Fl@devigen rapportser., 1, 1984. ISSN 0333-2594 The Propagation of Cod Gadus morhua L.
VARIATION OF PEAK SPAWNING OF ARCTO-NORWEGIAN COD
(Gadus morhua L.) DURING THE TIME PERIOD 1929-1982 BASED ON INDICES ESTIMATED FROM FISHERY STATISTICS
T. Pedersen
Department of Fisheries Biology, University of Bergen, P.O.Box 1839, N-5011 NORDNES Norway
ABSTRACT
Pedersen, T., 1984. Variation in peak spawning of Arcto- Norwegian cod ( G a d u s morhua L.) during the time period 1929-1982 based on indices estimated from fishery statis- tics. In: E. Dahl, D.S. Danielssen, E. Moksness and P.
Solemdal (Editors), The Propagation of Cod Gadus m o r h u a l . FlGdevigen rapportser., 1, 1984: 301-316.
The weekly amounts of roe and cod landed in the main spawning area at Vestfjorden are known since 1929. An index of the median of the spawning intensity curve was constructed on the basis of an analogy to the gonosomatic index using the weight ratio roe/cod.
The index of median reveals a delay of about one week during the time period from 1929 to 1982. This method is compared to the median of spawning curves based on net hauls of newly spawned eggs from the traditional spawning grounds for the time period 1976 to 1982.
Sources of errors in the fishery statistics are discussed.
INTRODUCTION
The Lofoten and Vesterhlen area is the main spawning area of the Arcto-Norwegian tribe of cod. The spawning period lasts from February to May with the main spawning in March and April (Sars, 1879; Rollefsen, 1932; Wiborg, 1957;
Ellertsen et al., 1981a). The cod appear at the spawning grounds in February and March after the spawning migration
f r o m t h e , B a r e n t s S e a ( R o l l e f s e n , 1 9 5 5 ) . The m o s t c o n c e n - t r a t e d s p a w n i n g i s o n t h e s h e l f o n t h e w e s t e r n s i d e o f V e s t f j o r d e n c l o s e t o t h e s h o r e (Wiborg, 1 9 5 2 ; E l l e r t s e n e t a l . , 1 9 8 1 a , 1 9 8 1 b ) . T h e r e i s a l s o s p a w n i n q o n t h e c o a s t a l b a n k s o u t s i d e L o f o t e n a n d V e s t e r a l e n a n d n o r t h t o S @ r @ y a
( H j o r t , 1 9 0 2 )
.
The c o d spawn i n t h e t h e r m o c l i n e b e t w e e n t h e c o l d c o a s t a l w a t e r a n d t h e warmer a n d more s a l i n e b o t t o m w a t e r o f A t l a n t i c o r i g i n . Spawning o c c u r s a t 3.5-6. ~ O C ( R o l l e f s e n , 1 9 5 5 ; E l l e r t s e n e t a l . , 1 9 8 1 b ) .
The o l d e r a n d b i g g e r f i s h g e n e r a l l y a p p e a r e a r l i e r i n t h e c a t c h e s t h a n y o u n q e r a n d s m a l l e r f i s h ( R o l l e f s e n , 1 9 3 7 , 1 9 3 9 , 1940; S u n d , 1 9 3 7 , 1 9 3 8 ) . The o l d e r f i s h a l s o l e a v e t h e L o f o t e n a r e a b e f o r e t h e y o u n g e r f i s h ( H y l e n , 1 9 6 2 ) .
The a g e a n d s i z e f r e q u e n c y d i s t r i b u t i o n o f t h e s p a w n i n g s t o c k h a v e c h a n g e d d u r i n g t h e p e r i o d 1929 t o 1 9 8 2 b e c a u s e o f t h e i n c r e a s i n g e x p l o i t a t i o n , a n d m o s t o f t h e s p a w n i n g s t o c k now c o n s i s t s o f young f i s h ( H y l e n , 1 9 6 2 ; S a t e r s d a l a n d H y l e n , 1964; Hylen a n d D r a g e s u n d , 1 9 7 3 ; Ponomarenko, 1 9 8 2 ) .
A d e l a y i n p e a k o f s p a w n i n g w o u l d b e e x p e c t e d b e c a u s e o f t h e combined e f f e c t s o f l a t e r s p a w n i n g t i m e o f y o u n q e r f i s h a n d t h e c h a n g e i n a g e s t r u c t u r e o f t h e s p a w n i n g s t o c k t o w a r d s more y o u n g f i s h d u r i n g t h e t i m e p e r i o d ( W i b o r g , 1 9 5 7 ; H y l e n , 1 9 6 2 ; S o l e m d a l , 1 9 8 2 ) .
The f i s h e r y s t a t i s t i c s from t h e L o f o t e n f i s h e r y h a v e e a r l i e r b e e n u s e d t o i n d i c a t e t h e l e n g t h o f t h e s p a w n i n g p e r i o d a n d p e a k o f s p a w n i n g ( W i b o r g , 1 9 5 7 ; C u s h i n g , 1 9 6 9 ) . B e s i d e s t h e f i s h e r y s t a t i s t i c s , e g g d a t a f r o m n e t h a u l s i n 1976-1982 i n t h e e a s t e r n p a r t o f L o f o t e n w e r e a v a i l a b l e
( S o l e m d a l , 1 9 8 2 ) .
The aim o f t h i s i n v e s t i g a t i o n was t o d e c i d e w h e t h e r f i s h e r y s t a t i s t i c s c a n b e u s e d t o e s t a b l i s h a t i m e s e r i e s o f i n d i c e s o f p e a k s p a w n i n q w h i c h c o u l d b e u s e d t o t e s t t h e h y p o t h e s i s o f a d e l a y i n p e a k s p a w n i n g d u r i n g t h e t i m e p e r i o d 1929-1982. The r e s u l t s f r o m t h e f i s h e r y s t a t i s t i c s w i l l b e e v a l u a t e d by c o m p a r i n g them t o r e s u l t s f r o m s p a w n i n g i n t e n - s i t y c u r v e s b a s e d o n v e r t i c a l n e t h a u l s f o r t h e p e r i o d 1976-1982.
MATERIALS AND METHODS
--
Estimation of spawning intensity curves 1976-1982
From 1976-1982 eggs were sampled with vertical Juday net hauls from the three subareas: Henningsvarstraumen, H@lla and Austnesf jorden (Solemdal, 1982)
.
These stations are located in the eastern part of Lofoten on the traditional spawning grounds. Samples were usually taken each second or third day from the middle of March until spawning was fin- ished in the beginning of May.The cod eggs were staged by a key given by Westernhagen (1970), modified by Str@mme (1977). The number of eggs less than 48 hours of development were sorted out of the samples and counted. Yearly spawning curves were calculated by pooling the data from the three subareas. The curves were smoothed by a three point moving mean.
Cumulative spawning curves were calculated by trapezoidal integration. The median (M), which is the point of 50 per cent of the total amount of eggs spawned, was estimated from these cumulative curves. Because eggs less than 48 hours of development were counted, one day was subtracted from the median, assuming that the mean age of the eggs was one day.
A mean curve was calculated from the spawning curves for 1976-1982. This curve represents the mean numbers of eggs less than 48 hours of development per square meter for the years 1976-1982. Day of year, which is the number of days
from 1 January, has been used as the time unit.
Fishery statistics
From 1929 onwards the fishery statistics include tables of the quantity of cod roe and cleaned gutted cod landed every week of the fishery in the Lofoten area. The values used are from the pooled statistics from all fishing stations in the area. The gonosomatic index (GSI) is the ratio (gonad weight/body weight)
-
loo%, and is defined for individualfish. GSI have been used to describe the maturity cycle and spawning period of a fish population (Eliassen and Vahl, 1982; de Vlaming et al., 1982). The roe index (RI) is defined as the ratio (weight of roe/weight of females). loo%, and is defined for the landed catch. RI is an analogy to GSI and is deduced below.
R - roe weight in the landed catch in tonnes
W,
-
weight of females in the landed catch in tonnesk
-
conversion factor between total body weight and the weight of cleaned gutted cod, assuming that cleaned gutted weight is 62.5 per cent of total body weightP ?
-
fraction of female body weight in the landed catchW
-
weight of cleaned gutted cod in the landed catch in tonnesIn the present paper it is assumed that Po=0.5 and was constant through the period studied.
From the fishery statistics the ratio (R/w) is known and equation (1) is used to transform the weekly points (R/w) so that the scale of RI can be compared to that of GSI. RI have then been used instead of GSI.
The weekly points of RI for 1978 are shown in Fig. 1 as an example. A cubic spline function has been fitted to the weekly points of RI. This fitting procedure has the advan- tage over linear interpolation in this case that it is not so sensitive to single deviating data points (Wold, 1974).
T I M E [day of y e a r )
Fig. 1. Comparison of the data from fishery statistics and the eggdata for 1978. Weekly points of RI are indicated by dots and the fitted cubic spline function is shown by solid line. Vertical solid line indicate index of median (MRI).
Cumulative spawning curve from egg net hauls is shown by broken line and the median (M) indicated by vertical broken line.
The upward trend of RI (Fig. 1) in the first part of the season is due to ovary growth which is rapid during this period (Sorokin, 1967). The sudden fall in RI from the middle of March is caused by the beginning of spawning.
Official criteria and control of the quality of roe that is acceptable at the fishing stations has existed since 1934 (Anon., 1981). Roes with many ovulated transparent eggs, being in the stage of spawning, are not suitable as human food and therefore in general not accepted at the fishing stations. These rejected roes are not included in the roe quantity (R) in the fishery statistics. The roe quantity (R) is an underestimate of the true quantity of roe in the catch before delivery to the fishing stations (Anon., 1929-1982).
RImax is defined as the maximum of the RI function. Since in the present computation it is assumed that PO=0.5, then
b o t h v a r i a t i o n s i n s e x r a t i o and r e l a t i v e o v a r y w e i g h t b e t w e e n y e a r s w i l l a f f e c t t h e computed RImax The i n d e x o f median ( M R I ) i s d e f i n e d a s t h e p o i n t o f t i m e when h a l f t h e RImax i s p a s s e d . T h i s d e f i n i t i o n would make MRI i n d e p e n d e n t o f v a r i a t i o n s i n s e x r a t i o and r e l a t i v e o v a r y w e i g h t b e t w e e n y e a r s . M R I h a v e b e e n e s t i m a t e d from t h e f u n c t i o n s f i t t e d t o t h e weekly p o i n t s o f RI f r o m 1929 t o 1982.
The R I c u r v e would b e d e l a y e d i f much r o e o f s p a w n i n g f i s h i s a c c e p t e d r e l a t i v e t o t h e o f f i c i a l c r i t e r i a f o r a c c e p t a n c e b e c a u s e o f v e r y g r e a t demand f o r r o e . T h i s would l e a d MRI t o b e d e l a y e d . The o p p o s i t e s i t u a t i o n would o c c u r i f r o e s a t p r e s p a w n i n g m a t u r i t y s t a g e s w e r e r e j e c t e d a t t h e f i s h i n g s t a t i o n s b e c a u s e o f g r e a t s u p p l y and low demand f o r r o e . I n t h i s c a s e t h e r e s u l t would b e v e r y e a r l y MRI.
I n t h e m i d d l e o f March t h e s u p p l y o f r o e e x c e e d e d t h e c a p a c i t y o f t h e f i s h i n g s t a t i o n s i n 1 9 4 4 , 1946 a n d 1 9 7 2 , c,ausing much r o e t o b e d i s c a r d e d . I n 1 9 4 7 , 1952 a n d 1 9 5 3 s a l t i n g o f r o e was p r o h i b i t e d and much r o e was dumped. T h e s e y e a r s show e x t r e m e R I ' s w i t h a s h a r p d r o p i n t h e m i d d l e o f March a n d h a v e b e e n o m i t t e d from t h e a n a l y s i s .
The mean d a t e o f c a p t u r e (MD) was u s e d by C u s h i n g ( 1 9 6 9 ) a s a n i n d e x o f p e a k s p a w n i n g f o r t h e t i m e p e r i o d 1893-1967.
MD h a s b e e n c a l c u l a t e d b y e q u a t i o n ( 2 ) f o r t h o s e y e a r s w h e r e e s t i m a t e s o f t h e median f r o m e g g d a t a (M) e x i s t .
i
-
number o f weeks i n t h e f i s h e r y n - t o t a l number o f weeks i n t h e f i s h e r y Ci - c a t c h i n t o n n e s l a n d e d week i Di-
d a y o f y e a r when C . i s r e p o r t e dT h r e e d a y s i s s u b t r a c t e d from t h e t e r m s w i t h i n b r a c k e t s i n e q u a t i o n ( 2 ) b e c a u s e t h e c a t c h i s r e p o r t e d o n t h e l a s t d a y i n t h e week.
RESULTS AND DISCUSSION
The spawning curves for 1976-1982 show that both the start and end of the spawning period is very stable (Fig. 2). The spawning intensity increases in the middle of March and decreases from the middle of April. This material has been published by Ellertsen et al. (1981a), Solemdal (1982). The median estimates from egg data 1976-1982 indicate a very stable spawning period in this period (Table 1).
The medians (M) have been read from the cumulative spaw- ning curves given by Smedstad and 0iestad (1974) for the years 1968-1972 (Table 1) (Fig. 3). These spawning curves are based on a combination of results from egg net samples and samples of cod where the maturity stages were determined.
However, the method is difficult to repeat from the descrip- tion given. The median is stable with the mean on day 90 (31 March).
When pooling the results from 1968-1972 given by Smedstad and 0iestad (1974) and the results from the egg data (i976- 1982), the mean date of the median (M) is day 90 (31 March) with a standard deviation of 2.2 days. This indicates a very stable peak spawning during the time period 1968-1982.
The curves of RI from different years have a very similar shape. The agreement between the median from the egg data (M) and the index of median (MRI) seems acceptable except for 1971, 1979 and 1980 (Table 1). There is a positive corre- lation between the index of median (MRI) from fishery statis- tics and the median (M) from the egg data, although this is not a very high correlation (Table 2). 1971, 1979 and 1980 are years with an extreme MRI (Table 1) (Fig. 3). The correlation coefficient improves slightly when these years are excluded (Table 2). his suggests that MRI are biased by nonbiological factors in 1971, 1979 and 1980.
The time series of MRI reveal a trend towards higher values from 1929 to 1982 (Fig. 3). This trend is indicated
1976-1982
0
50 60 70 80 9 0 100 110 120 130 140 TIME l d a y o f y e a r )
Fig. 2. The spawning curves for 1976-1982. The last curve is the mean curve for 1976-1982. Vertical line indicate day 91 (1 April).
by a five years moving mean. A nonparametric test against the trend w a s applied (Lehman, 1975), and the trend is sta- tistically significant, p<0.001.
TABLE 1
Comparison o f t h e median f r o m e g g d a t a ( M ) , t h e i n d e x o f median f r o m t h e f i s h e r y s t a t i s t i c s (MRI) and t h e mean d a t e o f c a p t u r e ( M D ) . M f r o m 1968-1972 a r e r e a d from t h e g r a p h s g i v e n b y Smedstad a n d 0 i e s t a d ( 1 9 7 4 ) . The d i f f e r e n c e s b e t w e e n MRI and M a n d b e t w e e n MD a n d M a r e g i v e n . Time u n i t i s d a y o f y e a r .
Y e a r M MRI MRI-M MD MD-M
S i n c e a b o u t 1955 t h e v a r i a n c e i n MRI seems t o h a v e i n - c r e a s e d ( F i g . 3 ) , and i s l a r g e compared t o t h e more s t a b l e m e d i a n ( M ) from e g g d a t a ( F i g . 3 ) ( T a b l e 1 ) . The l a t e MRI i n 1 9 5 5 , a n d 1957 i s p r o b a b l y c a u s e d by t h e g r e a t demand f o r r o e i n t h e s e y e a r s , c a u s i n g much p a r t l y s p e n t r o e t o b e a c c e p t e d l a t e i n t h e s e a s o n . I n 1 9 7 1 , 1972 and 1973 t h e c a t c h e s w e r e g r e a t compared t o t h o s e i n t h e s i x t i e s . Some r o e was d i s - c a r d e d i n 1 9 7 1 and 1973 and much i n 1972 (Anon., 1 9 2 9 - 1 9 8 2 ) . T h e s e y e a r s h a v e a n e a r l y MRI. 1979 a n d 1980 show a n e x t r e m e l a t e MRI, t h i s may b e c a u s e d by t h e g r e a t demand f o r r o e i n t h e s e y e a r s . The main s o u r c e o f e r r o r i n t h e i n d e x o f median
(MRI) from f i s h e r y s t a t i s t i c s seems t o b e v a r i a t i o n s i n demand f o r r o e b e t w e e n y e a r s a t t h e f i s h i n g s t a t i o n s . V a r i a t i o n s i n demand f o r r o e e x p l a i n much o f t h e v a r i a t i o n s
i n MRI s i n c e t h e f i f t i e s .
TABLE 2 .
C o r r e l a t i o n c o e f f i c i e n t s b e t w e e n t h e median f r o m e g g d a t a (M) and t h e i n d e x o f median f r o m f i s h e r y s t a t i s t i c s ( M R I ) , a n d b e t w e e n M and mean d a t e o f c a p t u r e (MD). The median (M) f r o m 1 9 6 8 - 1 9 7 2 a r e t a k e n from Smedstad and 0 i e s t a d ( 1 9 7 4 ) .
Time p e r i o d M v s MRI
1 9 6 8 - 1 9 7 2
1 9 6 8 - 1 9 7 1 0 . 2 3
1 9 7 6 - 1 9 8 2 0 . 6 5
1 9 6 8 - 1 9 7 2 a n d 1 9 7 6 - 1 9 8 2
1 9 6 8 - 1 9 7 1 a n d 1 9 7 6 - 1 9 8 2 0 . 5 2 1 9 6 8 - 1 9 7 0 a n d 1 9 7 6 - 1 9 8 2
e x c e p t 1 9 7 9 and 1 9 8 0 0 . 6 5
The t r e n d t o w a r d s a d e l a y e d MRI o c c u r s b e f o r e 1 9 5 5 ( F i g . 3 ) , a n d t h e r e seems t o b e l e s s v a r i a n c e i n MRI b e f o r e 1 9 5 5 t h a n a f t e r w a r d s . T h i s r e l a t i v e l y s m a l l v a r i a n c e c a n b e e x - p l a i n e d by a more s t a b l e demand f o r r o e i n t h e t i m e p e r i o d b e f o r e 1 9 5 5 . A l o n g t e r m t r e n d i n t h e demand f o r r o e b e f o r e 1 9 5 5 , s o t h a t t h e r o e was r e j e c t e d a t t h e f i s h i n g s t a t i o n s a t p r e s p a w n i n g m a t u r i t y s t a g e s , c o u l d s e v e r e l y b i a s MRI a n d b e t h e c a u s e o f t h e t r e n d o b s e r v e d i n MRI. T h e r e i s s p a r s e i n f o r m a t i o n a b o u t t h e demand f o r r o e b e f o r e 1 9 5 5 , b u t t h e r e i s n o s i g n o f a l o n g t e r m t r e n d i n t h e demand f o r r o e i n t h e a v a i l a b l e l i t e r a t u r e .
C u s h i n g ( 1 9 6 9 ) u s e d mean d a t e o f c a p t u r e (MD) a s a n i n d e x o f p e a k s p a w n i n g f o r t h e p e r i o d 1 8 9 3 - 1 9 6 7 . T h e r e i s a t r e n d o f a b o u t f i v e d a y s f r o m t h e t h i r t i e s t o t h e s i x t i e s t o w a r d s
YEAR
F i g . 3 . The t i m e s e r i e s o f t h e i n d e x o f m e d i a n (MRI) f o r 1929-1982 a n d t h e m e d i a n f r o m e g g d a t a ( M ) 1968-1972 a n d 1976-1982. A f i v e y e a r moving mean i s f i t t e d t o M R I . H o r i z o n t a l l i n e i n d i c a t e d a y 9 1 (1 A p r i l ) . 1) I n d e x o f m e d i a n (MRI) f r o m f i s h e r y s t a t i s t i c s , 2 ) m e d i a n ( M ) r e a d f r o m s p a w n i n g c u r v e s g i v e n by S m e d s t a d a n d 0 i e s t a d ( 1 9 7 4 ) a n d 3 ) m e d i a n ( M ) from e g g d a t a 1976-1982.
e a r l i e r MD ( C u s h i n g , 1 9 6 9 ) . T h i s i s t h e o p p o s i t e o f t h e t r e n d i n MRI f o u n d i n t h i s i n v e s t i g a t i o n . The MD f o r t h e y e a r s 1968-1972 a n d 1976-1982 o c c u r 11 t o 2 5 d a y s e a r l i e r t h a n t h e median f r o m e g g d a t a (M) ( T a b l e 1 ) . T h e r e i s a v e r y s m a l l c o r r e l a t i o n b e t w e e n MD a n d M ( T a b l e 2 ) . The c a t c h e s
l u s u a l l y p e a k i n t h e l a s t p a r t o f March a f t e r a n i n c r e a s e f r o m
l
t h e s t a r t o f t h e f i s h e r y . A f t e r t h e p e a k t h e c a t c h e s d r o p a n d t h e m i g r a t i o n f r o m t h e s p a w n i n g a r e a i s r a p i d . F i g . 4 shows t h a t t h e mean d i s t r i b u t i o n o f w e e k l y c a t c h e s i n t h e p e r i o d 1970-1979 i s skewed.
The s p a w n i n g p e r i o d o f A r c t o - N o r w e g i a n c o d was f i r s t d e s c r i b e d by G.O. S a r s ( 1 8 7 9 ) . A c c o r d i n g t o him t h e p e a k o f
T I M E ( w e e k o f f i s h e r y ) JAN+FEB+MARCH+APRIL+MAY
F i g . 4 . Mean d i s t r i b u t i o n o f w e e k l y c a t c h i n p e r c e n t o f t o t a l y e a r l y c a t c h f o r t h e p e r i o d 1970-1979.
s p a w n i n g o c c u r r e d a t t h e end o f March. A c c o r d i n g t o R o l l e f s e n ( 1 9 3 2 ) t h e main s p a w n i n g t o o k p l a c e i n t h e l a s t p a r t o f March. Wiborg ( 1 9 5 7 ) s t a t e s t h a t t h e main s p a w n i n g p e r i o d i s f r o m m i d d l e o f March t o t h e b e g i n n i n g o f A p r i l .
S i n c e 1 9 4 9 , c o d e g g s h a v e b e e n sampled by v e r t i c a l n e t h a u l s on a s i n g l e s t a t i o n l o c a t e d o u t s i d e t h e s h e l f n e a r t o H @ l l a ( E l l e r t s e n e t a l . , 1 9 8 1 a ) . The d a t e o f p e a k number o f e g g s f o r t h e y e a r s i n t h e p e r i o d 1949-1977 i n d i c a t e s t h a t t h e mean d a t e o f p e a k s p a w n i n g i s a r o u n d t h e f i r s t week o f A p r i l
( E l l e r t s e n e t a l . , 1 9 8 1 a ) .
B e c a u s e o f t h e t r e n d i n t h e i n d e x o f median ( M R I ) from t h e f i s h e r y s t a t i s t i c s , and a l s o t h e e a r l i e r r e c o r d s o f t h e s p a w n i n g p e r i o d , it seems t h a t t h e p o s s i b l e c h a n g e i n p e a k s p a w n i n g i s a maximum o f 10 d a y s t o w a r d s l a t e r s p a w n i n g . The d e l a y seems t o h a v e o c c u r r e d b e f o r e 1 9 5 5 .
1930 1940 1950 1960 1970 1980 YEAR
Fig. 5. The mean age of the spawning population of Arcto- Norwegian cod. 1) Taken from Satersdal and Hylen (1964), 2) taken from Ponomarenko (1967)
,
3) calculated from the age frequency distributions given by Hylen (1962) for the period 1958-1962 and Hylen and Dragesund (1973) for the period 1963-1969, 4) calculated from the age frequency distributions given by Jakobsen (1978a, 1978b), 5) from Ponomarenko and Yaragina (1981).The change in the age frequency distribution of the spawning population has been quite drastic during the time period from 1929 to 1982 (Fig. 5). The Norwegian data from 1929-1969 are from long line catches from Lofoten (Satersdal and Hylen, 1964; Hylen and Dragesund, 1973). The data given by Jakobsen (1978a, 1978b) are from purse seine catches from the same area. According to Ponomarenko (1982) the mean age in the period 1971-1981 was 8.1 years with 8.6 years in the period 1971-75 and 7.6 years in the period 1976-1981.
Comparison of Fig. 3 and 5 shows that the delay in peak spawning estimated by the index of median (MRI) from the fishery statistics occurred at a time of a change in the age structure of the spawning stock towards more young fish.
This delay can be explained with younger fish spawning later than older fish, and this is in accordance with the hypo- thesis mentioned in the introduction.
ACKNOWLEDGEMENTS
I would like to thank Per Solemdal for his supply of the egg data and his advice concerning the investigation and the manuscript, Svein Sundby for helpful critisism of both methods and manuscript, Elsa Strand and Per Bratland for their work with the egg material, and Pauline Dayrathne for help with the English language in the manuscript.
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