Some aspects of mortality.pdf (810.5Kb)

SOME ASPECTS OF MORTALITY, CONDITION FACTORS AND

LIVER

STATE WITH ANISAKIS-INFECTION I N BLUE WHITING I N THE NORTH-EAST ATLANTIC

T e r j e Monstad

I n s t i t u t e of Marine Research P.O.Box 1870, Nordnes N-5024 Bergen, Norway

ABSTRACT

T o t a l m o r t a l i t y c o e f f i c i e n t s of blue whiting i n t h e spawning a r e a a s well a s i n t h e feeding a r e a , were obtained f o r v a r i o u s y e a r s and f o r v a r i o u s y e a r c l a s s e s , by c a l c u l a t i n g t h e r e g r e s s i o n s of t h e n a t u r a l l o g a r i thms of age frequency a g a i n s t age. Toge t h e r w i t h es t a b l i s h e d f i s h i n g m o r t a l i t y c o e f f i c i e n t s , a mean value of 0.18 was c a l c u l a t e d f o r t h e n a t u r a l m o r t a l i t y c o e f f i c i e n t ; compared t o 0.2 used by t h e Blue Whiting Assessment Working Group.

The index of t h e l a r v a l Anisakis p a r a s i t e on t h e l i v e r s u r f a c e of blue whiting was found t o i n c r e a s e with h o s t ' s age. The burden of accumu- l a t i o n of t h i s p a r a s i t e d i d n o t seem t o a f f e c t n e i t h e r t h e condition f a c t o r nor t h e length growth i n blue whiting. While t h e condition f a c t o r increased with age i n t h e spawning a r e a , t h e c o n t r a r y was observed i n t h e feeding a r e a .

320 PARASITES, MORTALITY AND MIGRATION Some a s p e c t s of m o r t a l i t y , condition f a c t o r s a n d . . .

INTRODUCTION

The majority of t h e a d u l t blue whiting (Micromesistius poutassou) s t o c k i n t h e n o r t h - e a s t A t l a n t i c migrates every e a r l y s p r i n g from t h e

l

feeding a r e a i n t h e Norwegian Sea t o spawn a l o i g t h e c o n t i n e n t a l s h e l f west of t h e B r i t i s h I s l e s . During l a t e s p r i n g and e a r l y summer i t r e t u r n s again and d i s p e r s e s over v a s t a r e a s on t h e warmer s i d e of t h e Norwegian Sea, with congregations mostly i n t h e temperature f r o n t

l

a r e a s .

Estimates of annua1 m o r t a l i t y r a t e s f o r blue whiting, f o r f u l l y r e c r u i t e d age groups, show a g r e a t v a r i a b i l i t y . Bailey (1982) g i v e s v8lues from d i f f e r e n t a r e a s , years and obtained by d i f f e r e n t methods.

The instantaneous m o r t a l i t y c o e f f i c i e n t , Z , was found t o be higher than 0.9 i n t h e period 1969-70 based on c a t c h p e r u n i t of e f f o r t f o r t h e same y e a r c l a s s e s . However, based on p l o t of I n percentage frequency a g a i n s t age i n t h e period 1967-70 an average of 0.75 was c a l c u l a t e d

,

and f o r a l a t e r period 0.51, even which is a r a t h e r high value.

The S c o t t i s h d a t a was r e c a l c u l a t e d using samples from p e l a g i c t r a w l catches only, and a mean value of 2=0.34 f o r t h e period

1973-78

was then c a l c u l a t e d (Bailey

1978).

I n 1980 t h e ICES Blue Whiting Working Group c a l c u l a t e d an average value of Z= 0.27 f o r Norwegian samples from p e l a g i c t r a w l catches f o r t h e period

1973-79

(Anon. 1980).

The v a r i e t y between d i f f e r e n t y e a r s may be due t o some u n c e r t a n t i e s i n aging t h e o t o l i t h s , but a l s o because of t h e p o s s i b i l i t y of samples r e p r e s e n t i n g d i f f e r e n t components of t h e population. I n t h e p r e s e n t paper t h e m o r t a l i t y r a t e s a r e s t u d i e d f o r t h e period 1980

- 1988

^both

i n t h e spawning a r e a and i n t h e feeding a r e a . The m o r t a l i t i e s w i t h i n some of t h e y e a r c l a s s e s a r e a l s o d e a l t with.

Blue whiting

i s

i n f e c t e d by a number of d i f f e r e n t p a r a s i t e s ( H ~ y g a a r d 1980) The nematode l a r v a of Anisakis s p . i s found i n t h e v i s c e r a l c a v i t y , t h e muscles and on t h e l i v e r s u r f a c e . I n some specimen i t is found i n such a number t h a t s e v e r a l authors have thought i t might i n f l u e n c e t h e condition of t h e h o s t , However, no such evidence has been s t a t e d . The l a r v a of Anisakis, l i k e most o t h e r p a r a s i t e s i s acquired from t h e previous intermediate h o s t ( e u p h a s i i d s ) i n food, and t h e f i n a l h o s t i s normally a marine mamma1 (Bailey 1982). Some r e l a t i o n s h i p s of Anisakis-infection of t h e l i v e r s u r f a c e a r e d e a l t with by age and y e a r c l a s s e s i n various a r e a s , a s w e l l a s t h e c o n d i t i o n f a c t o r s and l i v e r s i z e .

I

MATERIAL AND METHODS

Samples were obtained from t h e surveys c a r r i e d o u t i n t h e spawning a r e a a s w e l l a s i n t h e feeding a r e a and, i n some cases c o l l e c t e d from commercial c a t c h e s . They were grouped a f t e r a r e a of o r i g i n shown i n Fig. 1.

While analysing t h e samples, r e g u l a r observations l i k e l e n g t h , weight, s e x , maturity s t a g e were made i n a d d i t i o n t o c o l l e c t i o n o f o t o l i t h s . I n most cases a d d i t i o n a l observations of t h e l i v e r s i z e i n r e l a t i o n t o t h e body s i z e were a l s 0 made by s u b j e c t i v e judgement a t a s c a l e from 1 t o

4 ,

and t h e i n f e c t i o n r a t e by Anisakis on t h e l i v e r s u r f a c e a t a s c a l e of O t o

4

(Anon.

1989) .

PARASITES, MORTALITY AND MIGRATION

321

Some aspects of mortality, condition factsrs

and...

322

PARASITES, MORTALITY AND MIGRATION

Some a s p e c t s of m o r t a l i t y , condition f a c t o r s a n d . . .

i

l

l

l Table 1. Age composition ( N %) of t h e blue whiting spawning

s t o c k based on Norwegian survey e s t i m a t e s .

1

age 1 2

4 3 5 6 7 8 9

1 O 11 12

13 14 15+

~ ~ 1 0 9

l

1980 1981 1982" 1983 1984 1985"" 1986 1987 1988

^l

0.6 0.4 5.4

0.1

1.4

0.1

0.6 6.3 0.6 l

l

10.0

6.4 5.7

1.1

53.0 4.6 5.3 9.4

12.0 1

11.1

20.3 7.4 7.9 7.3 36.2 39.5 18.5 15.1 i

12.2

8.7 16.1

10.2

7.8 4.8 40.3 51.5

21.0 l

16.8 9.8 11.4 24.3 8.3 7.6 6.4 10.8 34.7 l

15.5 12.3 12.4 14.0 9.0 16.1

2.1

0.7 12.6

14.3 12.5 11.5 13.7 5.5 12.8 1.4 1.0

1.2 l

9.2 9.8 11.4 11.8 2.7 7.4

2.0

0.9

1.1 ^I

4.5 7.0 7.9 8.5

2 . 1

3.2 0.8 0.4 0.6

1.9 4.8 4.5 4.4 1.7 2.4

1.0 0.1

0.7

2 . 1

3.6 2.3

2.0

0.4

2.1

0.4 0.3

0.2

1.1

2.4 1.6 1.3 0.6 1.5

0.1 0.1 0.2

0.3

1.0

0.5 0.3

0.1

0.6

+ +

-

0.2

0.6 1.4 0.3

0.1

0.3 + +

0 . 1

0.4 0.6

^0.1

- _0.3

_0.1

₊ -

33 37 - 27

2 1

- 18 43 58

* Weighted by echo i n d i c e s only.

* * Norw. comm. samples weighted by echo i n d i c e s from Faroes survey.

Table

2.

Age composition ( N % ) of t h e blue whiting s t o c k i n t h e Norw.

Sea observed during t h e i n t e r n a t i o n a l surveys i n summer.

Age O 1 2

3 4 5 6 7 8 9

10 11 12

13 14 15+

N X I O ~

1980" 1981" 1982 1983 1984 1985 1986 1987"" 1988"

- - 16.0 23.0 3.8 4.8 18.0 11.4 36.5 2.5 0.7 0.8 62.1 62.0 12.7 8.3 29.3 20.8 18.2 2.8 2.0

1.1

28.4 50.8 8.5 14.4

12.0

8 . 1 17.4 5.4 1.3 1.7 26.6 25.8 18.5

11.0

8.4 14.9 20.5

2.1

0.8 1.4 24.8 19.9 14.5 12.6 10.3 15.7 3.9

1.1

0.9 6.7 4.3 3.8 14.7 10.9 13.6 1.6

1.1

0.4 3.4 0.8 0.5 12.4 11.4

10.1

1.3 0.7 0.5

1 . 2

0.6 0.4 9.7 10.6 7.3 1.2

0.1

0.3

1.1 0.2 0.1

4.8 7.9 3.8 0.9

0.1

0.8 0.5 0.3

0.2

3.2 5.0 1.8 0.6

0.1

0.3 0.7

0.1

-

3.3 4.2

1.1

0.3

0.1 0.1 0.2 0 . 1

+

1.0

1.8

1 . 2

0.2

0.1 0.1

0.5 +

0 . 1

0.5 1.4 0.4

0.1

-

0.2 0 . 1 ^m

+

0.5 0.4

0.2 +

- o.

¹

+

^m

-

0.1

0.3

0.1

+ - -

0.1

- -

56 26

2

3 36 49 47 28 28 24

* Norwegian surveys only

* " Combined r e s u l t of Norwegian and GDR surveys

l

I

PARASITES, MORTALITY AND MIGRATION

Some a s p e c t s of m o r t a l i t y , c o n d i t i o n f a c t o r s a n d . . . .

...

The l i v e r s t a t u s f o r t h e s c a l e used i s d e s c r i b e d a s follows:

S t a g e 1) Thin and n e g l e c t a b l e , 2-4 mm t h i c k .

"

2 ) Easy t o s p o t by e y e , b u t t h i n ; f i l l s

114

of t h e c a v i t y .

" 3)

Voluminous with l o b e s ; f i l l s c a . 112 of t h e c a v i t y .

"

4 ) S w o l l e n , f i l l s 3 / 4 o r m o r e o f t h e c a v i t y .

The i n f e s t a t i o n s c a l e of Anisakis on t h e l i v e r s u r f a c e i s l i k e w i s e d e s c r i b e d :

S t a g e O ) No v i s i b l e p a r a s i t e s

"

1) From 1 t o a few p a r a s i t e s .

"

2 ) Ca. 112 of t h e l i v e r i s covered.

' 3 )

Ca.

314

o f t h e l i v e r i s covered.

" 4 )

The l i v e r s u r f a c e i s completely covered.

The c o n d i t i o n f a c t o r s used i s c a l c u l a t e d a f t e r t h e d e f i n i t i o n :

The method used f o r e s t i m a t i o n of t h e i n s t a n t a n e o u s m o r t a l i t y c o e f f i c i e n t , Z , i s d e s c r i b e d by Ricker ( 1 9 7 5 ) , and i s a l l based on age compositions from which a p l o t of I n frequency o r p e r c e n t a g e frequency a g a i n s t age was made. The v a r i o u s age compositions used a r e based on t h e same m a t e r i a l a s used and described by Monstad (1989).

RESULTS AND DISCUSSION M o r t a l i t y

Age compositions o f t h e spawning s t o c k , based on Norwegian survey r e s u l t s , f o r t h e y e a r s 1980

-

1988 a r e given i n Table 1 , and of t h e s t o c k i n t h e Norwegian Sea d u r i n g summer, based on i n t e r n a t i o n a l s u r v e y s , i n Table 2. The g r a p h i c i l l u s t r a t i o n s o f t h e same d a t a a r e given i n Monstad (1989)

.

The n a t u r a l logarithms of t h e age frequency were c a l c u l a t e d f o r each y e a r , and t h e i r mean v a l u e s f o r t h e p e r i o d p l o t t e d a g a i n s t age a r e shown i n Fig.2 A-B. On an average t h e r e i s of c o u r s e t h e d i f f e r e n c e of f u l l y r e c r u i t e d y e a r c l a s s e s i n t h e spawning s t o c k and i n t h e s t o c k i n t h e Norwegian Sea.

The spawning s t o c k had a high r e c r u i t m e n t l e v e l of t h e y e a r c l a s s e s a l r e a d y from t h e age of

3-4

y e a r s mainly because of t h e i n f l u e n c e of t h e r i c h 1982- and 1983-yearclasses. However, w h i l e p l o t t i n g each y e a r s e p a r a t e l y ( F i g . 3 ) t h e v a r i a n c e i s more c l e a r l y i l l u s t r a t e d , with t h e d i f f e r e n c e from

1983

t o

1984

e s p e c i a l l y n o t a b l e . Based on t h e s e p l o t t i n g s t h e t o t a l m o r t a l i t y c o e f f i c i e n t of t h e v a r i o u s y e a r s was o b t a i n e d from t h e r e g r e s s i o n s c a l c u l a t e d from

7

t o

14

y e a r s o l d f o r

1980-1985, and from

4

^{t o}

14

y e a r s f o r 1986-1988.

The p l o t t i n g of t h e mean I n

(%N)

f o r t h e s t o c k i n t h e Norwegian Sea i n d i c a t e s high r e c r u i t m e n t t o t h e y e a r c l a s s e s a l r e a d y from t h e age of z e r o . This i s a l s o due t o t h e i n f l u e n c e o f t h e 1982- and

1983-

y e a r c l a s s e s . Likewise, p l o t t i n g s f o r each y e a r show t h e i n d i v i d u a l d i f f e r e n c e s i n t h i s a r e a a s w e l l ( F i g . 4 ) . I n o r d e r t o o b t a i n t h e t o t a l m o r t a l i t y c o e f f i c i e n t f o r t h e Norwegian S e a , t h e r e g r e s s i o n s based on t h e s e p l o t t i n g s were c a l c u l a t e d from t h e age of 2 t o

14

y e a r s .

324

PARASITES, MORTALITY AND MIGRATION Some a s p e c t s of m o r t a l i t y , condition f a c t o r s and

...

Ln(%N)

2

-

1

-

0

-

-1

-

-2

-

2

-

1

-

0

-

-1

-

-2

-

A

S p a w n i n g s t o c k

o e

o

.

e

I 1 1 I l I I I I I I I I ^I

,

B

N o r w e g i a n s e a

8

& l I I I l a & l I I I ^{I l}

1 2 3 4 5 6

7

0 9 1 0 11 12 13 1 4 Y e a r s

Fig. 2. Mean of n a t u r a l logarithms of percentage c o n t r i b u t i o n of various age groups a g a i n s t age of blue whiting, 1980-1988.

A ) I n t h e spawning stock during s p r i n g , B ) i n t h e s t o c k observed i n t h e Norwegian s e a during summer.

PARASITES, MORTALITY AND MIGRATION

Some a s p e c t s of m o r t a l i t y , c o n d i t i o n f a c t o r s a n d . . .

Age i n Y e a r s Age i n Years

2 4 6 8 I 0 12 1 4 2 . 4 6 8 1 0 12 1 4

I I I I I ~ I ~ ~ I ~ I I *

Age i n Yearc

2 4 6 8 30 12 1 4

Fig.

3.

Natural logarithms of percentage age composition a g a i n s t age i n t h e b l u e whiting spawning s t o c k during s p r i n g

1980-1988,

with r e g r e s s i o n l i n e s f i t t e d t o d a t a f o r ages

7-14

y e a r s

(1980-85)

and ages

4-14

years

(1986-88).

326 PARASITES, MORTALITY AND MIGRATION Some a s p e c t s of m o r t a l i t y , condition f a c t o r s and..

...

From t h e t a b l e of f i s h i n g m o r t a l i t y c o e f f i c i e n t F, given i n t h e Working Group Report of 1988 (ANON. 1989). t h e F ' s of corresponding y e a r s and age groups f o r t h e various Z ' s , were c a l c u l a t e d . The v a r i o u s values of F and Z f o r the spawning s t o c k and f o r t h e s t o c k observed while i n t h e Norwegian Sea a r e shown i n t h e t e x t t a b l e below:

For t h e spawning s t o c k t h e Z-values vary from 0.43 t o 0.69 i n s e p e r a t e years. A s mentioned above such d i f f e r e n c e s could be a r e s u l t of b i a s i n t h e sampling o r i n t h e ageing of t h e o t o l i t h s . The F-values a l s o have l a r g e d i f f e r e n c e s i n i n d i v i d u a l y e a r s , and i n some cases thye a r e even higher than t h e corresponding Z-values, thus i n d i c a t i n g t h e u n c e r t a n t i e s . The mean values of F and Z may g i v e a u s e f u l r e s u l t f o r t h e n a t u r a l m o r t a l i t y : M ⁼ 0.57

-

^{0.39 = 0.18.}

I n t h e Norwegian Sea t h e F- and Z-values a l c o show g r e a t d i f f e r e n c e s from year t o y e a r . Here too t h e F-values a r e higher i n some c a s e s than corresponding Z-values. The mean values could be used t o c a l c u l a t e a n a t u r a l m o r t a l i t y o f : M = 0.43

-

^{0.31 = 0.12. a}

This "catch-curve" method of c a l c u l a t i n g Z t s , a c t u a l l y g i v e s values t h a t r e f l e c t t h e mean m o r t a l i t y i n previous y e a r s , and n o t t h e m o r t a l i t y i n m y i n d i v i d u a l yea. To r e l y on t h i s method, t h e population needs t o be i n an e q u i l i b r i o u s s t a t e , o r c l o s e t o such a s t a t e . V a r i a t i o n s i n recruitment may reduce t h e r e l i a b i l i t y f o r i n d i v i d u a l y e a r s , a s f . e x . t h e c o n t r i b u t i o n s of t h e numerous 1982- and 1983-yearclasses. Likewise changes i n t h e f i s h e r y may d i s t o r t t h e e q u i l i b r i o u s s t a t e and hence reduce t h e usefulness of t h e method.

However, a mean of Z-values over a number of years may be a b a s i s f o r consideration of M .

The Z mean value of 0.57 f o r t h e spawning s t o c k i n t h e y e a r s 1980-1988 i s more than t h e double of t h e Z-value, 0.27, c a l c u l a t e d by t h e

Working Group using t h e same method, f o r t h e y e a r s

1973-1979

^(ANON.

1980a). The S c o t t i s h r e s u l t of mean Z-value was 0.34 f o r t h e y e a r s

1973 -1978

(Bailey 1978).

The f i s h i n g r a t e increased l a r g e l y from

1975

onwards, and may t o some e x t e n t have a f f e c t e d t h e age composition and t h e s t a t e of equilibrium.

However, t h e f i s h i n g m o r t a l i t y was anyhow considered t o be r a t h e r low

PARASITES, MORTALITY AND MIGRATION

327 Some aspects of mortality, condition factors and...

328

PARASITES, MORTALITY AND MIGRATION Some a s p e c t s of m o r t a l i t y , condition f a c t o r s a n d . . .

Table

3.

Abundance ( N x 1 W 6 ) a t various ages of t h e

1978 - 1985-

y e a r c l a s s e s a s observed i n t h e Norwegian Sea during summer.

Yearclasses

compared t o t h e n a t u r a l m o r t a l i t y , a t l e a s t up t o

1978,

and t h e e s t i m a t i o n of mean Z was taken a s a f i r s t approximation of n a t u r a l m o r t a l i t y . It could of course be an overestimate, but M was agreed upon t o be i n t h e range of 0.2

- 0.3

(Anon.

1 9 8 0 4 .

I n

1979

t h e e x p l o i t a t i o n r a t e increased very much, and up t o

1982

t h e t o t a l c a t c h was a t a very high l e v e l , more than 1 m i l l . tonnes i n

1979

a s well a s

1980

o r approximately twice t h a t catch of

1978

(Anon.

1988,

Monstad

1989).

This a u s t have d i s t o r t e d the equilibrium of t h e s t o c k n o t a b l e , and may be r e f l e c t e d i n t h e d i f f e r e n c e of t h e c a l c u l a t e d F ' s of t h e two p e r i o d s .

Age O 1 2

3

4 5 6 7 8 9

10

The Working Group has l a t e r used M =

0.2

while running t h e VPA of t h e s t o c k , and t h e assuming of t h i s value was done a f t e r s e v e r a l t r i a l s with d i f f e r e n t values within t h e range. (Anon.

1983).

1978 1979 1980 1981 1982 1983 1984 1985

- - - - ^{3 731 8 403}

¹

⁸⁵⁷

²

²⁴⁶

1

378 184 188

22

629 30 547 6 o03

2

320

10

181 740 455 416 14 007 23 950

2

389 4 052 4 578

1

254 456 826

12

525 7 201 5 185

2

838 4 778 779 393 682 6 924 5 582

2

587

1

425 534 418

1

863

1

217 3 423 -

544 203 962 235 903 - - -

245 348 i68

120

- - -

317 066 091 - - - -

o79 017 - - - - -

o55 - - - - -

Calculations of t h e F-values used a l c o include t h i s M-value, and t h e r e f o r e t h e r e s u l t s a r e only suggestions i n t h e c o n s i d e r a t i o n of an appropriate M. The value of

0.18,

however, seems t o j u s t i f y t h e previous assumption of 0.2.

The "catch-curve" method was a l s 0 used t o study t h e t o t a l m o r t a l i t y of s e p a r a t e y e a r c l a s s e s from d a t a c o l l e c t e d i n t h e Norwegian Sea during t h e summer surveys i n

1980-1988.

I n Table

3

t h e abundance e s t i m a t e s of t h e y e a r c l a s s e s

1978-1985

a r e shown s p l i t on age groups. The coverage of t h e b l u e whiting s t o c k during t h e feeding period i s t o a l a r g e e x t e n t incomplete, and t h e e s t i m a t e s which a r e c l e a r l y underestimates, consequently a r e taken a s i n d i c e s only (Anon.

1985).

PARASITES, MORTALITY AND MIGRATION

Some aspects of m o r t a l i t y , condition f a c t o r s and..

...

Age i n Years Age i n Years

- 4

-

O

i

^{1 9 8 4} Year c l a s s Year c l a s s

I { . * .

^{1 9 8 5} Year c l a s s

Fig.

5.

Natural logarithms of age composition ( N x 10-9) a g a i n s t age f o r t h e 1978-1985 year c l a s s e s of blue whiting a s observed i n t h e Norwegian Sea during summer. Regression l i n e s f i t t e d t o d a t a f o r 2 years and o l d e r age groups f o r t h e year c l a s s e s of 1978-1983.

330 PARASITES, MORTALITY AND MIGRATION Some a s p e e t s of m o r t a l i t y , condition f a c t o r s and.

...

P l o t diagrams of I n frequency ( N ) a g a i n s t age f o r a l l of t h e year- c l a s s e s a r e shown i n Fig.

5.

The regressions f o r t h e 1978

- 1983-

y e a r c l a s s e s were c a l c u l a t e d from t h e age of 2 y e a r s t o t h e o l d e s t , i . e . t h e i r r e s p e c t i v e age i n

1988.

The r e s u l t s a r e given below t o g e t h e r with corresponding F ' s c a l c u l a t e d from t h e Working Group r e p o r t (Anon.

1989) .

Great d i f f e r e n c e s a r e found between t h e v a r i o u s y e r c l a s s e s . The h i g h e s t t o t a l m o r t a l i t i e s were c a l c u l a t e d f o r t h e 1978-, 1982- and 1983-yearclasses, which a r e a l s 0 t h e t h r e e most abundant y e a r c l a s s e s f o r t h e period (Monstad

1989).

The average n a t u r a l m o r t a l i t y f o r t h e s e y e a r c l a s s e s i n t h e period 1980

- 1988

can be c a l c u l a t e d t o 0.31.

Yearclass 1978

1979

1980

1981

1982 1983 Mean

Fig. 6. Monthly mean values of t h e l i v e r index

L

and t h e condition f a c t o r C of blue whiting over t h e period 1982-1988.

Z F

0.67 0.22

0.32 0.21

0.21 0.18

0.26 0.18

0.78 0.12

0.61 0.12

0.48

0.17

age- groups

2-10 2-9 2-8 2-7 2-6 2-5

PARASITES, MORTALITY

AND

MIGRATION 331 Some a s p e c t s of m o r t a l i t y , c o n d i t i o n f a c t o r s a n d . . .

The f i s h i n g m o r t a l i t y i n c r e a s e s with i n c r e a s i n g age, e s p e c i a l l y from t h e age of 7

- 8

y e a r s , i . e . a f t e r f u l l recruitment. For t h e y e a r c l a s s e s i n q u e s t i o n , t h e F ' s t h e r e f o r e came o u t a s r a t h e r low, e s p e c i a l l y f o r t h e 1982-and 1983-yearciasses. Taking i n t o account t h a t t h e s e two y e a r c l a s s e s have been a main b a s i s f o r t h e f i s h e r y i n r e c e n t y e a r s , one should expect higher v a l u e s . An explanation might be t h a t they could be even s t r o n g e r than recosded.

Again, t h e method used i s n o t r e l i a b l e enough f o r e s t i m a t i o n of t h e m o r t a l i t y . Neither i s t h e m a t e r i a l s u f f i c i e n t l y a c c u r a t e , and t h e r e s u l t s can only g i v e i n d i c a t i o n s of t r e n d s i n t h i s m a t t e r .

Condition f a c t o r , l i v e r s t a t e and p a r a s i t e i n f e c t i o n

The monthly mean v a l u e s of t h e l i v e r i n d i c e s i n t h e p e r i o d 1982

- 1988

g i v e a p i c t u r e of t h e annua1 c y c l e ( F i g . 6 ) . The d a t a used should r e p r e s e n t t h e main p a r t of t h e n o r t h - e a s t A t l a n t i c s t o c k , migrating within t h e a r e a from southwest of I r e l a n d t o t h e Barents Sea, but i n some c a s e s t h e proper age groups were not a v a i l a b l e i n t h e samples.

The l i v e r mass v a r i e s a g r e a t d e a l both w i t h i n a sample and within a month. The lowest values were observed i n March-April when spawning t a k e s p l a c e . The energy s t o r e d i n t h e l i v e r is then "transformed" t o spawning product. Highest values i s found i n summer and autumn when t h e f i s h a r e feeding and b u i l d i n g up new r e s e r v e s .

The d i f f e r e n c e i n t h e l i v e r i n d i c e s between male and female was n o t of a s i g n i f i c a n t c h a r a c t e r ( F i g . 7 ) . Dumke (1986) found t h e l i v e r - weight i n percentage of body weight t o be h i g h e s t i n September and t h e values were

9.5

and

8.8

f o r male and female r e g p e c t i v e l y .

Fig.

7.

Monthly mean v a l u e s of t h e l i v e r index L by sex of b l u e whiting over t h e period 1982-1988.

PARASITES, MORTALITY AND MIGRATION Some aspects of mortality, condition factors and...

I I I I I I I I I l I I n I

7 6 7 8 8 0 8 2 8 4 8 6 8 8 Y e a r

P _PARASITE - - - N o r w . Sea

3 -

2 -

1

-

4

C O N D I T I O N

N o r w . C o a s t

.-.

P o r c u p i n e B a n k

C

1975 7 6 7 8 8 0 8 2 8 4 86 8 8 Y e a r

l 1 l l I I l l ¹ I I I I I

7 6 7 8 8 0 8 2 84 8 6 8 8 Y e a r

Fig. 8. Annua1 mean values of the parasite index P (larval Anisakis

on liver surface), the liver index L and condition factor C

of blue whiting in various areas (Fig. 1) 1975-1988.

PARASITES, MORTALITY AND MIGRATION

333

Some a s p e c t s of m o r t a l i t y , condition f a c t o r s a n d . . .

Fig.

9.

Mean values of t h e p a r a s i t e index P and t h e r a t i o between t h e p a r a s i t e and l i v e r i n d i c e s P/L by age of b l u e whiting.

The c o n d i t i o n f a c t o r , expressing how w e l l nourished t h e f i s h a r e , show a c y c l e almost p a r a l l e l t o t h e l i v e r i n d i c e s ( F i g . 6 ) . The minimum value i s observed i n May and t h e maximum i n August. S i g n i f i c a n t changes t a k e s p l a c e i n t h e spawning period and decreases of up t o 25

%

have been observed (Bailey 1982). The build-up of r e s e r v e takes place gradually through t h e summer and autumn with most r a p i d i n c r e a s e observed i n J u l y and August. The drop i n both c o n d i t i o n f a c t o r and l i v e r i n d i c e s observed f o r September

-

November and again f o r January, may be due t o b i a s i n t h e sampling.

I n Fig.8 a r e shown t h e year t o year v a r i a t i o n s of p a r a s i t e i n d i c e s ( A n i s a k i s ) , l i v e r i n d i c e s and condition f a c t o r s from

1975

t o

1988

i n t h e a r e a s of Porcupine, Hebrides, Norwegian Sea and Norwegian Coast

( F i g . 1 ) . For t h e Hebrides and Porcupine t h e annua1 mean values r e p r e s e n t February-May and f o r Norwegian Sea and Norwegian Coast June- September

.

For t h e p a r a s i t e i n d i c e s only minor v a r i a t i o n s between t h e a r e a s were ^x observed, but t h e r e i s a decreasing tendency up t o t h e middle of t h e 1980's. Except f o r 1979 l e s s p a r a s i t e s were found i n t h e Norwegian Coast a r e a a l l of t h e years. This i n d i c a t e a p a r t of t h e population which only t o a c e r t a i n e x t e n t mix with f i s h from t h e o t h e r t h r e e a r e a s . I n t h e s e a r e a s t h e i n d i c e s v a r i e d i n a r e g u l a r way and a l t e r n a t i v e l y show t h e h i g h e s t value from year t o year.

The l i v e r i n d i c e s , however, show g r e a t v a r i a t i o n s between t h e various a r e a s a s well a s from one year t o another. Lowest value of

1.4

was observed i n Porcupine i n 1985 and highest value of

3.7

i n Norwegian Sea i n

1984.

The condition f a c t o r s show s i g n i f i c a n t l y smaller v a r i a t i o n s , and have a congruent tendency, a l s 0 towards t h e middle of t h e 1980

'

s

.

334 PARASITES, MORTALITY AND MIGRATION

Some aspects of m o r t a l i t y , condition f a c t o r s and

...

O 2

4'

6 8 1 O

Age

in

years .

Fig. 10. P a r a s i t e index P , l i v e r index

L

and condition f a c t o r C by age of blue whiting i n various a r e a s

1970-1987.

PARASITES, MORTALITY AND MIGRATION

Some a s p e e t s of m o r t a l i t y , condition f a c t o r s a n d . . .

P

+ Norwegian S e a

+

⁷⁶

o Norwegian C0aSt

2 +79-

+

^{86 +77}

7+8

^{o 79}

O 7 8 +82

+8 7 + B 3 $81

o86 +80 o8 l

+8 8 O88 O82 +84

+85

1 o83 O84

0 87

O85

3

o P o r c u p i n e +75

4-76

+

⁷⁷

2

+85

0 79

+

o 8 8 +84 o 0 ° 8 0

85 87

1

+

⁸⁷

F i g . 11. Annua1 mean values of t h e p a r a s i t e index P a g a i n s t c o n d i t i o n f a c t o r C of t h e blue whiting i n t h e Norwegian Sea and t h e Norwegian Coast a r e a s ( u p p e r ) , and t h e Hebrides and t h e Porcupine a r e a s ( l o w e r ) .

336

PARASITES, MORTALITY AND MIGRATION Some a s p e c t s of m o r t a l i t y , c o n d i t i o n f a c t o r s and...

The l a r v a l form of Anisakis which accumulates on t h e l i v e r s u r f a c e , a l s 0 i n f e c t s t h e v i s c e r a l c a v i t y probably by being e x p e l l e d from t h e l i v e r . The index v a l u e , however, i s s u b j e c t i v e l y set i n r e l a t i o n t o t h e l i v e r s i z e , which v a r i e s through t h e year and from y e a r t o year ( F i g . 8 ) . I n o r d e r t o s e e how t h i s would a f f e c t t h e p a r a s i t e index, i t s value by age was compared t o corresponding r a t i o of p a r a s i t e and l i v e r i n d i c e s , PIL, by age ( F i g . 9 ) . These o v e r a l l mean v a l u e s f o r t h e a r e a s i n question show t h e i n f e s t a t i o n r a t e i n c r e a s i n g with h o s t age.

Up t o t h e age of

5

y e a r s t h e d i f f e r e n c e was n e c l e c t a b l e i f any. For t h e o l d e r age groups t h e d i f f e r e n c e was n o t i c e a b l e , with t h e p a r a s i t e index being s l i g h t l y lower when n o t r e l a t e d t o t h e l i v e r s i z e .

P a r a s i t e i n d i c e s by h o s t t s age i n t h e

4

a r e a s a r e shown i n Fig.10 t o g e t h e r with t h e corresponding l i v e r i n d i c e s and c o n d i t i o n f a c t o r s .

Table

4.

The growth parameters of von B e r t a l m f f y ' s equation ( L i n f * K and t o ) , p a r a s i t e index of l a r v a l Anisakis, l i v e r index ( L ) and r a t i o of p a r a s i t e and l i v e r i n d i c e s

(PIL) of t h e 1971-1983 -blue whiting y e a r c l a s s e s a t 1-6 y e a r s o l d i n t h e Hebrides a r e a .

Yearclasses

1978 1979

1980

1981

i982 1983

t o P a r a s i t e

-2.83 -1.32 -0.99 -1.21 -2.13

-1.48

1.30 1.40 1.52 1.22 1.02 0.86

PARASITES, MORTALITY AND MIGRATION 337 Some a s p e c t s of m o r t a l i t y , c o n d i t i o n f a c t o r s a n d . . .

I n a l l a r e a s t h e amount of Anisakis on t h e l i v e r i n c r e a s e s with i n c r e a s i n g a g e , and t h e h i g h e s t v a l u e was observed i n t h e Norwegian Sea f o r t h e 10 and 11 y e a r s o l d s . Smith and Wotten

(1978)

a l s o found i n c r e a s i n g number of Anisakis with age of t h e b l u e w h i t i n g h o s t , and no s i g n i f i c a n t d i f f e r e n c e between t h e s e x e s . Bussmann and Ehrich ( 1 9 7 8 ) , however, found t h a t t h e i n f e s t a t i o n was d e c r e a s i n g i n t h e h i g h e s t l e n g t h groups.

The l i v e r index and t h e c o n d i t i o n f a c t o r t o a g r e a t e x t e n t follow each o t h e r a f t e r t h e youngest age groups. I n t h e Hebrides and t h e Porcupine a r e a s they a r e observed t o be r a t h e r even f o r most age groups, b u t a f t e r

8

y e a r s o l d t h e c o n d i t i o n f a c t o r i n c r e a s e s s i g n i f i c a n t l y with age i n t h e Hebrides a r e a . The l i v e r index show o n l y a s l i g h t i n c r e a s e f o r t h e o l d e s t age groups, i n d i c a t i n g t h a t t h e spawning products n a t u r a l l y a r e more s i g n i f i c a n t i n t h e o l d e r t h a n i n t h e younger age groups of t h e spawning s t o c k . I n t h e Porcupine a r e a t h i s phenomenon was n o t obvious t o t h e same e x t e n t .

On t h e c o n t r a r y , i n t h e Norwegian Sea a r e a t h e c o n d i t i o n f a c t o r d e c r e a s e s s i g n i f i c a n t l y with age from 2 y e a r s o l d . The l i v e r index was a l s o observed t o d e c r e a s e with a g e , b u t a t a l e s s e r r a t e . I n t h e Norwegian Coast a r e a t h e same tendency could be t r a c e d , b u t t h e r a p i d i n c r e a s e of t h e c o n d i t i o n f a c t o r of t h e o l d e s t age groups, a l t e r t h e p a t t e r n of t h e Norwegian Sea a r e a .

While t h e spawning product mass r e a r t h e c o n d i t i o n f a c t o r i n t h e spawning p e r i o d , t h e o l d e s t , and hence t h e l a r g e s t specimen, seem t o need more food p r o p o r t i o n a l l y p e r body l e n g t h u n i t t o g a i n a c o n d i t i o n e q u a l t o t h e smaller specimen d u r i n g t h e f e e d i n g p e r i o d .

The i n c r e a s i n g p a r a s i t e index w i t h age could g i v e reason t o b e l i v e t h a t t h e accumulation of t h e l a r v a l Anisakis on t h e l i v e r s u r f a c e would i n some way a f f e c t t h e b l u e w h i t i n g h o s t , and hence e x p l a i n t h e d e c r e a s i n g c o n d i t i o n f a c t o r with age d u r i n g t h e f e e d i n g p e r i o d . Bussmann and Ehrich (1978) and Smith and Wotten ( 1 9 7 8 ) , however, show t h a t i n b l u e w h i t i n g t h e r e l a t i o n between weight and l e n g t h , i . e . t h e c o n d i t i o n f a c t o r , i s n o t a f f e c t e d by t h e appearence of t h e l a r v a l Anisakis p a r a s i t e .

I n o r d e r t o v e r i f y t h i s n o n - r e l a t i o n s h i p t h e mean v a l u e s of t h e p a r a s i t e i n d i c e s f o r t h e y e a r s 1975

-

¹⁹⁸⁸ were p l o t t e d a g a i n s t corresponding c o n d i t i o n f a c t o r s f o r each s e p a r a t e a r e a ( F i g . 1 1 ) . The random d i s t r i b u t i o n shown on t h e graph could n o t j u s t i f y a r e g r e s s i o n f o r t h e r e l a t i o n s h i p .

Data a v a i l a b l e f o r t h e Hebrides a r e a p e r m i t t e d a f u r t h e r s t u d y of t h i s m a t t e r , and a comparison was made between t h e growth parameter K (von B e r t a l a n f f y ' s ) of t h e y e a r c l a s s e s

1971 - ¹⁹⁸³

and t h e i r Anisakis p a r a s i t e index. The m a t e r i a l and method f o r t h e growth c a l c u l a t i o n s a r e d e s c r i b e d i n Monstad (1989). To avoid a b i a s due t o i n f l u e n c e of o l d e r age groups i n some of t h e y e a r c l a s s e s , t h e growth parameter a s w e l l a s t h e p a r a s i t e index were based on l

- 6

y e a r s o l d f i s h only.

The parameters of von B e r t a l a n f f y ' s length-growth e q u a t i o n f o r t h e v a r i o u s y e r a r c l a s s e s a r e given i n Table

4

t o g e t h e r w i t h corresponding p a r a s i t e and l i v e r s i z e i n d i c e s .

338

PARASITES, MORTALITY AND MIGRATION Some a s p e c t s of m o r t a l i t y , condition f a c t o r s a n d . . .

P

K

- 8

.6

. 4

.2

Y e a s - c l a s s e s

Fig. 12. Regression of growth parameter K (von B e r t a l a n f f y ) , c i r c l e and s t i p p l e d l i n e , and of p a r a s i t e index P , c r o s s and f u l l l i n e , a g a i n s t year c l a s s e s of blue whiting i n t h e Hebrides a r e a during s p r i n g , based on values f o r t h e s i x f i r s t years of l i f e .

The r e g r e s s i o n s i l l u s t r a t e d i n Fig. 12 were c a l c u l a t e d t o : Y~ ⁼ -0.03

.

^X

y e a r c l a s s ⁺ 0.6

f o r t h e growth, and

"p ' X y e a r c i a s s ⁺ 2.2

f o r t h e p a r a s i t e index.

The l i n e s were almost p a r a l l e l with decreasing values with i n c r e a s i n g y e a r c l a s s e s .

Although t h e r e i s v a r i a t o n i n t h e y e a r c l a s s ' K-values, t h i s a n a l y s i s gives no reason t o b e l i e v e t h a t t h e burden of l a r v a l Anisakis para- s i t e s on t h e l i v e r s u r f a c e a f f e c t s t h e blue whiting h o s t , regarding t h e l e n g t h growth i n t h e f i r s t s i x years of l i f e .

REFERENCES

Anon. 1980. Report of t h e Blue Whiting Assessment Working Group, Bergen 5-10 May 1980. ICES, C.M. 1 9 8 0 / ~ : 5 , 65 pp.

Anon.

1983.

Report of t h e Blue Whiting Assessment Working Group, Copenhagen 15-21 September 1982. ICES, C.M.

19831

Assess:3,

51

pp.

PARASITES, MORTALITY AND MIGRATION 339

Some aspects of mortality, condition factors and.... ...

Anon. 1985. Report of the Workshop on the International Acoustic Surveys of Blue Whiting in the Norwegian Sea. Bergen 6-11 May 1985. ICES, C.M. 1985/~:6, 27 pp.

Anon. 1989. Instruks for pravetaking og koding av data. "Manual for fish sampling and coding of data". , Centre of Marine Resources, Institute of Marine Research, Bergen, 1 January 1989, 32 PP.

Bailey, R.S. 1978. Changes in the age coaposition of blue whiting in the spawning area west of Scotland, 1967-1978. ICES, C.M.

19781 ~ 1 5 2 , 6pp.

Bailey, R.S. 1982. The population biology of Blue Whiting in the North Atlantic. Adv. Mar. Biol., Vol.19: 257-355.

Bussmann, B, and Ehrich, S. 1978. Larval nematodes Anisakis sp. in blue whiting from waters around the Faroe-Islands. ICES, C.M. 1978/H:13, 11 pp.

Dumke, A. 1986. Results of investigations of liver Weights from blue whiting (Micromesistius potassou RISSO) from NE-Atlantic.

ICES,C.M 1986/H:56, 13 pp.

HØjgaard, D.P. 1980. Parasitter hos blåhvilling. "Parasites of blue whiting". Thesis for the cand. scient-study, Biologisk Institut, Odense Universitetscenter and Zoologisk

Laboratorium, Universitetet i Bergen, November 1980.70pp.

Monstad, T. 1989. Distribution and growth of blue whiting in the north-east Atlantic 1980 - 1988. IV Sovjet-Norwegian symposium on "Biology and fishery of blue whiting in the Northeastern Atlantic and Atlanto-Scandian herring, Bergen 12-16 June 1989* 42pp.

Smith, J.W. and Wotten R. 1978. Further studies on the occurence of

larval Anisakis in blue whiting. ICES, C.M. 1978/H:53,9pp.