De Clerck, R.

Redant, F.

Open Marine Archive

/This paper not to be cited without prior reference to the
International Council for the
,Exploration of the Sea
C.H o 1974/K : 23
Shellfish and Benthos Committee
The evolution of the shrimp stock along the Belgian coast in 1969 - 1973
by
•
R. De Clerck and F. Redant (*)
Fisheries Research Station, Ostend, Belgium
INTRODUCTION
During the period 1966 - 1970 the Belgian landing~ of commercial
shrimps were stabilized at the rather high level of about 1200 tons on the
average. In 1971 the landings decreased to 905 tons. 1972 was an even wOrse
.- --==-=
year for the shrimp fishery with landings of only 865 tons.
The contribution describes and explains the fluctuations of the
landings during the period July 1969 - December 19730 As a matter of fact
special attention is paid to the period 1971 - 19720 Besides, the immediate
and long-term interactions betwecn shrimp and cod stock are examined.
MATERIAL AND METHODS
The' study is based partiallyon statistical data supplicd by the
.. Fisheries Division and partiallyon results of experimental fishing on cod.
The statistical data include the monthly landings of cod and shrimps
from the Belgian coastal waters (otatistical rectangle G 1) and the monthly
fishing effort of the shrimp fleet. Figure 1 shows the landings of cod
during the period July 1969 - December 1973. The monthly yields (catch/hour
fishing) of the shrimp fishery during the period July 1968 - December 1973
are illustrated in figure 2.
The experimental part of the study consisted of monthly samplings on
five fixed stations in the Westdiep. The positions of the stations are
given in figure 30 These samplings were started in May 1970. Experimental
(*) Contribution to the Belgian Research and Development Programme on the
Physical and Biological Environment; Prime Minister Service, Scienti-
fic Policy Programmation.
••
----- I~
2.
fishing was always performed with an otter trawl. The mesh size of the net
was 18 mm. On each station the catch of a 15 minutes' haul was analysed.
Special attention has been paid to the densities of juvenile cod. The
densities of 1 year old cod (number/hour fishing) are mentionod in figure 40
RESULTS AND DISCUSSION
The immigration of the O-group cod of the 1969 year-class in the
Dutch coastal wators during winter 1969 - 1970 caused a docreaso of the
shrimp landings during summer and early autumn 1970 (Boddeke, 1971). These
young cods mainly feod on shrimps of 35 - 45 mrn (Boddeke, 1971 and Boddeke
and Daan, 1971)0 Thc economic consequences of such a predation will only
be perceived after a long term, namoly in the period that the shrimps of the
decimated length-class roach a commorcial size and thus become important
for the finhery (Boddeko, 1971).
Along tho Bolgian coast howover, tho situation during the winter
1969 - 1970 was ontirely differento The groatest concontration of the 1969
cod year-class belonging to the Southern Bight population, remained north
of Hoek van Holland (Boddeke, 1971)0 The immigration of these young cods
in the Belgian coastal waters was rather small. The yield of the Belgian
shrimp fleet during this season remained high and was even better than
during the preceding winter (i.e. winter 1968 - 1969) (figure 2)0 This
finding harmonizes perfectly with the Dutch experienceso Because of the
absence of a great number of the O-group cod, the undersized shrimps were
spared, resulting in fairly good ~lds during summer and early autumn 1970
(figure 2)0
In October 1970 the immigration of the I-group cod started (figure 4)0
The cod belonged to the Pas de Calais population as well as to the Southern
Bight population (De Clerck, 1973)0 The majority of them had a length of
between 35 and 45 cm (De Clerck, 1973). These fish feed on shrimp less than
the :~ottnger ones but the shrimps they feed on have a commercial size
(Boddeke, 1971). This competition between natural and fishing predation
has an immediate effect on the shrimp landings. From November 1970 the
yield of the shrimp fishery decreased sharply (figure 2). The mere fact
of such a decrease of the yield in winter is absolutely normal but in 1970
it was clearly mOre abrupt than in the preceding years (figure 2). In April
••
1971 the yield reuched an extreme mini~um vulue of only ItoO kg/hour fishing.
This was much less than during the previous years (figure 2)0
In the Dutch coastul areu an immigration of the O-group cod of thc
1970 year-class was observed in the same period (winter 1970 - 1971) (Boddeke,
1971). This year-class was less importunt than the preceding one (Anonymus,
1974 and Boddeke und Daun, 1971) but the simultaneous appeurunce of both
clusses (1969 and 1970 year-classcs) had a catastrophical effect on the
Dutch shrimp stock (Boddeke, 1971) 0 l\.guin the undersized shrimp population
along the Belgian coast was spared from an intensive thinning out by the
O-group cod. So landings "lere normal during summer and eurly autumn 19710
The yield of the shrimp fishery in October 1971 reached the same level as
in September 1970 (figure 2).
During the winter 1971 - 1972 only poor concentrations of the I-group
cod (year-class 1970) were observed ulong the Belgian coast (figure 4).
The landings of cod from thc coastal watcrs were much higher than during
the previous winter (figure 1), but consisted mainly of 2 years old cod from
the strong year-class 19690 These older fishes live in deepcr water at the
outer edge of the shrimp zoneo This was confirmcd by the absenoe of II-group
cod in thc experimental catches. Moreover, the contribution of shrimps to
the food of these cods is negligible (Daan, 1973)0 So thc stock of 2 years
old cod hud no immediate effect on the shrimp stock.
The yield of the shrimp fishery reached aguin a very low level during
spring 1972 (figure 2)0 However at thut moment no intensive predation on
the commercial shrimp stock by 1 year old cod was found. Pro!?~.p.!:r_.~]1is
phenomenon can be explained by a lo~~term effect of~in±ensive-predation
..-..-
during the winter 1970 - 1971.
-_·_;~-~~~e-~;·;~;,ü;-~ur'vesof shrimps (Tiewr" 1954 and Rcdant, 1972) it
can be deduced that thc commercial shrimp stock (~ 50 mm) during the pe-
period December 1971 - May 1972 mainly consisted of 1 year old females
born in winter 1970 - 1971, 2 years old females born in winter 1969 - 1970
\ rod spring 1970 and 2 years old males or secundary females (Redant, 1972) l
~
born in winter 1969 - 1970. Taking into account their little change to
survive, thc contribution of 3 years old shrimps is probably very smallo
Shrimps born in winter 1970 - 1971 originatcd from thc reproduction period
of autumn 1970 (Havinga, 1930 ; Mcyer, 1935 and Tiews, 1954). The fraction
•of the shrimp population which had to guarantee this reproduction was how-
over largely decimated by the intensive predation by the I-group coda
Consequently the recruitment was very~all. So the contribution of females
of this poor recruitment to the commercial shrimp stock in winter 1971 -
1972 and spring 1972 was reduced to a large extento This also explains
the low yield of the shrimp fishery during this periodo
From June 1972 the yield of the shrimp fleet reached about its former
level (figure 2)0 The 1971 cod year-class was very poor (Anonymus, 1974) so
that during winter 1972 - 1973 only small densities of the I-group cod were
observed (figure 4). From that moment the situation of the shrimp stock
could be considered as normalized.
The record landings of 1615 tons in 1973, which was the highest since
1956, show sufficicntly that the shrimp stock recovered completely from the
crisis period 1971 - 19720 In 1973 large densities of shrimps were noted
along the Belgian coast (De Clerck and Redant, 1974 ; De Clerck, Cloet and
Redant, 1973 and 1974)0 This is probably due to the strong succesive brood
classes, the absence of juvenile cod and favourable hydrological and climato-
logical conditionso
CONCLUSIONS
In contrast with the Dutch shrimp stock, the population along the
• Belgian coast has, neither during vlinter 1969 - 1970 nor during winter
1970 _ 1971, suffored from an intensive predation by 0 year old cods of the
1969 and 1970 year-classeso
The abundance of the 1969 cod year-class during winter 1970 - 1971
had an immediate effect on the shrimp landings during this season and a
long-term negative influence on the catches during the winter 1971 - 1972
und spring 1972.
The 1970 and 1971 cod year-classes were only observed in small densi-
ties during the winter 1971 - 1972 and 1972 - 1973. Their influence on the
shrimp stock was limitedo
Since the second half of 1972 the shrimp stock seemes to have re-
covered completely from its crisis period.
BIBLIOGRAPHY
Anonymus (1974) : Report of the North Sea Roundfish W)rking Group.
ICES, 1974, C.M~ F : 5, 37 pp.
Boddeke, Ro (1971) The influencc of the strang 1969 and 1970 yearclasses
of cod on the stock of brown shrimp along the Netherlands coast in
1970 and 1971.
ICES, 1971, C.M. K : 32 , 11 pp.
Boddeke, R. and Daan, N~ (1971) : Waar zijn de garnalen geblevcn ?
Visserij, XXIV, pp. 323-335.
~ Daan, N. (1973) : A quantitative analysis of thc food intake of North Sea
cod, Gadus morhua.
Netherlands Journal of Sea Research, VI, 4, pp. 479-517.
De Clerck, R. (1973) : Tagging experiments on cod off the Belgian Coast.
ICES, 1973, C.M. F : 33, 11 pp.
De Clerck, R., Cloet, N. and Redant, F. (1973) Bestandsopname langsheen
de Belgische kust (7)0
Ed. Ministry of Agriculture, Commission ToW.O.Z., Publ. nr. 81-B/10/
1973, 41 pp.
De CLrck, R., Cloet, N. and Redant, F. (1974) Bestandsopname langsheen
de Belgische kust (8).
Edo Ministry of Agriculture, Commission T.W.O.Z., Publ. nr. 87-B/12/
~ 1974, 45 pp.
De Clerck, R. and Redant, F. (1974) : Belgian shrimp research in 1973.
Annales Biologiques 1973, XXX (in press).
Havinga, B. (1930) : Der Granat (CranRon vulgaris Fabr.) in den holländischen
Gewässern.
Journal du Conseil, V, 1, pp. 57-87.
Meyer, P.F. (1935) : Ein Beitrag zur Frahe der Laichperiodizitüt bei der
Nordseekrabbe (Granat) Crangon vulgaris Fabr.
Zool. Anz., 109, 1/2, pp. 23-32.
Redant, F. (1972) : Groei en geslachtsverandering bij Crangon crangon (Lo)
voor de Belgische kust.
Licentiaatsvcrhandeling, Vrije Universiteit Brussel, 112 pp.
Tiews, K. (1954) : Die biologischen Grundlagen der Büsumer Garnelenfischerei.
Ber. Dtsch. Wisse Komm. Meeresforsch., XIII, 3, pp. 235-2690
,>, "
,
Months
N J M 'M J S, N
1973
N J M M J S
1972
J M M J S
1971
"
Fig. 1. Monthly landings of cod from the Belgian ,coastal waters.
Period July 1969 - December 1973.
• Kg/l,l0ur'fiSbing r ,
50
40
30
20
10
o
Months
J S N J MMJ S N J M M·J S N J M MJ S:N J M MJ S N J·M MJ S'N
1968 1969 1970 1971 19721973
Fig.
l,
2. Monthly yields of the Belgian shrimp fishery.
Per~od'Ju1y1968 Deoember 1973· i
, t
Um
--
51' 07'30·
Positions of the Westdiep
sampling stations (in circ1es).
Depths in decimetres.
Fig. 3."
r. '
. '
Number/hour fishing
200
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·.
·.
·..
"..:.. , .
..
.. I, • L
·.
,e1.50 .. .... ...... .. ....
..
....
..
.. .. .. ..
.. ..
..
.. .. ..
.. .. ..
..
.. .. ..
.. .... ..
..
.. .... ..
100 .. .. .. .... .. ..
.. .. ..
.. .. .. ..
.. .. ..
.. ..
.. .. .. ..
.. .. ..
.. ..
.. .. ..
..
.. .. ..
.. ..
..
.... .. ....
..
.... .. ..
.... .. .. ..
..
50 ..
.... .. .. ..
.. .. .. .. ..
..
..
.. .. .. .. ..
..
.... .. .. ..
..
.... .. .. ..
..
.. .. .. .. .. cn cn
..
.. .. .. .. ..
.: .:.. .. .. .. ....
.. .. .. .. .. Ci... .... .. .. .. E.. .. .. .. .. .. .... ...
.. .. .. ..
.... .. ....
.. .. .. .. ....
..
.. .. .. .. ....
.. C>
0 .. .. .. ...... .. c:
M J S N J W M J S N J M M J S N J M M J S N
1970 197,1 1972 1973
.! .
Fig. 4. Densities (number/hour fishing)'of jüveni1ecod on the
fishing ground West~iep. .
Period Hay 1970 - Decernber 1973.
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The evolution of the shrimp stock along the Belgian coast in 1969-1973

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