ICES mar. Sei. Symp., 198: 244-253. 1994
Cod distribution and temperature in the North Sea
H en k Heessen and Niels D aan
Heessen, H. J. L ., and D aan, N. 1994. Cod distribution and tem perature in the North
Sea. - ICES mar. Sei. Symp., 198: 244-253.
Survey data on the distribution of individual age groups of cod in the North Sea exhibit
marked annual differences in the ambient tem perature. There is no indication that
these differences are related to a certain preferred tem perature, not at least for the
juveniles. Juvenile cod are exposed to the highest water tem peratures in late summer
and autumn, and the lowest tem peratures in winter. This analysis may provide a better
basis for the study of annual growth differences and the effect of tem perature on
digestion rates.
H enk Heessen and Niels Daan: Netherlands Institute fo r Fisheries Research, PO Box
68, 1970 A B Umuiden, The Netherlands.
Introduction
O ne approach to the cod an d climate problem is p ro ­
vided by th e behaviour of th e cod population in relation
to annual variations in w ater te m p e ra tu re , with particu ­
lar reference to distribution p atterns. M ountain and
M urawski (1992), for exam ple, attrib u ted changes in the
distribution of several fish species on the northeast
continental shelf o f the U S A to changes in te m p eratu re.
In the N orth Sea, young herring surveys w ere initiated in
F ebruary 1965, which have evolved over the years to
highly standardized and internationally coordinated sur­
veys (now called In ternational B o tto m Traw l Surveys,
IBTS), aim ed at estim ating the year-class strength of a
variety of species, including cod. A lthoug h prelim inary
analyses of th e effect of w ater te m p e ra tu re on the
distribution of cod have b een p resen ted earlier (Buijsse
and D a a n , 1986; H eessen, 1993), an in-depth analysis is
h am p ered by the fact th a t the hydrographical data
collected during these surveys are e n tered in a different
database at IC E S th a n th e fish data, and so far it has not
b een possible to link the stations properly. A nalyses
have therefore been limited to subsets of stations, for
which te m p e ra tu re d a ta have b e e n m a d e available on a
national basis. C onsequently these studies covered only
som e of the stations. H ow ever, upon req uest from the
Multispecies A ssessm ent W orking G ro u p , IC E S has
now m ade available a com prehensive d a ta set giving the
range, average, and stand ard deviations of the recorded
te m p eratu res in th e datab ase by statistical rectangle,
q u a rte r, and year. A lth ough this set includes sources o f
inform ation o th e r th an just the IB TS, it provides an
excellent o p portu nity to study th e relationship betw een
the average cod catches during the surveys and the
te m p e ra tu re regime in each year. T h e objective of this
p a p e r is to relate annual changes in distribution to
variations in tem p eratu re.
M ethods
T he d a ta available include the catches p e r h o u r fishing
by age group by statistical rectangle fo r th e IB T S in
F ebruary 1977-1991. In addition to these d ata on the
w inter distribution o f cod, quarterly surveys w ere car­
ried o u t in 1981 and 1991 in co nnection w ith large-scale
stom ach sam pling projects. D uring these additional su r­
veys a variety of o th e r gears was used, although in 1991
m ost of the countries involved em ployed th e G O V
trawl, which was also th e stand ard g ear for the F eb ru ary
surveys (IC E S, 1992). T he catch d a ta from the F eb ru ary
surveys w ere m ade available through IC E S ; for the
o th e r surveys, data from the individual countries were
sent directly to the N etherland s Institute fo r Fisheries
R esearch. N o corrections w ere m a d e for possible differ­
ences in catchability b etw een gears.
A ltho ugh inform ation on hydrographic p a ram eters
could be included in the exchange form at fo r the survey
d ata, this is n ot do ne as a m a tte r of ro utine, largely
because such inform ation is stored within IC E S in a
C od and climate in the N orth Sea
IC E S m a r . Sei. S y m p ., 198 (1 9 9 4 )
E5
E6
E5
245
E6
52
51
Quarter 1
Quarter 2
1977 / 1991
1991
50
49
48
47
V
46
7
45
58 N
44
43
57 N
42
41
56 N
40
11-
39
55 N
38
37
36
35
53 N
34
33
52 N
32
31
51 N
30
29
28
27
52
51
Quarter 3
Quarter 4
1991
1991
50
49
60 N
48
47
59 N
46
45
58 N
44
10
43
42
41
40
39
38
55 N
,•12
16.
37
36
14
T
13.
■ 11
18.
54 N
14 14.
35
53 N
34
33
52 N
32
31
51 N
30
29
50 N
28
27
4W
2W
0
2E
4E
6E
8E
10E
12E
4W
2W
0
2E
4E
6E
8E
10 E
12E
Figure 1. Quarterly distribution charts of mean bottom tem perature by rectangle, based on data from the ICES Oceanographic
Data Bank. The position of the Dogger Bank is indicated. A. Winter 1977-1991; B. Spring 1991; C. Summer 1991; D. Autumn
1991.
246
I C E S m a r . Sei. S y m p .. 1 9 8 (1 9 9 4 )
H. H eessen an d N. Daan
separate datab ase. T hese two d atabases cann ot be
directly linked an d consequently te m p e ra tu re infor­
m ation is no t available for all individual hauls. H ow ever,
th e IC E S O cean ographic D a ta B an k can export m e an
quarterly b o tto m te m p e ra tu re s by statistical rectangle
with th e associated ranges of observ ed te m p e ra tu re s and
stan d ard deviations. In o rd e r to m e et th e objective of
this study, w e replaced all te m p e ra tu re d a ta in th e IBTS
database with the m ean te m p e ra tu re d a ta from the
O ceano grap hic D a ta B ank . T h e w inter te m p e ra tu re
d ata cover alm ost th e entire N orth Sea, bu t in o th e r
quarters the re arc considerable gaps, particularly for
1981 and the last q u a rte r of 1991. In cases of missing
values, we in terp o lated te m p e ra tu re d a ta if d ata w ere
available for at least tw o neighbouring rectangles.
N evertheless, a large com p o n en t of th e cod population
could not be associated with the am bien t te m p eratu re
a nd th e refo re th e results for q u arters o th e r th an th e first
one may be unreliable.
In the sub sequ en t analysis, the rectangles w ere classi­
fied in 1°C te m p e ra tu re ban ds. A ll observations of
te m p e ra tu re s below 0°C w ere p ooled in th e te m p e ra tu re
b an d of - 1 ° C . Subsequently, the n u m b e r of rectangles
in each te m p e ra tu re b and and the average catch-at-age
p er h o u r p e r te m p e ra tu re band were calculated. T he
results w ere analysed in two ways: as densities in relation
to te m p e ra tu re , and as percentages o f the total p o p u ­
lation, relative to the surface area covered by each
te m p e ra tu re band.
Results
T e m p e ra tu re
In o rd e r to in terp ret th e results, the qu arterly te m p e ra ­
ture regim e is indicated in Figure 1 A -D . T h e d a ta for
the first q u a rte r cover 15 years and therefo re can be
taken to describe the average situation fairly well. F or
the o th e r q u arters, only 1991 d a ta have b een used,
because of the even larger gaps in th e 1981 set. D uring
the w inter season (Fig. 1A ), the average te m p e ra tu re in
the n o rthw estern N o rth Sea is b etw een 6 an d 7°C, and
the D og ger B an k m arks th e b o rd e r o f th e colder w aters
found along th e continental coast. T h e Shetland area,
w here A tlantic W a te r enters the N o rth Sea, is c h aracter­
ized by the highest te m p e ra tu re s, and an inflow of
slightly higher te m p e ra tu re s can also be observed
th rou gh th e C hannel.
T h e average b o tto m te m p e ra tu re in th e N o rth Sea
during w inter (Fig. 2) varies b etw een 4.5 a nd 7.5°C, with
notably high average values during the period 1988—
1990. T he years 1979 and 1986 were relatively cold. For
the entire N o rth Sea, how ever, the te m p e ra tu re signal is
w eak.
60
50
40
R
30
20
1977
1980
1983
1986
1989
Figure 2. Indices (R) of abundance of 1-group cod (bars) and
average North Sea bottom tem perature in °C (squares) during
winter 1977-1991.
T h e w inter of 1991 was fairly mild and probably
affected th e te m p e ra tu re s reco rd ed in th e la ter seasons
(shown in Figure 1 B -D ) co m p ared to the average winter
situation (shown in Figure 1A). T h e sou th eastern N o rth
Sea has started to w arm up in th e second q u arter. T he
available d a ta indicate m a jor differences b etw e e n neigh­
bouring squares, which is u ndo u b te d ly caused by the
fact th a t th e IC E S O cean ographic D a ta B a n k is based
n o t just on b o tto m trawl survey d ata b u t also on o th e r
sources, an d the d a ta are integrated o ver three m onths
of observation. D e p en d in g on th e actual d a te on which
th e d ata have b een collected, considerable differences
can be expected. In the third q u a rte r th ere is a small strip
of higher te m p e ra tu re s along the Scottish coast running
southw ards. S outheast o f th e D o g g er B a n k , te m p e ra ­
tures above 10°C are record ed th ro u g h o u t. A s far as
te m p e ra tu re d a ta are co n cerned, nothing has changed
very much in th e fourth q u a rte r c o m p ared to th e third.
A ltog e th er, these d ata imply only m ino r seasonal differ­
ences in te m p e ra tu re over a large cen tral are a n orth of
the D og ger B ank.
D istribution
T h e re is a m a rk e d overall age d e p e n d e n t relationship
betw een cod density and te m p e ra tu re during the F e b ru ­
ary surveys (Fig. 3). O n average, th e highest densities of
1-group cod are fo und at the low er end of the range,
even below 0°C. T he highest density o f 2-group cod is at
som ew hat higher te m p e ra tu re s, w hereas the older part
of the p o p u lation has its highest densities at the u p p er
end of th e te m p e ra tu re range. T h e general p attern
ap pears to be consistent from year to year, although
there are annual variations (Fig. 4). T hese observations
are obviously strongly related to th e general o b ser­
vations th a t 1- and 2-group cod are found p redom inantly
C od a n d clim ate in the N orth Sea
I C E S m a r . Sei. S y m p .. 198 (1 9 9 4 )
247
5
140
4
120
100
a 98 2
80
age 3
3
2
N/hr
60
1
40
0
20
1
0
1
2
3
4
5
6
7
8
9
1
0
1
2
3
4
5
6
7
8
9
1
0
1
2
3
4
5
6
7
8
9
1
0
1
2
3
4
5
6
7
8
9
5
4
Figure 3. Average numbers-at-age of cod per hour fishing by
1°C tem perature bands in February 1977-1991.
3
2
1
in the southeastern N o rth Sea, particularly along the
continental coast, w here te m p eratu res are lowest. It has
to be borne in mind w hen interpreting these d ata that
fish are n o t free in selecting preferred te m p eratu res,
because the te m p e ra tu re range d epends on the severity
of the winter. In addition, the distance o v er which cod
usually migrates is restricted (D a a n , 1978).
T o circum vent this p roblem , we com pared th e frac­
tion of the N orth Sea characterized by the different
te m p eratu res with the p ropo rtion of the population
found at each te m p e ra tu re for the years 1977-1991 (Fig.
5). T he percentage of surface area of the N orth Sea
within each 1°C te m p e ra tu re band is based on the
n u m b e r of rectangles within th a t band. T h e percentage
o f the total population of each age group is given for ages
1 -4 + . This provides a b e tte r indication of the im p ort­
ance of a particular te m p e ra tu re in relation to the total
distribution of the fish than the observed densities,
because a large catch could be restricted to a single
square. In that case, the particular te m p e ra tu re o f that
square would be overem phasized. Figure 5 shows that
the largest fraction of 1-group cod is consistently found
at the lower end of the range. T o a lesser extent this is
also true for 2-group fish, w hereas the d istribution o f the
older age groups is m ore variable. In general, how ever,
the o lder age groups seem to be found tow ards the high
side of the available te m p e ra tu re distribution.
T hese pictures can be sum m arized by plotting the
m ean am bient te m p e ra tu re of the p opulation by age
g roup in each year against the m e an N orth Sea te m p e ra ­
ture (Fig. 6). T h e am bient te m p eratu re o f 1-group fish is
indeed consistently below th e average, and to a lesser
extent this appears to be true also for 2-group fish. In
contrast, 3-group and 4-1- cod have always been found in
a slightly higher am bient te m p e ra tu re than the average.
Figures 7 and 8 provide similar d ata for all q uarters in
1981 and 1991. N ote that the te m p e ra tu re d ata for the
0
5
4
3
2
1
0
5
4+
4
3
2
1
0
1
0
1
2
3
4
5
6
7
8
9
T e m p e ra tu re (°C)
Figure 4. Numbers-at-age of cod per hour fishing (log scale) by
1°C tem perature bands in individual years, February 19771991.
2nd, 3rd, and 4th q uarters o f 1981 an d th e 4th q u a rte r of
1991 are far from com plete and may be biased. N ever­
theless, th e picture em erging h ere is th a t th e p a ttern of
am bien t vs. average te m p eratu re (Fig. 8) shifts m a rk ­
edly during the year. In au tu m n , 0- and 1-group cod live
at higher a m bien t te m p eratu res than the average N orth
Sea values, w hereas for the oldest age groups the
opposite is th e case.
248
I C E S m a r . Sei. S y m p ., 198 (1 994)
H . H e e sse n a n d N . D a a n
100
100
100
■
1982
1987
75
75
75
% 50
% 50
% 50
25
25
25
L
-1
0
1
2
3
4
5
6
1 0
7
1
2
3
4
5
6
7
8
1
9
0
1
2
3
4
5
6
7
8
9
1 00
100
1988
1978
75
75
% so;
% 50
25
25
1
0
1
2
3
4
5
6
7
8
100
-1
9
1979
0
1
2
3
4
5
6
7
8
100
1
9
1984
75
% 50
% 50
% 50
25
25
25
2
3
4
5
6
7
8
1 0 1
9
2
3
4
5
6
7
8
100
% 50
-1
0
1
2
3
4
5
6
7
8
100
% 50
% 50
25
25
0
1
2
3
4
5
6
7
8
% 50
% 50
25
25
25
3
4
5
6
7
8
9
7
8
9
1
2
3
4
5
6
7
8
9
2
3
4
5
6
7
8
9
1991
% 50
2
6
1986
75
1
5
100
100
75
0
0
1 0 1
9
75
1
4
1990
75
1
3
10 0
75
9
1981
1
9
1985
1980
2
1989
75
0 1
1
100
75
1
0
1
0 1
2
3
4
5
6
7
8
9
1
0
1
2
3
4
5
6
7
8
9
EH bottom temperature
□
age 4+
H age 3
Figure 5. Fractions of the total cod population by age group observed in 1°C tem perature bands
by year in February 1977-1991. The shaded area represents the fraction of the total area
characterized by each band.
Ü age 2
H age 1
C od a nd climate in the N orth Sea
IC E S m a r . Sei. S y m p ., 198 (1994)
249
9
8»
8
O
o
®
7
80
k.
3
4-«
«k.
o
■
age 1
□
age 2
♦
age 3
o
age 4+
88
82
6
Q.
E
CD
®
5
c
®
nm
7»
jQ
E 4
<
3
87
2
2
3
4
5
6
7
8
9
Average temperature (°C)
Figure 6. Average ambient tem perature of different age groups of cod in relation to the average North Sea bottom tem perature by
year in February 1977-1991.
Discussion
T he available d ata strongly suggest that th ere are
m a rk e d differences in the te m p e ra tu re s at which the
different age groups of cod live in the N orth Sea.
H ow ever, it is unlikely th a t th e re is a specific p reference
involved for low er or higher te m p eratu res, n o t at least in
the case o f the juveniles because of th e ir exposure to the
lowest te m p eratu res in w inter and to th e highest te m ­
peratu res in late sum m er and au tum n. T he o lder fish
seem to avoid areas w here te m p eratu res fluctuate
strongly an d to buffer them selves against these by stay­
ing in those areas w here th e annual fluctuation in te m ­
p e ra tu re is minimal. This suggestion is su p p o rted by the
average distribution pattern s o f th e d ifferent age groups
(Fig. 9). This, of course, n eed no t be a direct cause and
effect relationship, because th ere are o th e r p aram eters
re lated to te m p e ra tu re , such as d ep th and the avail­
ability of foo d, which are m ore likely to be responsible.
A lso, for fish aged 3 and o lder the distribution will be
affected by spatial differences in exploitation rate.
W h en com paring the situation in different regions, it is
difficult to detect general pattern s in th e relation b e ­
tw een distribution of cod and te m p e ra tu re . F o r
exam ple, Sinclair (1991) found tha t in su m m er on the
E a ste rn Scotian Shelf am b ient te m p e ra tu re decreased
with increasing age, just as we found in the N o rth Sea,
w hereas R ose et al. (1994) found older L a b ra d o r cod
occurring in relatively w arm er water. O bservations in
the fall in the Southern G ulf of St L aw rence showed
distribution to be in d e p en d en t of age (Swain, 1991), as
did R ose et al. (1994) for the N o rth east N ew foundland
Shelf.
In studies of cod distribution in relation to te m p e ra ­
tu re, conclusions are often draw n ab out the ‘p re fe r­
ences’ involved w ithout the annual and seasonal vari­
ations in possibilities for selecting the ‘preferred '
te m p e ra tu re being taken into account (Sinclair, 1991;
Swain, 1991). E ven if the density o f 1-group cod in
relation to te m p e ra tu re (Fig. 3) suggests th a t, in winter,
these fish prefer the 1-2°C te m p e ra tu re b an d , it must be
ta k en into account th a t in w arm winters their choice is
restricted to a m inim um of 6°C. M oreo v er, if 1-group
fish prefer to rem ain close to the continental coast
250
H. H eessen and N. Daan
I C E S m a r . Sei. S y m p ., 198 (1994)
100
100
1981 Q3
1981 Q1
75
75
% 50
%
50
25
25
1
0
1
2
3
4
5
6
7
8
9
5 6
7
8 9 10 1 1 1 2 13 14 15 16 17 18 19
100
100
1991 Q3
1991 Q1
75
75
%
% 50
50
25
25
1
0
1
2
3
4
5
6
7
8
5 6 7 8 9 1 1
0 1
9
1 1 1 1 1 1 1 1
2 3 4 5 6 7 8 9
1 00
1981 Q4
1981 Q2
75
%
5 0
25
R-n, jflaUjja
0
5
6
7
8
9
1011
5
12131415
6
7
8
9
1011
12131415
100
100
1991 Q4
1991 Q2
75
75
% 50
%
50
25
25
0
5
6
7
8
9
1 0 1 1 1 2 1 3 1 4 1 5
5
6
7
8
9
1 0 1 1 1 2 1 3 1 4 1 5
il bottom temperature
LJ age 4+
m
Figure 7. Quarterly fractions of the total population by age group of cod
observed in 1°C tem perature bands in 1981 and 1991.
age 3
C od an d climate in the N orth Sea
IC E S m a r . Sei. S y m p ., 198 (1994)
251
*
I D
"
■
/ '
£1^4
2- 12
®
fc.
■
■
A age O
11/ 4 m
3
81/3
4-*
a
km
®
Q.
E
«
■
age 1
□
age 2
♦
age 3
91/
8
9
O
c
91/3
81/2
«
O age 4+
A
E
<
■
81/1
▲
0
4
▲
8
12
16
Average temperature (°C)
Figure 8. Average ambient tem perature of different age groups of cod in relation to the average North Sea bottom tem perature by
quarter in 1981 and 1991.
irrespective of the type of w inter, the ap p a re n t effect will
be high densities in cold w aters, because these are
necessarily restricted to the coastal area at these lati­
tudes. B ecause the w arm er w aters encom pass a large
area, w here 1-group fish are not found, th e average
density in those is reduced. T h erefo re, differences in
am bient te m p eratu re betw een age groups o r years are
not necessarily explained by differences in p reference in
the true sense of the w ord; much m ore intricate analyses
are required if we are to arrive at any firm conclusions in
this respect.
T he approach p resented in this p a p e r m ay be useful
also because it provides a b e tte r basis fo r studying
annual grow th differences o r effects o f te m p eratu res on
digestion rates than a point estim ate for the average
te m p eratu re in the sea. T h e am bient te m p eratu res of the
individual age groups deviate quite unpredictably from
the average. If the w inter is cold, 1-group fish are
exposed to much low er average values than w hen the
w inter is relatively w arm . T h e physiological effect is thus
m uch stronger th a n indicated by the average.
W ithin the context o f “cod and climate chang e” , it
w ould seem unlikely that the distribution of cod in the
N o rth Sea w ould be very much affected by a gradual
increase in te m p e ra tu re , because the re is no direct
evidence tha t the cod actively avoid high te m p eratu res,
at least n ot in th e range observed so far. This contrasts
with the findings for several fish species, including cod,
on the northeast continental shelf o f th e U S A ; these
were show n to com pensate, at least partly, for the
interan n u al te m p e ra tu re changes by m igration (M o u n ­
tain and M uraw ski, 1992).
W ater te m p e ra tu re may of course have o th e r effects
on the p opula tion dynamics o f cod, particularly on
recruitm ent, an d in this context it is w orth noting that
the m ature cod population ( 4 + ) ap p ears to perceive
a very limited te m p e ra tu re signal during spawning
(Jan uary /F eb ruary ). Since the N o rth Sea is well mixed
at this tim e of th e year, th e surface and b o tto m te m p e ra ­
tures are n ot th a t different and th erefo re it is unlikely
th a t a severe w inter will directly affect egg m ortality to
any large extent. If the re is an effect, it is m o re likely to
be found in the larval an d early 0-group phase. H o w ­
ever, the rem ark ab le change in recruitm en t during the
1960s to 1980s (see D a a n et a l. , this volum e) w ould seem
very much stronger than could ever be explained by
252
I C E S m a r . Sei. S y m p ., 198(1 9 9 4 )
H. H eessen an d N . Daan
F2
FS ■ F 6 1 F 7 1 F 8 [ F 9 l G 0 l G 1 1 G 2
F 5 I F6 l F 7 i F8 | F 9 | G 0 | G 1 | G 2
52
49
•
1-9
•
10-24
•
•
60
47
25-99
48
2100
45
58-
58-
44
44
57-
57-
42
55-
55-
38
34
29
Age 2
Age 1
E 5 i E 6 i E7 i E 8 1 E 9 i FO i F1 i F 2
F3
E5 i E 6 i E7 i E8 i E9 i FO i F1 i F2 i F3 i F 4 1F5 i F6 i F7 i F8 i F9 i GO i G11 G 2
F 4i F5 i F 6 i F7 i F8 i F9 i G 0 i G 1 1G 2
52
52
48
47
47
59
45
45
58-
58-
44
44
43
57-
57-
42
40
55-
55-
38
Age 3
28
Age 4+
27
To—i— r r
Figure 9. Average distribution of age groups 1, 2, 3, and 4 + of cod for the period 1983-1987 (from Heessen, 1993).
I C E S m a r . Sei. S y m p .. 198 (1994)
any te m p e ra tu re signal, and to that extent th e effect of
climate change on the N orth Sea cod can be expected to
be su bordinate to o th e r m uch m o re im p o rtan t factors.
Acknowledgments
W e are grateful to D r H arry D ooley and G arry H opwood from the IC E S Secretariat for providing us with
pre-processed te m p eratu re d a ta from th e IC E S O ce a n o ­
graphic D a ta B an k and to various colleagues from
national laboratories aro und th e N orth Sea participating
in the Intern atio n al B otto m Traw l Survey for providing
us with the necessary data.
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253
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