plateau this is dominated by the N orth Atlantic
Current (Fig. la ), which in most regions circles the
Plateau anticyclonically. In the shallower parts the
Shelfwater has an even more persistent anticyclonic
circulation around the islands (Fig. 7).
The horizontal distribution of larvae in late Ju n e early July is available from the Faroese 0-group surveys
and these show clearly (Jâkupsstovu and Reinert, 1994)
that the main habitat of the juveniles is the shallow part
of the Faroe Shelf (shallower than 100 m). The transport
Discussion
Environmental conditions of spawning
Cod seem to spawn throughout the Faroe Plateau, but
the two main spawning grounds are located to the north
and west of the islands (Fig. 7). In most years the
spawning occurs close to the 100 m depth contour. This
bottom depth seems to coincide fairly well with the
boundary between the Atlantic waters off the shelf and
the well-mixed Shelfwater over the shallow parts of the
Plateau. Thus Faroe Plateau cod in parallel with other
Northeast Atlantic cod stocks (e.g. Icelandic and Arcto
Norwegian) spawns in the boundary region between an
Atlantic and a fresher water mass.
A fter fertilization, the cod eggs ascend towards the
upper layers, where development up to hatching occurs
after about 16-20 days. During this time the spawning
products are advected by the flow. Over the deeper parts
Figure 7. Bottom topography of the Faroe Shelf and residual
flow vectors based on m easurem ents from long-term current
m eter moorings at the sites shown (H ansen, 1992). Hatched
areas indicate the main spawning grounds.
Physical effects on recruitment o f Faroe Plateau cod
ICES mar. Sei. Sym p., 198 (1994)
525
able sizes for the various larval stages and in the N orth­
east Atlantic Calanus finmarchicus seems to be the
dominant prey item. No investigations have been made
on the food of cod larvae in Faroese water, but the
dominance of C. finmarchicus among zooplankton in
Faroese waters during spring (Gaard, in prep.) indicates
that this species may play a similar role for the Faroe
Plateau cod as for other cod stocks.
This copepod species has been found in extensive
areas in the Northeast Atlantic including the Norwegian
Sea. It is known to overwinter as copepodit stages IV
and V in deep water. During winter, C. finmarchicus is
almost absent from the Faroe Plateau (Gaard, in prep.),
which is consistent with this life cycle. There therefore
has to be an import of these animals to the plateau
during spring, and observations on the Plateau in the
spring of 1992 and again in 1993 indicate that the import
occurs close to the cod spawning area (Fig. 8).
of spawning products from the spawning grounds into
the shallow areas must be a critical process.
In most regions on the outer parts of the Faroe Shelf
(Fig. 7) the residual current follows the bottom topogra­
phy, and transport onto the shelf must occur mainly by
mixing; but just south of the northeastern corner of the
plateau (site E3 in Figure 7) the current meter obser­
vations indicate a flow on to the shelf (Hansen, 1992).
Using current meter observations from the area (H an­
sen, 1992), a drift from the northern main spawning
ground to the E3 site can be estimated to require in the
order of 2-4 weeks.
The food items of cod larvae
It is generally assumed that a key factor for the survival
of cod larvae is the availability of zooplankton of suit­
l o oo
500
200
200
1 00 '
*i5
100
E
Copepod nauplii
C. finmarchicus
y
Cl-
C. finmarchicus
. CIV-VI
- •
/
200
2 001
2001
n
150
100
■"Copepod nauplii
_ C . finmarchicus
Cl-lll ,
_ C . finmarchicus
CIV-VI
Figure 8. Concentrations of various stages of C. finm archicus around the Faroes, 26 Feb-3 M ar 1993 (a-c) and 12-20 M ar 1993
(d-f). From G aard (in prep.).
526
B. Hansen , E. Gaard, and J. Reinert
Faroe Plateau
I C E S m a r . Sei. S y m p ., 198 (1994)
Faroe Plateau
North
20
10
01
Faroe Plateau
Figure 9. Concentration of C. finm archicus on the standard sections, 5—8 Feb 1993. Stages are indicated by hatching as shown on
one of the graphs. From Gaard (in prep.).
The most likely source region is indicated by Figure 9,
showing concentrations of C. finmarchicus in the upper­
most 50 m in offshore waters some weeks before spawn­
ing in 1993. The concentration of C. finmarchicus in the
Faroe Bank Channel and over the shelf edge north of the
Faroe Islands was about 10 times higher than in the other
areas around the islands. The increase was mainly due to
higher concentrations of copepodit stage V in these
areas. These observations, together with the dominant
flow patterns of the upper layers, point to the Faroe
Bank Channel as a main supplier of C. finmarchicus to
the Faroe Plateau. The animals may be transported to
the channel either from the open N orth Atlantic through
advection by the North Atlantic Current (Fig. la ) or
from the deep Norwegian Sea by the deep currents
ending in the outflow through the channel (Fig. lb).
Recent investigations have shown high concentrations
of C. finmarchicus in the deep water in February, con­
firming this as a major pathway for C. finmarchicus to
the Faroe Plateau (G aard, in prep.).
Effect of wind changes on recruitment
To some extent it is a matter of interpretation when the
period of bad recruitment started. Since 1984 the re­
cruitment values have been low but the most abnormal
development has been from 1989. For 1988 the 0-group
index was fairly high, while the other two recruitment
estimators indicated a small year class. Thus it is not
clear whether 1988 should be included in the period of
collapse.
It was shown above (in “Variations in winds”) that this
period of recruitment collapse broadly coincided with
increased winds from the southwest. In Figure 10 the 0group index is plotted against the wind energy input and
the windstress component towards the northeast. We
would expect windstress in this direction to affect the
transport of cod-spawning products and of copepods
from the spawning grounds to the shelf negatively. Wind
energy input to the sea also might affect the depth of the
mixed layer and hence the primary production in waters
Physical effects on recruitment o f Faroe Plateau cod
ICES mar. Sei. Sym p., 198 (1994)
1000
1000
100
100
• "
■ 86
S- 10 ■■
527
■ 86
■ 89
■8 9
0.1
0
500
1000
1500
2000
2500
Wind energy input (m/s)
3000
-20
3
0
20
40
60
Wind stress towards NE (m/s)
80
2
Figure 10. 0-group index plotted against wind energy input to the sea (a) and windstress towards the northeast (b) for the M archMay period each year. Exceptional year classes are m arked by the year of spawning.
off the shelf. As the zooplankton drift for most of the
time in these waters, a high primary production is
probably necessary for sufficient production of nauplii.
The correlation coefficients between the logarithm of
the 0-group index and these two wind indices are just
below significance at a 95% level in both cases. Also the
1991 cod year class, which was the smallest one on
record according to the 0-group index, experienced
much smaller wind forcings than the 1990 year class
which, although small, seems to have been much better
than the 1991 year class. Therefore, wind by itself does
not explain all variations in year-class strength of Faroe
Plateau cod; but as a whole there are indications that
some of the recruitment collapse may be linked to
abnormal winds. Thus the five lowest 0-group indices on
record occurred in years (1986, 1989, 1990, 1991, and
1992) when the wind energy input was high, and all of
these but the 92-year class had large windstresses
towards the northeast.
Hansen et al. (1990) discussed the relative stability of
cod compared to haddock recruitment on the Faroe
Plateau, indicated from the data available at that time.
They suggested a larger sensitivity of the planktonic
stages of haddock to high wind speeds compared to cod
owing to different horizontal distributions. This might
seem incompatible with the results quoted here, but it
must be noted that haddock recruitment on the Faroe
Plateau has also collapsed in recent years. Unfortu­
nately, less reliable 0-group indices for haddock make it
difficult to compare the cod and haddock collapse quan­
titatively.
Effects of a weakened flow of Atlantic water
Winds were apparently not the only physical param eter
behaving abnormally in this period. Although the avail­
able data set does not allow firm conclusions, we have
presented time series of two independently estimated
parameters (Figs. 4 and 6); both indicate a weakened
influence of Atlantic water on the plateau.
The reduced dominance of Atlantic water north of the
Faroes (Fig. 4) may well have resulted in a smaller
import of copepods from the Faroe Bank Channel to the
shallow parts of the shelf. It should also lead to a less
stable transport path from the spawning grounds to the
Shelfwater. Such a weakening would make the transport
more vulnerable to external forcing, especially from
windstress towards the northeast.
With changing conditions north of the Faroes, a
change in the coupling between the Faroe Bank Channel
and the shelf can also be expected, and Figure 6 indicates
that this may have occurred. The change in Figure 6 may
be interpreted differently; but a weakened coupling
between the Faroe Bank Channel and the shelf is at least
one of the possible explanations. If this explanation is
supported by further investigations, this is one more
indicator of a weakening in the processes transporting
spawning products and copepods to the shallow parts of
the shelf. The hypothesis of a weakening Atlantic water
influence is not as well documented as the abnormal
wind situation, but all the available data support this
hypothesis.
Conclusion
If the spawning stock of Faroe Plateau cod had been in a
good condition, we might well have concluded that the
abnormally low recruitment that has persisted since the
end of the 1980s was an effect of the changes in winds
and Atlantic water influence documented in this paper.
W hen the actual state of the spawning stock is taken into
528
B. Hansen, E. Gaard, and J. Reinen
account (Jâkupsstovu and Reinert, 1994), purely bio­
logical effects cannot be excluded. On the other hand, it
is not obvious either that purely biological effects suffice
as an explanation for the drastic decline in recruitment
observed, and we suggest that a combination of in­
creased winds and reduced Atlantic flow be considered a
main hypothesis in the further efforts to explain the
recruitment collapse of the Faroe Plateau cod stock.
IC ES mar. Sei. Sym p., 198 (1994)
References
Hansen, B. 1992. Residual and tidal currents on the Faroe
Plateau. ICES CM 1992/C: 12.
Hansen, B., Kristiansen, A ., and R einert, J. 1990. Cod and
haddock in Faroese waters and possible climatic influences
on them. ICES CM 1990/G: 33.
Jâkupsstovu, H. i, and R einert, J. (1994). Fluctuations in the
Faroe Plateau cod stock. ICES mar. Sei. Symp., 198: 194—
211 .