This paper not to be cited without prior reference to the author
C.H. 1974/F:3 0
Demersal Fish (Northern)
Committee
International Council for the
Exploration of the Sea
On the effects of temperature on sole recruitment.
by
R. De Clerck
Fisheries Research Station, Ostend
INTRODUCTION
The situation of the sole fishery may be considered as
critical to such an extent that the importance of reliable
forecasts will be more and more prominent in the near future.
As those forecasts however are generally based on the
den~ity
and the composi tion of the parent stock cOl:lbined wi th the
fishing effort, they are neglecting the strengths of the incoming year-classes. Taking into account that first of all
about two years elapse between the spawning and the recruitment to the fishery and necondly that the production rate is
known to bc very irregular year by year, it becomes clear that
the forecasts only based on tho parent stock can be biased.
Thc crisis in the Nortll Sea population as well as the biologists'
dcmand for the conservation of the rathor prosperous condition
of other sole populations was roflected vize in the proposcd
catch quota (ICES, 1973). It is self-evident that tho strength
of the incoming recruits will influence considerably these
quotas baned on forecasts on the parent stock. For this reason
the study on the causes leading to a succes or a failure of a
new year-class has to be intensified.
The temperature was considered by De Veen (1965) as boing the
most important factor for the yoar-class strength, although other
unknown factors lliay also be of importance. His hypothesis was based
on sole data in the North Sea. The aim of this study is to test
the rclationship temperature-breeding on other sole grounds, i.e.
2.
the Irish Sea, thc Bristol Channel and the English Channel.
I1ATERIAL AND I iEI'iIODS
Due to the lack
o~
su~~icient
international duta the
Belßian age distribution of the sole catches in thc Irish
Sea, the Bristol Channcl and the English Channol for the
1971, 1972 and 1973 '\;Tere used. The mean nUr.lber per
years
100 hours fishing was calculated for each year-class on
the data compiled during tho period 1971-1973. Tlle numbers
per hours fishing may be used as a standard becausc no substantial changes occurred in the Belgian fishing effort in
thc pcriod undcr review.
The valuos thus obtained ,,,ere trunsI'erred into a logari thmic scale and were plotted against the corrosponding ycar. The
results are given in thc
~igures
l-J. The regression was cal-
culated as well as the confidence bolts .vith the probability
of
%.
95
From the distancc of each value to the regression line
the year-class strength was defined on the following sewiquantitative scale
Poor : position in the lowcr area of the confidence belts.
Hoderate : position bet,Yeen the con.fidence bclts.
Good : position in the upper area oI' the confidence belts.
Strong
position in the upper area of tho confidence belts,
but extrer.lely distant from the regression line.
The sur.face teupcratures used in this study are tuken
from the ICES oceanographic data lists and Annales Biologiques.
RESULTS
A previous study (De Clerck,
1973) has indicated the mar-
ked dif.fercnces betwoen tlle sole population in the Irish Sea
and in thc Bristol Channel. These di.fferoncos are mainly exprcssed in the growth rate. Tllo third solo stock, vize the
English Channel stock which will also bo considered in this
study, can also be treated as different because of its proper
growth parameters (L~=
3J,3 cm, K
= 0,33,
t
0
=
1,75 for thc
3.
males and ~= 38,3 cm, K
= 0,57,
=
to
-0.39 f'or the f'ema1es)
1. The year-c1ass strength in the Irish Soa, tho Bristo1
Channe1 and tho Eng1ish Channe1.
From the position of' the laoan number/100 h. f'. per yoarc1ass in tho figures 1-J the re1ativo yoar-c1ass strongth was
semi-quantitatively deterr:lined. These f'igures indicate great
variations in and botwoen thc aroas and considorab1e f'lnctuations from year to year.
During the poriod 1946-1969 tllG 1'0110,üng c1assif'ication
of' the year-class strenßths can be wade : l'or the Irish Sea :
7 poor, 10 moderate and 7 good ; f'or the ßristo1 Channel : S
poor, 12 l;loderate and 4 good ; f'ar the English Channe1 : 10
poor, 10 moderate
Thcre
SCCI.1S
J good and 1 strong.
to bo no re1ationsllip betwcon tho strength
of' the year-c1asses and tho areas, e.g. during 1957 and 1963
the strengths per area were very uuch divergent.
Anothcr phenoLlünon is tho fact that the quantitative dif'f'erenccs betwecn poor, moderate and good year-c1nsses in tho
Bristol Channel and Irish Sea are very limited. This is also
ref'lected in the hiGh correlntion (r
Channe1 and r
=
= 0.97
in the Bristo1
0.95 in the Irish Sea). In thc Eng1ish Channe1
the nurabers per yoar-c1ass sho,'1 grea t f'luctua tions (r = 0.76).
e
2. The relation ycar-c1ass strcnE;th-telaperaturo.
Thc strength of' tho year-c1asses was testod with the corresponding t empera tures bel'ore and durine the sp.HlTning of' each
year. This relationship is shown in figure 4. The corre1ation
was ca1cu1ated for tho months January, February, iiarch, April
and 11ay separarc1y and f'or the suu of' tho teuporaturos fror:a
January to
l'~ay
in order to obtain a global vie,.,.
These ca1cu1ations lead to the f'ollowing tnb1e
4.
Aren
Correlation
Highost cor.
during
Irish Sea
positive
Feb.
:Ha.
~0.43~
0.44
Lowest cor.
during
Cor. period
Jan.-Hay
Nay (0.14 )
0.42
f-o-----------. ------------ ------------- ------------ ------------Bristol Ch.
pos./neg.
-
-
Jan. ~ -0. 32 ~
Feb. -0.39
Apr. ( -0 . 14 )
0.04
1------------ ------------ ------------- ------------- ------------English Ch.
negative
As appcars
~rou
this table and
fro~
-0.27
figure 4 the influenco
of the teclperaturo before and during spawning is quito opposito
according to the area.
In the Irish Sea thore is astrang tendency that good yearclasses are related to highor
~enperaturos.
In this rospect the
nost striking figura is tho poor 1963 year-class born during the
severest winter of the poriod 1946-1969, with a February temporature
o~
1.4 0 C, whoreas
all the good year-classes are achie-
ved at high Janu~ry-Fobruary tomperatures (about 6° C).
The data of the Bristol Channel whore the yearly anomalies
are rather small show no correlation at all. It was already pointed out in the first part
_
this study that the recruitment
o~
in the Brü, tol Channel area has neurly the same level every year.
This must be due to the ve;:y stable mean ter.lperatures during the
months January to tiay (respectively 10.6 °C, 10.0
10,2 °C und 11,4
°C) uhereas.
oc,
9.8
oc,
the abso lu te minimum ,,,as 8.7 °C
and the absolute maxiuum 12.8 °C. No changes is temperature reflected by no changes in year-class strengths might in a certain
respect also bc considorod as an arguMent in favour
o~
De Veen's
hypothesis.
J
On tho otller hand the English Channol population follows
vory good Do Voen' s hypothcsis1'or tho North Sea stock. Al though
thc Fcbruary
te~peraturo
of 5.4 °C o.g. during 1963 'Jas not so
low as was tho cnso in thc Irish Soa (2,1
f
oc)
it resultod never-
theless in a strong English Channol year-class in contrast with
the poor 1963 rocruitment in the Irish Sea.
DISCUSSION
Considering the rosults of tho English Channel as well as
the De Veen's rusults in the North Sea, thore is a strong tendency
to believe that there is a relationship botweon the temperature
before
s~awning
and the strength of the corresponding year-class,
in the way that good und strong year-classes coincide ,vith low
water temperatures. The Irish Sea data givo also arguments to
this reL..tionship, but in a adVerse sence (good year-classos
coinciding w·ith high water temperatures) • Finally, the sole in
the Bristol Channel lives in a very regular water temperature
syster:l resul ting in a permanent breeding amount nearly lvi thout
ups and do,vns.
This study provided consoquently further arguments in favour
of the wuter temperuture hypothosis but in view of the fact that
some contrasts occur according to the area the relationship
during the larval phase with predators and (or) competitors
(also influonced by the wuter temperature) muy be seriously interfering.
•
BIBLIOGRAPHY
De Clerck, R.
(1973) Belgian sole and plaice fisheries in the
Irish Seu and the Bristol Channel. ICES, C.H. 1973/F : 32
De Veen, J.
(1968) On the strongth of yeur-classes in sole.
ICES, C.H. 1965 Near Northern Seas Committee No 62.
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••
62
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f.
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68
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70
YEAR - CLASSES
Figure 1 - The year-c1ass strengths in the Irish Sea sole stock during the period 1946-1969.
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3
+
(j)
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e
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62
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. .__.. __..
_ - <....
__~...._
64
65
615
67
68
69
70
YEAR - CLASSES --_-.:;::-
Figure 2 - The year-class strengthsin the Bristo1 Channel· sole stock during the period 1946-1969.
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60
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62
63
64
65
66
67
68
69
YEAR - CLASSES
70
~-:::-
Figure 3 - Thc year-c1ass strengtlls in thc English Channel sole stock cluring the periocl 1946-1969.
.1
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15
IR i S ~-l
r
10
=0.308
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ENGLISH
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f.l
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Sur,l OF
r,1AR
JAN
FEß
APR
JMJ - MAY
Figure 4 - Thc relationship bctwccn thc strcngth of thc ycar-c1asses and. the tcmpcraturc In thc Irish Sca,
TI ri stol Channcl 'lnd. Engli sh ChanneL
(x)
Significance 95
%
probability
5