F l 6 d e v i g e n r a p p o r t s e r . , 1, 1 9 8 4 . ISSN 0333-2594 T h e P r o p a g a t i o n o f Cod Gadus morhua L .
0-GROUP COD INVESTIGATIONS ON THE NORWEGIAN SKAGERRAK COAST
S . T v e i t e
F l 6 d e v i g e n B i o l o g i c a l S t a t i o n
N-4800 ARENDAL Norway
ABSTRACT
T v e i t e , S . , 1 9 8 4 . 0 - g r o u p c o d i n v e s t i g a t i o n s o n t h e N o r w e g i a n S k a g e r r a k c o a s t . I n : E. D a h l , D.S. D a n i e l s s e n , E. Moksness a n d P. S o l e m d a l ( E d i t o r s ) , T h e P r o p a g a t i o n o f Cod G a d u s morhua L . F l @ d e v i g e n r a p p o r t s e r . , 1 , 1 9 8 4 : 5 8 1 - 5 9 0 .
S i n c e 1 9 1 7 0 - g r o u p c o d h a v e b e e n s a m p l e d b y b e a c h s e i n e a l o n g t h e N o r w e g i a n S k a g e r r a k c o a s t . T h e a v e r a g e number o f O- g r o u p p e r h a u l i s f o u n d t o g i v e a r e l a t i v e l y r e l i a b l e e s t i m a t e o f y e a r c l a s s s t r e n q t h .
T h e r e was a p e r i o d o f r e l a t i v e l y l o w a b u n d a n c e i n a l l a r e a s f r o m 1 9 3 0 t o 1 9 5 0 . The b e s t p e r i o d f o r t h e o p e n a r e a s o n t h e s o u t h c o a s t was f r o m 1 9 5 0 t o 1 9 7 0 , w h i l e i n more l a n d l o c k e d a r e a s t h e a b u n d a n c e was h i g h e s t b e f o r e 1 9 3 0 a n d a f t e r 1 9 7 0 . I n t h e L a n g e s u n d a r e a a n d i n n e r p a r t s o f t h e O s l o f j o r d t h e r e h a s b e e n a g e n e r a l d e c l i n e s i n c e 1 9 5 0 , a t l e a s t p a r t l y c a u s e d b y e n v i r o n m e n t a l c h a n g e s d u e t o pollution.
INTRODUCTION
From 1 9 0 3 t h r o u g h 1 9 0 5 G . M . D a n n e v i g ( 1 9 0 6 ) a n d K . D a h l ( 1 9 0 6 ) u s e d b e a c h s e l n e s f o r e s t l m a t l n g t h e y e a r c l a s s s t r e n g t h o f c o d . T h e l n v e s t l g a t l o n s w e r e p e r f o r m e d I n o r d e r t o p r o v e o r d i s p r o v e a n y b e n e f i c ~ a l e f f e c t o f liberated y o l k s a c c o d l a r v a e . T h e y r e a c h e d different conclusions b a s e d o n t h e s a m e material.
A l f D a n n e v l g ( 1 9 2 1 ) was o f t h e o p l n l o n t h a t l o n g e r s e r l e s o f
d a t a w e r e n e c e s s a r y b e f o r e a f i r m c o n c l u s i o n c o u l d b e d r a w n . A new s e r i e s o f b e a c h s e i n e h a u l s a t d i f f e r e n t l o c a t i o n s a l o n g t h e S k a g e r r a k c o a s t was t h e r e f o r e s t a r t e d i n 1 9 1 7 , p e r f o r m e d a n n u a l l y a n d i s s t i l l c o n t i n u i n g .
NAUTICAL MILES
1 10
SKAGERAK
F i g . 1. D i s t r i c t s i n v e s t i g a t e d l ) T o r v e f j o r d , 2 ) T o p d a l s f j o r d , 3 ) S t e i n d a l s f j o r d , 4 ) V e s t r e V a l l e s v a r , 5 ) B u f j o r d , 6 ) F l f l d e - v i g e n , 7 ) S a n d n e s f j o r d , 8 ) S f l n d e l e d f j o r d , 9 ) R i s f l r S k e r r i e s , 1 0 ) S t o l e f j o r d , 11) K i l s f j o r d , 1 2 ) S o p p e k i l e n , 1 3 ) L a n g e s u n d , 1 4 ) Vrengen-Tjflme, 1 5 ) H v a l e r , 1 6 ) H o l m e s t r a n d , 1 7 ) Drflbak 1 8 ) N s r s n e s - B y g d f l y .
G. Dannevig (1963) used parts of the material for the origi- nal purpose. He showed that in the Oslofjord area there seemed to be a correlation between liberation of cod larvae and the number of 0-group cod. However, when using all the data avail- able, artificial propagation could not be significantly sepa- rated from other sources of variation (Tveite, 1971). It was also shown that number of 0-group cod in the seine hauls gives a relatively good measure of subsequent yearclass strength (Tveite, 1.c.).
In this paper the more longterm trends will be considered.
MATERIALS AND METHODS
The seine hauls are performed during September-October. The total area covered is split into 18 districts as shown in Fig.
1. In each district there are two to nine localities for operating the seine.
In districts 1 to 12 the sampling has been carried out since 1917. In district 13 (the Langesund area) the investigations started in 1953 and in the remaining districts (Oslofjord) in 1936. The results from 1917-1919 are omitted since many of the hauls had not yet found their permanent position. During the war 1940-1944 only the hauls in Fl@devigen were worked. Only localities sampled over the entire period are considered here.
The hauls are taken at the exact same localities and cover the same bottom area each year. During the 66 years, only two persons have been in charge of the investigations on the
cruises. The hauls are taken during daytime. The greatest depth of the different hauls varies from 3 to 15 m. There is hardly any tide in the area. Up to 1961, seines made of cotton were used. They were 38 m long and 3.8 m deep, the mesh size was 14 mm stretched mesh. From 1962 nylon seines of the same size and construction have been used. No comparative experiments with
t h e two t y p e s h a v e b e e n p e r f o r m e d , b u t no c h a n g e i n l e n g t h d i s - t r i b u t i o n of t h e f i s h was d i s c o v e r e d . The s e i n e s were t h e r e f o r e c o n s i d e r e d t o h a v e t h e same s e l e c t i v i t y .
I n e a c h h a u l t h e number of e a c h f i s h s p e c i e s i s c o u n t e d . The c o m m e r c i a l s p e c i e s c o d , p o l l a c k ( P o Z Z a c h i u s poZZachius ( L . ) ) , w h i t i n g ( M e r Z a n g i u s merzangus ( L . ) ) and c o a l f i s h ( P o Z Z a c h i u s v i r e n s (L.)) a r e measured f o r l e n g t h . I n t h i s p a p e r o n l y
r e s u l t s on cod a r e p r e s e n t e d . S i n c e 0 - g r o u p cod m i g h t s o m e t i m e s b e d i f f i c u l t t o s e p a r a t e from t h e I - g r o u p c o d , some o t o l i t h s h a v e o c c a s i o n a l l y b e e n s a m p l e d .
F i g . 2 . C a t c h e s of 1922-1937 y e a r c l a s s e s a s 0 - g r o u p i n f o u r b e a c h s e i n e h a u l s compared w i t h c a t c h e s of I - I V g r o u p i n t r a p s i n T o p d a l s f j o r d ( r = 0 . 7 8 ) .
Fig. 3. Comparison of two "rich" hauls (1) and two "poor" hauls ( 2 ) in Steindalsfjord. A average for 1 , B average for 2.
RESULTS AND DISCUSSION
Dahl (1906) and Dannevlg (1906) slmply assumed that the strength of a yearclass was determined at the 0-group stage and that the number of cod per haul was a rellable measure of the abundance of the 0-group. Dannevlg (1963) showed the good relatlonshlp between the 0-group and the I-group cod caught In the selne the following year. Tvelte (1971) showed a statlstlc- ally slgnlflcant correlatlon between 0-group per seine haul and the later catches of the yearclass In traps as I-IV group cod.
An ex2rnple of the correlatlon 1s here shown In Flg. 2. The baslc assumption LS therefore supported by observatlons.
0 J I l I I I
1920 1930 1940 1945 l950 1960 1970 1980
FIRST YEAR OF 5 YEARS MEAN
Fig. 4. Five year moving average of percent of average cod per haul for 1920-1982. 1: three landlocked fjords: Topdalsfjord (2) Sondeledfjord (8) and Kilsfjord (11). 2: four more open:
Torvefjord (l), Bufjord (5), Flodevigen ( 6 ) and Risor Skerries ( 9 ) .
The average number of cod per naul differs between locali- ties. For example, of four localities in Steindalsfjord (dis- trict 3) the same two usually give higher numbers of 0-group cod than the two others (Fig. 3). The fluctuations follow the same pattern (r = 0.55), but the absolute differences are big.
This shows that new localities should not be used in calcula- tions without compensating for the average level of cod per seine haul for that locality.
The nearly tideless sea in this area might make it more suitable for the beach seine method than areas with big tidal differences. Most of the hauls are more than 5 m deep. This reduces possible effects of variations in the distribution of young cod due to differences in the light conditions.
The long term trends are shown in Fig. 4 , 5 and 6. In Fig.
m I -
1936 1940 1945 1 9 9 1960 1970 1980
F I R S T YEAR OF 5 YEARS M E A N
F i g . 5 . F i v e y e a r moving a v e r a g e o f p e r c e n t o f a v e r a g e c o d p e r h a u l f o r 1 9 3 6 - 1 9 8 2 . 1: O u t e r a r e a s o f O s l o f j o r d : Vrengen-Tj@me ( 1 4 ) a n d H v a l e r ( 1 5 ) . 2 : I n n e r a r e a s : H o l m e s t r a n d ( 1 6 ) , D r @ b a k ( 1 7 ) a n d N z r s n e s - B y g d e y ( 1 8 ) .
4 t h e more l a n d l o c k e d f i o r d s s o u t h o f t h e L a n g e s u n d a r e a a r e c o m p a r e d w i t h t h e more o p e n o n e s . T h e r a p i d d e c r e a s e d u r i n g t h e 1 9 2 0 ' s i n t h e l a n d l o c k e d a r e a s h a s b e e n c o r r e l a t e d w i t h t h e d i s a p p e a r a n c e o f Z o s t r a m a r i n a L . H o w e v e r , t h e f a l l i n g t r e n d s t a r t e d l o n g b e f o r e t h e e x t i n c t i o n o f t h e e e l g r a s s , w h i c h was m a i n l y o b s e r v e d i n 1 9 3 3 ( ~ a n n e v i g , 1 9 5 4 ) . From t a g g i n g e x p e r i - m e n t s a n d a g e - c o m p o s i t i o n o f t r a p - c a u g h t c o d , i t m i g h t b e c o n - c l u d e d t h a t t h e e x p l o i t a t i o n r a t e w a s q u i t e h i g h b e f o r e 1 9 4 0 . H o w e v e r , t h e r e i s no s i g n i f i c a n t e x p l a n a t i o n f o r t h e s m a l l e r y e a r c l a s s e s f r o m 1 9 3 0 o n w a r d s ( D a n n e v i g , 1 9 5 4 ) . A f t e r 1 9 4 4 t h e r e a r e n o d a t a a v a i l a b l e t o show a n y c h a n g e s i n e x p l o i t a t i o n r a t e .
I I l
1950 1960 1970 1980
FIRST YEAR OF 5 YEARS MEAN
F i g . 6 . F i v e y e a r m o v i n g a v e r a g e o f p e r c e n t o f a v e r a g e c o d p e r h a u l f o r 1 9 5 3 - 1 9 8 2 . 1: The L a n g e s u n d a r e a ( 1 3 ) , 2 : S t @ l e f j o r d ( 1 0 ) , K i l s f j o r d ( l l ) , S a n d n e s f j o r d ( 7 ) , S @ n d e l e d f j o r d ( 8 ) a n d R i s @ r S k e r r i e s ( 9 ) .
A f t e r 1 9 5 0 t h e r e h a s b e e n a g r a d u a l i m p r o v e m e n t i n t h e l a n d - l o c k e d a r e a s o n t h e s o u t h e r n p a r t o f t h e c o a s t . T h i s i s i n c o n - t r a s t t o t h e i n n e r O s l o f j o r d ( d i s t r i c t s 1 6 - 1 8 ) a n d t h e Lange- s u n d ( d i s t r i c t 1 3 ) a r e a , w h e r e t h e r e h a s b e e n a g r a d u a l d e - t e r i o r a t i o n d u r i n g t h e l a s t 2 5 y e a r s ( F i g . 5 a n d F i g . 6 ) .
R u e n e s s ( 1 9 7 3 ) a n d Bokn a n d L e i n ( 1 9 7 8 ) h a v e d e s c r i b e d c h a n g e s i n t h e a l g a l f l o r a i n t h e O s l o f j o r d . A more r o u g h p i c - t u r e o f t h e s i t u a t i o n i n t h e L a n g e s u n d a r e a , b a s e d o n d e s c r i p - t i o n s o f t h e b o t t o m c o n d i t i o n s g i v e n e v e r y y e a r f o r e a c h h a u l , i s shown i n F i g . 7. D u r i n g t h e s i x t i e s t h e e e l g r a s s ( Z o s t r a m a r i n a L . ) d i s a p p e a r e d a n d t h e g r e e n a l g a e UZva Z a c t u c a L.
b e c a m e a d o m i n a n t s p e c i e s . T h i s w a s m o s t p r o b a b l y c a u s e d b y c h a n g e s i n t h e p o l l u t i o n t o t h e a r e a ( B o k n , 1 9 7 9 ) . S i m u l - t a n e o u s l y t h e a b u n d a n c e o f 0 - g r o u p c o d was d r a s t i c a l l y r e d u c e d ( F i g . 6 ) . W h e t h e r t h i s r e d u c t i o n i s c a u s e d d i r e c t l y b y p o l l u - t i o n , f o r i n s t a n c e o n t h e l a r v a l s t a g e , o r i n d i r e c t l y t h r o u g h
Fig. 7. Observations of bottom conditions at the nine
localities in the Langesund area. Based on descriptions given for each haul every year.
1. Eel grass ( Z o s t r a m a r i n a ) 2. Brown algae
3. Green algae (mostly U Z v a Z a c t u c a ) 4. Blue mussels
X : Observed in a haul
m :
So much that it made sampling of fish difficult orimpossible.
1 2 3
the deterioration of the littoral zone as a habitat for 0-group cod, is not known. However, the trends in these areas are so clearly different from the trends in less polluted areas that pollution must be considered as the original reason for reduced recruitment.
In the Holmestrand (district 16) area a similar development has taken place without the obvious changes in the bottom flora, In some of the other areas in the Oslofjord the trend is the same, although some rich yearclasses have occurred in between the poor ones also during the later years.
1953 7 1955
-
-- -
-
1960- - - - 1965-
- - - 1970 -
1975-
- 1980 - 1982
-
X X X X X -
-
X X X
X X X
-
X X X X
X X X X X X
-
X X X X X X X - X X X X X
-
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.
X X X -
X X -
x x x -
X X X X X X
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X X
-
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X X X X X X X X X X X X X X X
X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X
X X
-
X
-
X -
X X
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X X
- -
X X X X X X X
-
X
-
-X
-
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-
X X X X
- -
-
--
- - X X X-
X X -
X
-
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X -
X X X
-
X
- -
X X X X
-
X X - -
-
X X
-
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-
m
-X x
-
.X X
U X X -
N X X X
-
N.NXXX
X X
-
x x x x
X X X X X X
-
X X X X X
-
X X X -
X X X X -
X X X X X -
X X X X X
-
W.. X X X X X - X X X X X X X -
CONCLUSION
A t a n a g e o f s i x m o n t h s t h e a b u n d a n c e o f c o d m e a s u r e d b y t h e b e a c h s e i n e m e t h o d c a n b e u s e d t o p r e d i c t t h e a b u n d a n c e o f t h e y e a r c l a s s r e c r u i t i n g l a t e r t o t h e f i s h e r y a l o n g t h e N o r w e g i a n S k a g e r r a k c o a s t .
F o r t h e L a n g e s u n d a r e a a n d t h e i n n e r p a r t s o f t h e O s l o f j o r d , p o l l u t i o n h a s d i r e c t l y o r i n d i r e c t l y h a d a n e g a t i v e e f f e c t o n r e c r u i t m e n t . I n t h e r e s t o f t h e a r e a s i n v e s t i g a t e d n o s u c h e f f e c t c a n b e t r a c e d .
REFERENCES
B o k n , T . , 1 9 7 9 . K l a s s e r a v f a s t s i t t e n d e a l g e r b r u k t som i n d i - k a t o r p i e u t r o f i e r i n g i e s t u a r i n e o g m a r i n e v a n n r n a s s e r . N o r s k I n s t . V a n n f o r s k . , S r b o k 1 9 7 8 : 5 3 - 5 9 .
B o k n , T . a n d L e i n , T . E . , 1 9 7 8 . L o n g term c h a n g e s i n f u c o i d a s s o c i a t i o n s o f t h e i n n e r O s l o f j o r d , N o r w a y . Norw. J . B o t . 2 5 : 9 - 1 4 .
D a h l , K . , 1 9 0 6 . U n d e r s 0 g e l s e r o v e r n y t t e n a f t o r s k e u d k l z k n i n g i ( Z l s t l a n d s k e f j o r d e . N o r g e s F i s k e r i e r A a r s b e r . , 1 9 0 6 : 1 - 9 7 . D a n n e v i g , G . M . , 1 9 0 6 . U n d e r s 0 g e l s e r o v e r n y t t e n a f t o r s k e u d -
k l z k n i n g i ( Z l s t l a n d s k e f j o r d e . N o r g e s F i s k e r i e r A a r s b e r . , 1 9 0 6 : 1 0 1 - 1 2 1 .
D a n n e v i g , A., 1 9 2 1 . F o r e l G b i g m e d d e l s e l s e orn u n d e r s 0 k e l s e r vedkommende a a r s y n g e l a v t o r s k e f i s k i s t r a n d r e g i o n e n p a a S @ r l a n d e t , s a r l i g i f o r b i n d e l s e med u d s l i p n i n g a v t o r s k e - y n g e l . N o r g e s F i s k e r i e r A a r s b e r . , 1 9 1 9 : 2 5 8 - 2 9 3 .
D a n n e v i g , A., 1 9 5 4 . T h e l i t t o r a l c o d o f t h e N o r w e g i a n S k a g e r r a k c o a s t . R a p p . D.-v. ~ 6 u n . C o n s . I n t . E x p l o r . M e r , 1 3 6 : 7 - 1 4 . D a n n e v i g , G . , 1 9 6 3 . A r t i f i c i a l p r o g a p a t i o n o f c o d . Some r e s u l t s
o f t h e l i b e r a t i o n o f l a r v a e . F i s k D i r . S k r . S e r . H a v U n d e r s . , 1 3 : 7 3 - 7 9 .
R u e n e s s , J . , 1 9 7 3 . P o l l u t i o n e f f e c t s o n l i t t o r a l a l g a l communi- t i e s i n t h e i n n e r O s l o f j o r d , w i t h s p e c i a l r e f e r e n c e t o A s c o p h y l Z u m nodosum. H e l g o l a n d e r w i s s . M e e r e s u n t e r s . , 2 4 : 4 4 6 - 4 5 4 .
T v e i t e , S . , 1 9 7 1 . F l u c t u a t i o n s i n y e a r c l a s s s t r e n g t h o f c o d a n d p o l l a c k i n s o u t h e a s t e r n ~ o r w e g i a n c o a s t a l w a t e r s d u r i n g 1 9 2 0 - 1 9 6 9 . F i s k D i r . S k r . S e r . H a v U n d e r s . , 1 6 : 6 5 - 7 6 .
