sel-barentshavet1987.pdf (319.7Kb)

l

i I

I tute of Marine

,

N-5024 Bergen, Norway

ÿ apport SPS 8815 30 April 1988

FEEDING HABITS OF NORTHEAST ATLANTIC HARP SEALS (Phoca groenlandica) IN THE BARENTS SEA, AUGUST-SEPTEMBER 1987.

Må ikke siteres uten etter av- tale med. Sj~pattedyrseks jonen ,

Havforskningsinstituttet,Bergen

Not to be cited without refer-

ence to the Sea Mamma1 Section,

BY

Institute of Marine Research,

Bergen.

LARS ANKER ANGANTYR

Institute of Marine Research.

P.O. Box 1870, N-5024 Bergen, Norway.

ABSTRACT

:

STOMACH SAMPLES FROM 59 HARP SEALS HAVE BEEN EXAMINED FOR A QUALITATIVE AND QUANTITATIVE ANALYSIS OF THEIR FEEDING HABITS IN THE BARENTS SEA. THE MOST FREQUENT FOOD ITEMS WERE THE AMPHIPOD Parathemisto libellula (55% vol.), FISH (34%

vol.) AND THE DECAPOD Pandalus borealis (9% vol.).

OF THE FISH POLAR COD (Boreogadus saida), NYBELIN'S SCULPIN (Triglops nybelini) AND GREENLAND HALIBUT (Reinhardtius hippoglossoides) MADE UP THE MAJOR PART.

REMAINS OF THE SQUID Gonatus fabricii WERE PRESENT IN

29%

OF THE STOMACHS, BUT MADE UP ONLY 1% OF THE VOLUME ON AN

AVERAGE.

INTRODUCTION:

Repeated invasions of harp s e a l s on t h e Norwegian c o a s t i n t h e 1970- i e s and 1980-ies have caused concern about t h e e c o l o g i c a l r o l e of t h e harp s e a l s i n t h e Barents Sea. Previous attempts by t h e I n s t i t u t e of Marine Research, Bergen, t o sample feeding harp s e a l s i n t h e Barents Sea i n

1981

and

1983,

were not succesful, mainly because of f a l i u r e t o develope techniques f o r capturing s e a l s i n t h e water. I n another attempt i n August-September

1987

stomach contents were c o l l e c t e d from

59

harp s e a l s i n t h e northern Barents Sea e a s t of Svalbard (Wiig, 1987) These samples have been thoroughly examined and t h e r e s u l t s a r e summarizes i n t h i s paper.

I MATERIAL AND METHODS:

l I

Between august 20th and september 5 t h a t o t a l of

59

harp s e a l s , 1 bearded s e a 1 and

4

ringed s e a l s were caught i n t h e a r e a e a s t of Svalbard ( 7 8 ' ~

-

8 0 ' ~ ~ 2 0 ' ~

-

5 0 ' ~ )

.

A l l s e a i s were s h o t i n t h e water.

The s e a l s were d i s s e c t e d immediately a f t e r capture t o minimize t h e degradation of t h e stomach content. Stomachs were opened, r i n s e d and t h e content frozen f o r l a t e r examination. Lower jaws were c o l l e c t e d f o r agedetermination and female reproductive organs f o r determination of t h e reproductive s t a t u s . A l l s e a l s were weighed and measured f o r assessment of condition. These d a t a have not been analyzed y e t , b u t a l l s e a l s appered t o be i n good condition. The blubber thickness was between

3

and 6 cm i n most s e a l s and only t h r e e s e a l s had less than

3

cm of blubber measured over t h e sternum.

I n t h e laboratorium t h e drained stomach content was weighed. (The contents were drained i n t h e f i e l d t o minimize t h e volume of t h e samples). This means t h a t only t h e s o l i d m a t e r i a l i s weighed and t h i s weight should only be seen a s a minimum.

The thawn sample was then c a r e f u l l y examined, looking through t h e whole material under a d i s s e c t i n g microscope. A l l f i s h , crustaceans and squids were i d e n t i f i e d o r kept f o r l a t e r i d e n t i f i c a t i o n . A l l o t o l i t h s were kept.

l

The c o n t r i b u t i o n of a l l major groups were estimated a s a volume

%.

l

This i s e a s i e r than t r y i n g t o s e p e r a t e t h e m a t e r i a l f o r weighing, and

l

assuming a d e n s i t y of 1.0 f o r a l l groups t h e v o l . % can be d i r e c t l y converted t o biomass.

When r e f e r r i n g t o "vol. %" i n t h e t e x t and t h e t a b l e s i t means: t h e average of a l l t h e volume percentages f o r t h a t group.

"% of biomass'' means: t h e

%

t h a t group make up of t h e t o a 1 biomass ( c a l c u l a t e d by adding t h e weights of a l l t h e stomach c o n t e n t s ) . The weight of each group i s estimated from t h e v o l . % of t h e group f o r a l l stomachs s e p e r a t e l y and then summarized. (Density of a l l groups = 1 . 0 ) .

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! Stomachparasites were c o l l e c t e d f o r l a t e r i d e n t i f i c a t i o n . This m a t e r i a l w i l l be analysed by B. Berland, Zoological Laboratorium,

1

University of Bergen.

RESULTS AND DISCUSSION:

The average weight of t h e contents of a l l stomachs was

259

g. Five stomachs contained less than 10 g of food. The average without t h e s e almost empty stomachs was

283

g. Since t h e content had been drained before weighing i n t h e l a b these averages w i l l underestimate t h e contents i n t h e stomachs. Therefore they should only be taken a s a minimum.

The d a t a can be looked a t i n d i f f e r e n t ways ( s e e t a b l e 1 + t a b l e 2 ) , but i n any case t h e dominating food items a r e t h e amphipod Parathemisto l i b e l l u l a , t h e fishspecies: p o l a r cod (Boreogadus s a i d a ) , Nybelin's s c u l p i n (Triglops n y b e l i n i ) and Greenland h a l i b u t

(Reinhardtius hippoglossoides), and t h e decapod Pandalus b o r e a l i s .

The d a t a have not y e t been looked a t f o r eventual e f f e c t of a r e a , s e x and age of t h e s e a l s .

FISH :

On an average

34.4%

(vol. ) of t h e stomach content was " f i s h " ( f r e s h f i s h , fishremains and o t o l i t h s ) . The f r e s h f i s h found i n t h e stomachs were dominated by p o l a r cod

(6%

of t o t a l biomass,

7%

v o l . and 50.6%

of no. of f r e s h f i s h ) and Nybelin's s c u l p i n

(3%

biomass and v o l . , 28.3% of of no. f r e s h f i s h ) . A l a r g e number of young Greenland h a l i b u t were p r e s e n t i n some stomachs (2% biomass and v o l . ,

14.5%

of no. of f r e s h f i s h ) , s e e Table 1 + Table 2.

I f one looks a t t h e o t o l i t h s ( s e e t a b l e 2 ) t h e dominance of p o l a r cod becomes even g r e a t e r

- ^65.8%

of t h e number of f i s h estimated from t h e o t o l i t h s . Nybelin's s c u l p i n contributed with 10.7% and Greenland h a l i b u t with 6.4% of t h e f i s h estimated from t h e o t o l i t h s .

O t o l i t h s of L i p a r i s spp. were found

(8.7%

of o t o l i t h s ) , b u t s i n c e no o t h e r remains of these species were found i t is d i f f i c u l t t o a s s e s s

t h e s i g n i f i c a n c e of t h e s e s p e c i e s a s a source of food of harp s e a l s .

A small number ( l e s s than 2% v o l . and biomass) of r e d f i s h (Sebastes marinus), long rough dab (Hippoglossoides p l a t e s s o i d e s ) and c a p e l i n

(Mallotus v i l l o s u s ) were present.

A 1 1 o t o l i t h s (2.2%) from t h e c a p e l i n were found i n one stomach. This s e a 1 was caught f u r t h e r south

( 7 4 ' ~ .

3 0 ' ~ ) than t h e r e s t of t h e s e a l s .

AMPHIPODS:

Parathemisto l i b e l l u l a was present i n

95%

of t h e stomachs and contributed on an average

55%

of t h e volume. It represented

58%

of t h e t o t a l biomass examined.

There was no s i g n of o t h e r s p e c i e s than P. l i b e l l u l a among t h e many Parathemisto examined. Very few o t h e r amphipods were p r e s e n t (less than 0.2% v o l . ) , but a few gammarids and one isopod ( I d o t e a granulosa) were found.

DECAPODS :

Pandalus b o r e a l i s made up t h e major p a r t (8.8% vol. and 12.6% of biomass) of t h e decapods found i n t h e samples. Other decapods made up only on an average 0.5% of t h e volume. Among t h e s e Sabinea septemcarinatus (0.2% v o l . ) was t h e most dominating species.

CEPHALOPODS:

Remains of cephalopods were found i n

31%

of t h e stomachs. The squid Gonatus f a b r i c i i was present i n 29% of t h e stomachs, b u t made up only 1% of t h e volume i n average. Of t h e t o t a l biomass examined i t made up only 0.7%. Remains of t h e octopus Bathypolypus a r t i c u s was found i n 2 stomachs (one might have been Elodone c i r r h o s a ) .

BIVALVES :

I

^Two stomachs contained remains of bivalves. These might have been secondary material from f i s h eaten by t h e s e a l s . Anyway they only made up 0.03% of t h e volume.

CONCLUSIONS:

From t h e samples c o l l e c t e d i n

1987

i t seems t h a t p o l a r cod and t h e amphipod Parathemisto l i b e l l u l a a r e very important food items f o r Northeast A t l a n t i c harp s e a l s , making up a t l e a s t

60%

of t h e vol. and t o t a l biomass. I n t h e samples from 1983 only amphipods (Parathemisto s p . ) were found (n=8, s e e enclosure 1 ) . Polar cod and Parathemistos sp. has a l s 0 been found i n stomachs from harp s e a l s i n t h e Northwest A t l a n t i c . Samples from West Greenland (unpub. d a t a ) and from Eastern Canada (Finley and Gibb, i n p r e s s ) have contained l a r g e q u a n t i t i e s of p o l a r cod and Parathemisto sp.

I n our study only two commercially f i s h e d s p e c i e s were found i n s i g n i f i c a n t q u a n t i t i e s . This was t h e decapod Pandalus b o r e a l i s

(8.8%

v o l .

,

12.6% of biomass) and young Greenland h a l i b u t (2.1% vol.

, 1.8%

of biomass).

Remains of L i p a r i s spp.

(8.7%

of o t o l i t h s ) and t h e squid Gonatus f a b r i c i i were found i n many stomachs, but contributed very l i t t l e t o t h e volume o r biomass (less than 1%). Therefore i t i s hard t o say how important t h e s e species a r e .

It must be emphasaized t h a t t h e conclusions i n t h i s paper a r e based on a r e l a t i v e small number of s & p l e s from a l a r g e area. More samples a r e needed before one can g e t a good i d e a of t h e feeding h a b i t s of harp s e a l s i n t h e Barents Sea. More information about t h e d i s t r i b u t i o n and population s i z e of t h e harp s e a l s i n t h e Barents Sea a r e a l s o needed before one can make c a l c u l a t i o n s on t h e t o t a l consuption of t h e population. Information about t h e d a i l y food i n t a k e of t h e s e a l s a r e a l s 0 needed.

TABLE 1. DESTRIBUTION OF THE FOOD ON MAJOR GROUPS OF ANIMALS:

FISH :

Boreodadus s a i d a T r i g l o p s n y b e l i n i Reinhardtius hippo.

Sebastes marinus Hippoglossoides p l . Mallotus v i l l o s u s Other f i s h

Unknown

1 X of % v o l .

7.0

%

2.9 % 2.1

%

0.2

%

2

F i s h ⁼ 100

%

20.3

% 8.4 91

3

% o f biomass 6.2

% 3.3 % 1.8

0.1

%

-

FISH TOTAL

34.4

%

99.9 %

27.7

%

CRUSTACEA:

Parathemisto l i b e l l u l a

54.9 %

Gammaridae spp. 0.1

%

Other amphipods 0 . 1

%

Pandalus b o r e a l i s

8.8 %

12.6

%

Sabinea septemcarinatus 0.2 %

Other decapoda

0.3 %

...

CRUSTACEA TOTAL

64.4 % 71.3 %

...

CEPHALOPODA:

Gonatos f a b r i c i i 1.0

%

Octapoda spp. 0.2

%

CEPHALOPODA TOTAL 1 . 2

%

0.9

%

BIVALVIA TOTAL 0.03 %

-

...

"X of

%

v o l . " i s t h e mean of how much t h e s p e c i e s c o n t r i b u t e d t o t h e volume i n

g.

Column 2 shows t h e d i s t r i b u t i o n i n t h e group " f i s h t t when f i s h = 100

g.

Column

3

sh8ws t h e

%

of each group of t h e t o t a l biomass.

TABLE 2. COMPOSITION OF THE FISH CONSUMED BY HARP SEALS.

1 2

3

FRESH FISH FISH FROM OTO. F. FISH + OTO.

P o l a r cod (B. s a i d a ) 50.6

% 65.8 %

^64.0

%

Nyb. s c u l p i n (T. nyb.) 28.3

% 10.7

% ^12.8

%

G. h a l i b u t ( R . hippo. )

14.5

%

6.4 % 7.4 %

Redfish (S. marinus) 1.8

%

^2.0

%

2.0

%

L . r . dab ( H . p l a t e s . )

1.8

%

1.3 % 1.4 X

Capelin ( M . v i l l o s u s ) 0.0 % 2.2

% 1.9 %

L i p a r i s spp. 0.0

% 8.7 % 7.7 %

Other f i s h

Unknown

TABLE 2. COMPOSITION OF THE FISH CONSUMED BY HARP SEALS I N THE BARENTS SEA AUGUST-SEPTEMBER

1987.

Column l shows t h e composition of t h e f r e s h f i s h found i n t h e stomachs.

Column 2 shows t h e composition of t h e number of f i s h c a l c u l a t e d from t h e o t o l i t h s . Column

3

shows t h e com- p o s i t i o n c a l c u l a t e d by adding t h e number of f r e s h f i s h and t h e number c a l c u l a t e d from t h e o t o l i t h s .

TABLE 3. LIST OF SPECIES FOUND IN STOMACHS OF HARP SEALS CAUGHT IN THE BARENTS SEA AUGUST-SEPTEMBER 1987.

FISH

:

Polar cod (Boreogadus saida)

Long rough dab (Hippoglossoides platessoides) Spotted snake blenny (Leptoclinus maculatus) Sea snails (Liparis spp.)

Capelin (mallotus villosus)

Greenland halibut (Reinhardtius hippoglossoides) Redfish (Sebastes marinus)

Nybelin' s sculpin (Triglops nyb&ini) CRUSTACEA:

Gammaridae sp.

Parathemisto libellula Idotea granulosa

Lebbeus polaris Pandalus borealis Pasiphaea tarda

Sabinea septemcarinatus CEPHALOPODA:

Bathypolypus arcticus

Elodone cirrhosa (unsertain det.) Gonatus fabricii

MOLLUSCA

:

Astarte elliptica

Leda pernula

ENCLOSURE 1. SAMPLES FROM THE BARENTS SEA 1981 AND 1983.

BARENTS SEA AUGUST 1981.

1 harp s e a 1 (male, I22 cm Pong,

53

kg) was caught

17.

august

1981.

The stomach was empty.

8

harp s e a l s were caught e a s t of Svalbard and south of

" K v i t ~ y a " (80' N 31' E)

,

11.

-

18. september

1983.

When t h e stomachs were examined t h r e e were empty and t h e rest contained

4 ,

3 , 3 ,

3

and

6

d e c i l i t e r of amphipods, r e s p e c t i v e l y (Parathemisto s p . , T. Ø r i t s l a n d , probably Parathemisto l i b e l l u l a ) .

no. d a t e sex length weight volume content

amphipoder amphipoder amphipoder amphipoder amphipoder

-