I
This paper not to be cited without prior reference to the authors.
INTERNATIONAL COUNCIL FOR TUE
EXPLORATION OF TUE SEA
C.M. 1978/H: 8
Pelagic Fish Committee.
•
DIFFERENT GROWTU PATTERNS IN MACKEREL WEST OF TUE BRITISU ISLES.
by
A. Corten & G. van de Kamp
Netherlands Institute for Fishery Investigations,
P.O. Box 68, 1970 AB Ymuiden, The Netherlands.
i
•
Thispaper not to be ci ted without prior reference to the authors.
INTERNATIONAL' COUNCIL FOR THE
C.M.1978/H : 8
EXPLORATION OF THE SEA
Pelagic Fish
Committee
DIFFERENT GROWTH PATTERNS IN MACKEREL WEST OF THE BRITISH ISLES.
================================================================
by
A. Corten & G. van de Kamp
Netherlands Institute 'for
Fishery Investigations,
P.O. Box 68~ 1970 AB Ymuiden,
The Netherlands •
•
Introduction
Kästner (1977) has described two groups of macker el with a different growth pattern in the waters westof Scotland. This was an
indication that the so-called "\vestern Stock" of mackerel (ICES
1977) may not be ahomogeneouspopulation, but a combination of
several subpopulations with different spawning and feeding areas.
A difference in growth pattern has been evident in mackerel sampled
from the Dutch fishery in the Celtic Sea during different seasons.
Mackerel caught during autumn and winter are general~y larger for
their age than mackerel caught during the following spring and summer. These difference~ are most conspicious in the weights per age
group, and the Netherlands have in recent years always presented
separate weight/age dati for each quarter of the year to the ICES
Mackerel Working Group.
Because the weight of the fish is not only determined by its annual
growth, but also by the condition factor at the moment of capture,
weight/age data are not the most suitable material to study differences in growth pattern. For this purpose, it is better to examine
length/age data. In the present report, this type of data are presented for the Celtic Sea, and a comparison is made with' the mackerel in other waters around Britain.
•
Material and Methods.
\
SampIes of mackerel were obtained from the Dutch fishery in the
various seas around Britain (figure 1). In moat areas, mackerel is
(or used to be) ,a by-catch in the fishery for herring (Hebrides,
Celtic Sea) or for roundfish (North Sea). The length composition of
the macker el landed from the North Sea was dependant on the market
price of small mackerel and the mesh size used in the cod end. In
,the herring fishery, many of the small mackerel used to be discarded and only the medium and large sized mackerel were landed normally. As these fishing and sorting practices may have caused a
bias in the mean length per age of the younger age groups, only the
fish of 4 years and older were considered for this report.
r
,- 2 In"the D~tch market sampling system, a large number of length measurements is taken in the fish market, and from,these length data,
,
the age composition of the catch is calculatedby means of a length/
age key. This is done for each area and quarter of the year separately.
,The mean length for age was derived from the total length/age composition of the catch, calculated as described above. Since the mean
length for age was derivedfrom transformed data, the variance of
the mean could not be calculated in the usual way, but only by a
complicated procedure. As the differences in mean,length between
various ,seasons were obvious, and followed a consistent pattern
from year to year, it was considered unnecessary to test the differences statistically.
.
Because the growth pattern may differ between year-classes of the
same sub-population, all data were grouped by year-class.
•
Results.
Figure 2 demonstrates the seasonal changes in mean length for age
for a number of year-classes in the Celtic Sea. The same pattern
emerges for all year-classes at all ages sampled, vize a high mean
length in the last quarter of the year and sometimes also in the
first quarter of the next year, followed by a sharp drop in the
2nd and '3rd quarter.
Figure 3 shows length frequency distributions of a particular yearclass during the various seasons in a number of successive years.
This figure is given in order to indicate,the spread of values around
the mean, in the absence of a calculated variance. Year-class 1968
was taken for this purpose, as this was the most num~rous year-class ,
in the Dutch material from the Celtic Sea. It is seen tha± the largest
fish in the sampIes from the winter fishery areabsent during spring',
and summer. On the other hand, so me of the smallest size categories
found in the 2nd, and sometimes already in the 1st quarter, were
not present in the catches made during the last qUdrter of the preceeding year.
•
The growth pattern of mackerel in the Celtic Sea is compared with
that in other areas in figure 4 and 5 and in table I. Again yearclass 1968 has been used because of the high number of observations
on this year-class. An increase in mean length during the 4th quar-'
ter, similar to the one in the Celtic Sea can be detected in the
material from west Scotland and the southern North Sea. The data
for the central and northeastern North Sea show a more regular pattern, without a sudden increase during 'the winter.
The winter values for the Celtic Sea are comparable to those from
, west Scotland, and also to the ones from the southern and central
North Sea (figure 4). The slow growing macker el found in' the Celtic Sea during spring and sum~er can only be compared to the southern North Sea mackerel in summer and,autumn, and to the west
Scottish mackereI' in autumn.
Discussion
The different growth patterns found in macker el of sub-area VII
and division VIa are a strong indication that the macker el of the
so-called "Western Stock" do not consti tute a horrogeneous uni t stock.
- 3 Dutch length/age data confirm the observation of Kästner (1977)
that a group of slow growing mackerel is found west of Scotland
during the 3rd quarter of the year. The differences in growth pattern, however, are even more pronounced in 'the Celtic Sea and the
area off Cornwall, and they also occur in the southern North Sea.
Bolster and Burd (1972) .have already attempted to split the mackerel of the western English Channel into two components; one that
, spawns in the Celtic Sea before June and also overwinters in this
area, and o'ne that spawns' in the southern North Sea in June-July
and overwinters off the coast of Cornwall.
•
The theory of Bolster and Burd was mainly based on differences in
age composition and maturity stage, and they do not nention a difference in growth pattern. In some aspects the theory is difficult
to reconcile with the age 'length data presented in this report.
We have found no great differences in growth between mackerel overwintering off Cornwall, and those overwintering off the Irish south
coast. If there are two distinct sub-populations present in the
Celtic Sea in winter, one of them must be out of reach for the
Dutch fishery in that area.
Secondly, the mackerel caught during summer in the southern North
Sea are of the slow growing type, whereas the overwintering mackerel in the Celtic Sea are fast growers •.
On the basis of the age/length data presented in this report we
would like to formulate a new hypothesis concerning the stock composition and migration habits of the western mackereI. We assume
that there is a northern, fast growing component of western mackerel
and a southern, slow growing one (figure 6)~.
The northern component does not feed and spawn in the Celtic Sea,
but in areas further north such as the Porcupine Bank, off northwest Ireland, and near Rona (Walsh 1976). Part of it mayaIso penetrate into the North Sea via the English Channel. Only during wintertime it migrates south and is caught in the Celtic Sea.
•
The mackerel that actually spawn and feed in the Celtic Sea are
the more southern, slow growing component of the western mackereI.
Part of this stock would overwinter in areas even further south
than the Celtic Sea, such as the Bay of Biscay. By the end of the
winter'when the fast growing mackerel are leaving the Celtic Sea,
the slow growingmackerel move into this area to spawn and feed.
Subsequently also apart of this slow growing group migrates north
towards west Scotland, and another group migrates east through the
Eng~ish Channel into the North Sea.
The slow growing mackerel arrive in the west Scottish waters much
later than the fast growing ones (Kästner 1977), and they also leave
the area before the fast growing mackereI. Our length/age data show
the presence of slow growing macker el in division" VIa only during
the 3rd quarter of the year.
Part of the slow growing mackerel also enter theNorth Sea in summer
and retreat again towards the Celtic Sea in autumn. The increased
"mean length of mackerel in the 4th quarter in the southern North Sea
may indicate a small proportion of fast growing western mackerel
which has also penetrated into the North Sea before the slow growing
Iones, and which is retreating only after the slow growing mackerel
have left the area.
- 4 Gf course this hypothesis does not assume a very sharp division
between the slow growing component and the fast growing one. Tagging experiments have demonstrated a considerable mixing of mackerel from all areas, even between the North Sea and the western stock
(Hamre, 1975). Still, the changes in growth type of mackerel in the
Celtic Sea follow a consistent pattern, and point to a distinct heterogenity of the western mackerel stock.
Sucha heterogenity of the western mackerel should'be taken into
consideration during the assessment of the stock. If our hypothesis about the migration of the fast growing macker el is correct,
this macker el will generally spawn outside the Celtic Sea. Any
stock assessment based on egg surveys in the Celtic Sea and the
Bay of Biscay will therefore not include the fast growing component
of the stock.
It will be very useful to test this hypothesis by me ans of a tagging experiment on mackerel caught during winter in the Celtic Sea •
•
.e
TABLE I - Mean length of year-class 1968 per area and quarter for the period 1972-1977.
Corre~ponding
numbers cf age readings are given as an indication of sampling effort.
1973
1974
1975
1976
1977
5 year
6 year
7 year
8 year
9 year
1.972
4 year
~
L'
N
-
-
North eastern North Sea
IVa
.1st
2nd 35.02 30
3rd
4th 35.19 10
Central North Sea
IVb
1st
2nd 33.08
·3rd
4th
-
West Scotland
VI
1st
2nd
3rd
4th
Southern North Sea
IVc
1st
2nd 32.09
3rd
4th 32.73
South of Ireland
VII
-
1st
2nd
3rd
4th 32.48
-
29
-
-
63
-
41
-
32
L
N
L
N
L
-
-
-
-
-
-
-
-
-
-
-
36.08 .13
35.35 17
35.87 27
-
33.42 43
32.76 56
34.23 17
-
32.79
32.54
34.74
-
31.48
32.48
34.23
-
21
20
15
36.85
9 38.22
36.14 15 38.76
36.03 . 10 38.16
35.35
34.42
-
-.
9
17
-
35.92 13
34.17 52
36.10' 26
-
-
32.98 20
33.36 29
34.71 . 24
- 34.52
45 32.12
23
41 . 36.02
-
16
29
-
34
N.
35.82
19
-
59
33.45
34.92 54
-
34.18
34.25
35.17
N
L
- 37.10 4
6 38.64
17 37.43
4 36.10 . 3 38.48
7 40.09
9 37.47
- - -35.22 25
-
L
-
-
35.71
35.24
35.25
38.67
-
-
9
-
9' 36.70
39 36.14
18 36.39
-
34.10 - 15
36.43 22
-
12 34.18
24 35.39
19 36.70
33.41 35
32.44 33
33.25 27
6
34.39
-
-
-
34.82
-
-
23 37.44
13 37.18
11 37.35
35.34 40
33.80 34
1
33.34
37.19 30
34.58
34.41
33.25
. 36.51
N
-
9
11
6
-
11
18
20
-
-
12
-
23'
16
12
38
32
7
14
.\11
- 6 References
Bolster, O.C. &
Burd, A.C., 1972
•
The mackerel of the western English Channel.
lCES, Doc. C.M. 1972/J : 7 (mimeo).
Hamre, J., 1975
The effect of recent changes in the North
Sea mackerel fishery on stock and yield.
lCES, Symposium on the changes"in the North
Sea fish stocks and their causes, No. 22
(mimeo).
lCES, 1977
Provisional information and data for allocation of resources under the new extended
national fisheries jurisdiction regime.
Prepared by the lCES Mackerel Working Group.
lCES, Doc. C.M. 1977/H : 2 - appendix (mimeo).
Kästner, D., 1977
Preliminary results of the occurrence of two
mackerel groups (Scomber scombrus L.) with
different growth pattern westof Britain •
lCES, Doc. C.M. 1977/H : 38 (mimeo).
Walsh, M., 1976
Mackerel spawning to the west of the British
lsles.
lCES, Doc. C.M. 1976/H : 33 (mimeo) •
•
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FIGURE 1.
Positions of oge-samples from Dutchmackerel catches in the period 1972 -1977
+
54
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..... ..........
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In cm
•
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1968
..I..-
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•
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•
30-: ~.~ ~ 304 t: r t 3-4 t-: r ~ f 1:. ~Il ~ 3 c: f' 2~ :r r. ~. 2 ~PJ t: r :r 3
N
I 1970, ~
1971
: 1972 : 1973
M
N
(:
~ot ~
:r
tlQuarter
I . 1974
: 1975 : 1976 : 1977 :VEAR
. Mean lengtlis of yearclasses 1966 -1971
South of Ireland per quarter
FIGURE 2.
for the years 1970 -1977.
lonly means of at least 50bservationsl
IL__ '
_ ............_~
.....-.....:r=::l_....
4 QUARTER
1972 _
no data
~,--'- - - - - 2 - - 1973
____
~'--
3_:_-4
2
~------197
no data
4
3
4
__r=edJ--n
~
_
2
___d1L'--=----3
no data
•
--1975
4
---~---...==-------.......;cCb---n'--'--=t:-"J--'----'-----2---1976
no data
3
4
----~.....- - - j , - -
---dlD-o:-,-----2-- 1977
no data
3
4
, 28 ' 29' 30' 31 ' 32 ' 33' 34 ' 35' 36' 37' 38' 39' 40' 41 ' 42'
Length in cm-below
FIGURE 3
Percent0ge length distributions of yearclass 1968 in area VII ISouth of Irelandl,
l
LENGTH
in CM.
•
•
38
I
I
35
•
•
•
I
•
•
0
•
+
32
V
V
•
V
rd
3
4th
~
nd
2
V
•
0
V
•
'!l
V
•
I
I
I
I
I
I
I·
I
I
st
1
nd
2
td
3
1974
4th
:
I.
st
1
I
I
I
nd
0
(only means of at least 10 observations)
I
,
I
I
I
I
I
I
,
I
I
1975
4
Mean length of yearclass 1968 per area and quarter for the period
I
I
I
td
1972 -1 9 77.
4 th
I
I
I
3
,
I
0
I
2
I
I
I
• •
I
I
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I
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'1
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,
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I
I
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4th
I
I
,
+
I
. jd
I
I
I
I
•
I
I
I
I
•
V·
~
,
V
I
I
I.
1973
1972
FIGURE
st
1
V
V
0
+
+
V
•
-j>d
~
0
0
+
•
V
+
0
+
V
01
34
33
V
•V
37
36
I
I
st
1
nd
2
td
3
1976
• Na
+ Nb
D~
V
Ne
-w
4th
I
I
I
I
I
st
1
ttd
1d
1977
North Eastern North Sea
Central North Sea
West ScoHand
Southern North Sea
South of Ireland
th
4 OUARTER
YEAR
I
LENGTH
lENGTH
in CM
•
38
In
•
CM
38
V
37
37
•
36
35
V
V
V
•
•
•
••
•
35
V
V
3'
3'
JJ
33
V
V
35
V
V
V
V
V
North Eastern North Sea
32
i"
)"
1972
," ,"
2~
l'
1973
," ,"
i"
)"
1974
," ,"
2~
l'
1975
,"
I"
1"
l'
,no
~
I'.
1976
i"
J2
l'
1977
l'
," ,"
2~
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,Ir'!
1'1
1973
t"
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1974
i ,"
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,'h:
1"
1975
1"
)"
1976
L''': ,"
,
1"
j'
,"C....t.R'!ER
'H.t.R
'977
lENGTH
LENGTH
In
Southern North Sea
1972
YEAR
V
V
1"
""Oi.JARTER
V
V
eH
InCH
38
38
..
37
..
36
35
..
..
·
.. ·
·
.:
37
.. .
+ :
•
35
3'
3'
33
33
• •
i.
Central North Sea
32
i"
)"
1972
," ,"
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l'
1973
,"
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t"
j'
' 1974
,"
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t"
," i,"
j'
1975
i"
j'
1976
,lIl.:
,
·
lENGTH
In CM
111
•
1"
j'
1977
""OUARTER
YEAR
1"
)"
1972
,'. ,"
•
•
•
2~
j'
'973
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11
1"
,
j'
1974
,'" i ,'I'
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36
South of Ireland
j'
1975
L'"
i
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1"
i'
1976
.
,''':
,li
i"
l'
1977
3B
37
36
,,
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3'
0
0
0
JJ
," ,"
2~
l'
1973
··
0
:
West
i'
0
," ,"
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i'
1974
,"
I"
2~
l'
1975
Scotland
"'1'"
1"
3"
1976
,I.. : ,'I
·
i"
l'
1977
5.
Mean length of yearclass 1968 per quarter for the
period 1972 -1977 in different areas.
,
32
i"
1
c:·
·!
0
1972
FI GU RE
0
··
0
0
""OlJARTER
f[AR
,t'Gt,':"'G:TER
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,
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" il l l l l l~I I I I I·
NOVEMBER- FEBRUARY
FIGURE
1
MARCH-JUNE
11111
JULY -OcrOBER
6,
Hypothet ical distribution of mackerel subpopulations in varlous seasons,'
1111111111111
Western mackerel(slow growin 9 type}.
-,
,
Western mackerel (f~st growing type).
North Sea_ mackerel .