ir_harpseals1990.pdf (166.8Kb)

T h i s p a p e r n o t t o be c i t e d w i t h o u t p r i o r r e f e r e n c e t o t h e a u t h o r .

I n s t i t u t e of Marine Research Sea Mammal S e c t i o n

Bergen

i 3 i ~ g k o a a t e ~ i&@b&&&ay

Report SPS 8 91 0 Bergen, 1 4 . 1 1 . 8 9

s S e a m t e Sea,

Oyvind U l l t a n g

I n s t i t u t e of Marine Research P b . 1870, 5024 Bergen, Norway

A s d i s c u s s e d i n t h e r e p o r t of t h e ICES Working Group on Harp and Hooded s e a l s (Anon 1 9 9 0 ) , some d r a s t i c e v e n t must have l e d t o a s i g n i - f i c a n t i n c r e a s e i n n a t u r a l m o r t a l i t y of t h e B a r e n t s Sea and White Sea h a r p s e a l s t o c k i f t h e d e c r e a s e i n t h e p o p u l a t i o n of b r e e d i n g f e m a l e s from a b o u t 140000 i n 1985 t o a b o u t 71000 i n 1988 a s measured by t h e S o v i e t a e r i a l p h o t o g r a p h i c s u r v e y s i s r e a l . One c o n t r i b u t i n g f a c t o r may h a v e been a s u b s t a n t i a l m o r t a l i t y i n f i s h i n g g e a r s d u r i n g t h e y e a r s of h a r p s e a l i n v a s i o n s t o t h e Norwegian c o a s t a l w a t e r s . I n t h i s p a p e r s t o c k s i z e development c o n s i s t e n t w i t h t h e a e r i a l s u r v e y s i s c a l c u l a t e d by a s i m u l a t i o n model, showing t h e s i z e o f e x t r a n a t u r a l m o r t a l i t y which has t o b e added t o c o n v e n t i o n a l v a l u e s i f s u r v e y e s t i m a t e s of s t o c k s i z e a r e t a k e n a t t h e i r f a c e v a l u e s .

The p o p u l a t i o n model and n o t a t i o n s u s e d a r e i d e n t i c a l w i t h what was p r e s e n t e d i n two working p a p e r s t o t h e ICES Working Group on Harp and Hooded s e a l s ( U l l t a n g , 1989 a and b ) . The s t a r t i n g y e a r f o r t h e s i r n u l a t i o n s was 1967. Catches of one y e a r s o l d and o l d e r s e a l s

(C ) were d i s t r i b u t e d on a g e g r o u p s i n p r o p o r t i o n t o t h e i r abundance ( i . e . e x p l o i t a t i o n r a t e l+ U = C /N f o r a l l 1 y e a r

It l +

o l d and o l d e r s e a l s ) . S t a r t i n g p o p u l a t i o n f o r a g i v e n l e v e l of pup p r o d u c t i o n i n 1967 was c o n s t r u c t e d i n a s i m i l a r way a s e x p l a i n e d i n U l l t a n g (1989a) f o r t h e West I c e h a r p s e a l s t o c k i n 1946, t a k i n g i n t o a c c o u n t t h e v a r i a b l e pup c a t c h e s d u r i n g t h e y e a r s 1962-1966.

The f o l l o w i n g v a l u e s f o r t h e b i o l o g i c a l p a r a m e t e r s were used:

N a t u r a l m o r t a l i t y on one y e a r s o l d and o l d e r s e a l s : M = 0 . 1

N a t u r a l m o r t a l i t y f i r s t y e a r of l i f e : M = 3M = 0.3

0

P r o p o r t i o n r e c r u i t e d ( p

,

F e r t i l i t y r a t e : f = 0.94

These a r e i d e n t i c a l t o t h e p a r a m e t e r v a l u e s u s e d i n one s e t of runs f o r t h e West I c e h a r p s e a l s t o c k i n U l l t a n g ( 1 9 8 9 a ) .

Results

F i g u r e 1 shows t h e r e s u l t s when s t a r t i n g w i t h a pup p r o d u c t i o n i n 1967 of 110000. T h i s i s a l e v e l s i m i l a r t o what was e s t i m a t e d by Benjaminsen (1979) by t h e s u r v i v a l i n d e x method f o r 1964-1965. During t h e 1970es, pup p r o d u c t i o n i n c r e a s e s by a r a t e of a b o u t 2.1% p e r y e a r , which i s l o w e r t h a n c a l c u l a t e d by Benjaminsen (1979) u s i n g more o p t i m i s t i c v a l u e s of b i o l o g i c a l p a r a m e t e r s . Pup p r o d u c t i o n r e a c h e s a

l e v e l of a b o u t 136000 i n 1980, s i m i l a r t o t h e USSR a e r i a l e s t i m a t e of 139000 b r e e d i n g f e m a l e s , a n d s t a y s a t a r a t h e r s t a b l e l e v e l u n t i l 1985 when pup p r o d u c t i o n i s c a l c u l a t e d t o 138000. The USSR s u r v e y gave an e s t i m a t e of a b o u t 140000 i n 1985. Thus, w h i l e t h e p o p u l a t i o n p a r a m e t e r s u s e d by Benjaminsen (1979) would p r e d i c t a f u r t h e r i n c r e a s e i n p o p u l a t i o n from 1980 onwards, t h e p r e s e n t s i m u l a t i o n s show a r a t h e r s t a b l e s t o c k d u r i n g t h e p e r i o d 1980-1985 c o n s i s t e n t w i t h t h e USSR a e r i a l s u r v e y e s t i m a t e s . Up t o 1985 t h e development i s "normal" i n t h e s e n c e t h a t no e x t r a n a t u r a l m o r t a l i t y i s r e q u i r e d f o r e x p l a i n i n g o b s e r v e d development.

From 1985 onwards, c o n s t a n t n a t u r a l m o r t a l i t i e s w i l l g i v e a s l i g h t d e c r e a s e i n pup p r o d u c t i o n t o about 131000 i n 1988 (Run ( l ) , f i g u r e 1 ) . Run ( 2 ) , f i g u r e ^1, shows t h e r e s u l t i f an e x t r a n a t u r a l m o r t a l i t y c o e f f i s i e n t of 0.02 i n 1985, 0 . 3 i n 1986 and 0.15 i n 1987 i s added f o r a l l a g e g r o u p s ( T a b l e 1 ) . Pup p r o d u c t i o n w i l l t h e n d e c r e a s e t o about 81000 i n 1988. T h i s i s s t i l l somewhat above t h e USSR a e r i a l e s t i m a t e of 71000, b u t s a t i s f a c t o r i l y c l o s e f o r t h e p u r p o s e of t h e p r e s e n t e x e r c i s e . T a b l e 2 shows t h e d e t a i l e d r e s u l t s of r u n ( 2 ) f o r t h e p e r i o d 1980-1990.

The e x t r a m o r t a l i t y added c o r r e s p o n d s t o t h e f o l l o w i n g f i g u r e s f o r e x t r a m o r t a l i t y i n terns of number of s e a l s :

I t s h o u l d b e n o t e d t h a t i n t h e s i m u l a t i o n model t h e s t a r t of t h e y e a r i s s e t a t t h e s t a r t of t h e b r e e d i n g / h u n t i n g season. Thus, f o r example 1985 i n t h e model s t a r t s a t b r e e d i n g s e a s o n 1985 a n d e n d s a t t h e s t a r t of t h e b r e e d i n g s e a s o n i n 1986. By comparing t h e f i g u r e s above w i t h t h e number of s e a l s drowned i n f i s h i n g g e a r and r e c o r d e d f o r compensation i n t h e Norwegian governmental programme t h e c a l e n d a r y e a r s 1986, 1987 a n d 1988, w e g e t t h e f o l l o w i n g f i g u r e s f o r t h e r a t i o

( e x t r a m o r t a l i t y i n s i m u l a t i o n / n u m b e r s r e c o r d e d f o r compensation ⁾ :

A more accurate comparison could be made by using monthly statistics of numbers recorded for compensation, but this was not considered worthwhile for the present exercise since it is the accummulated number of seals dying between breeding season in 1985 and 1988 which is of primary importance for the results. The exercise shows that an extra mortality of about 2.6 times the number recorded for compensation is sufficient for explaining the decrease observed in the USSR surveys between 1985 and 1988.

The present paper claims in no sence to describe the "true"

development of the White Sea harp seal population. Its only purpose is to show that

a) Values of population parameters which has been used for other harp seal populations give a stock development consistent with Benjaminsens' (1979) estimates of pup production in the 1960es and estimates of number of breeding females fromthe USSR aerial surveys in 1980 and 1985.

b) An extra mortality (distributed over all age groups) between the breeding seasons in 1985 and 1988 of about 2.6 times the number of seals drowned in fishing gear and recorded for compensation in the Norwegian governmental programme gives the decline observed by the USSR aerial surveys from 1985 to 1988. It is not considered unrealistic that the real number of seals which drowned in fishing gears or died of starvation during the invasions to Norwegian coastal waters 1986-1988 is of this magnitude or even higher.

Anon. 1990. Report of the Working Group on Harp and Hooded seals.

Bergen, 16-19 October 1989. ICES C.M. 1990/Assess:

Benjaminsen, T. 1979. Pup production and sustainable yield of White Sea harp seals. Fisk.Dir. Skr. Ser. HavUnders., 16:531-539.

Ulltang, 0. 1989a. Simulations of development in stock size and pup production for harp seals in the Greenland Sea ("West Ice") 1946-1989. E ^{S E A}

-

O c t n h ~ r 1989. 15 pp. (rnimeo)

Ulltang, 0. 1989b. Stock projections for Harp seals in the Greenland Sea 1990-1999. W SRA

-

S . 0c.t- 1989. 17 pp. (mimeo)

TABLE 1

Instantaneous natural mortality coefficients used in Run ( 2 ) .

AGE Y E A R

1 9 6 7 - 1 9 8 4 1 9 8 5 1 9 8 6 1 9 8 7 1 9 8 8 - 1 9 8 9

0 0 . 3 0.32 0.6 0 . 4 5 0 . 3

1 and older 0 . 1 0 . 1 2 0 . 4 0.25 0 . 1

TABLE 2

SIMULATION 1 9 8 0

-

LISTING OF: Total population

AGE Y E A R

1 9 8 0 1 9 8 1 1 9 8 2 1 9 8 3 1 9 8 4 1 9 8 5

AGE

1 9 8 6

Y E A R 1 9 8 7 1988

Y E A R Figure 1. Calculated pup production 1967 - 1990.

(l): Constant natural mortalities.

(2): Higher natural mortalities 1985-1987.

o : Estimated number of breeding females from USSR aerial photographic surveys.