This paper not to be cited without prior reference to the author
International Council for the Exploration of the Sea
C.M. 1979/H: 33 Pelagic Fish Committee
Abstract
Preliminary Results of Norwegian Blue Whiting Survey Northwest of Scotland
i n A p ri l 19 7 9
by
Terje Monstad
Institute of Marine Research P . b a x 18 7 0 - 7 2, N- 50 ll Be rg e n , N o rw ay
A brief report is given on the result from an acoustic survey with the R/V "r1ichael Sars'' to the blue whiting spawning area in second half of April 1979.
Rather dense concentrations of blue whiting were found from west of Scotland to the Faroes. Length- and age- composition are given. The majority of the stock consisted of 3 - 8 year old fish. Length-weight relationship is found to be different from previous years, and half of the stock was either ripe or spent.
A rough and preliminary figure of 4 mill. tonnes was cal- culated.
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Introduction
The norwegian directed fishery for blue whiting has developed rather sharply during the last few years. From March to May when the blue whiting stock congregates for spawning in the area west of the British Isles~ 12 vessels landed 40 000 tonnes in 1977, 35 vessels landed 153 000 tonnes in 1978 and 50 vessels landed near 200 000 tonnes in 1979. In addition there is a
mixed fishery in the North Sea where the by-catch of blue whiting increased from 20 000 tonnes in 1977 to 40 000 tonnes in 1978.
The size of the spawning stock in the period 1972-74 is esti- mated by Buzeta and Nakken (1975) to be in the order of 6 mill.
tonnes. They analysed the results from the norwegian acoustic surveys in 1972 (Jakupsstovu and Midttun,l972), in 1973
(Jakupsstovu, Olsen and Midttun, 1973) and in 1974 (Buzeta et.
al. 1974). Only part of the spawning stock was measured in 1975 and 1976 which gave results of 1.8 and 3.1 millp tonnes respec- tively (Jakupsstovu and Midttun, 1977)p In 1977 Norway had no acoustic survey on the spawning grounds, but a scouting vessel operated in the area.
In order to study the blue whiting concentrations in the spawning period, the new research vessel "Michael Sars" undertook an
acoustic survey from 18 April to 4 May 1979, with attempt to estimate the abundance in the area from west of Scotland to the Faroes.
Methods
The acoustic recordings were made with a Simrad 39 kHz echo- sounder, connected to a two channel integrator. The integrator worked mainly from 10 to 400 m depth, but this was varied in
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correspondance with the location of the recordings. To avoid saturation the gain was set to 20 dB, and the values (mm de-
flection per nautical mile) were multiplied by 20 to be expressed as 40 dB-values ..
The theoretical basis for acoustic abundance estimation is given by Forbes and Nakken (1972). The relation between fish density and integrator deflection is:
where
jJ
is number of fish per unit sea surface (square nauticalmile)~ M is the integrated echo intensity in mm per mile and C the coefficient of fish density that causes 1 mm deflection on the echo integrator. This depends on fish species and size.
The constant b is a threshold density~For picturing the distri- bution and relative abundance the average integrator value per
n.mile~ over 5 n.mile, were slided for each 25 n.mile and mapped along the cruise trackQ For estimation of the abundance, the area with blue whiting recordings was divided into subareas of 0.5° latitude and 1° longitude size, and the mean integrator deflection per n.mile within each subarea used.
0 u e t o d i f f i c u 1 t con d it i on an d g re at de p t h of t he re c o r d i n g s.. n o opportunity was given to make any single fish counts for obtaining
C-values~ Therefore a probable fish density coefficient~ cal- culated from the one obtained with R/V "G.OQ Sars" in 1976 (Blind- h e i m an d J
a
k up ss t o vu .. 19 7 7 ) was us e d ~ How eve r, y et t he re h as been no ship to ship calibration between "Michael'Sars" and"G. D. Sars'' ~ but from other ship to ship calibrations., a new relationship could be establish. This is preliminary, and based on relationshipsbetween "G.D. Sars" and "Johan Hjort'' and between
"Johan Hjort" and "Dr. Fridtjof Nansen" which has the same type of acoustic systems as that of "Michael Sars"a
!
I
l~
-
4For identifying of the recordings and biological analysis both pelagic trawl and bottom trawl were used~ Obtaining of hydro- graphical data were made by use of a CTO-sonde~
Results and discussion
Survey track with stations are shown in Figij 1, and the hydro- graphical situation in the sea-surface and at 400 m depth in Figs. 2 and 3 respectively.
<I
fl' J (l
~.
The area was covered from south to north. and blue whiting was recorded more or less continously within a relative wide belt along the shelf up to southwest of Faroes, In Fig. 4 is given the distribution and relative fish densityn During night time the blue whiting was scattered mostly between 250 and 500 m depth, while during daytime it congregated and gave echo re-
cordings as a dense layer around 350 m depth. At some localities the concentrations of blue whiting were rather high, and the best one was in second half of April found in the area north of St.
Kilda.
The stock migrated northwards, and the fishing vessels, that followed the best concentrations, were in first week of May operating at the shelf southwest of Faroes. While the majority of the stock, consisting of adult fish, was located outside the continental shelf at great depthsp blue whiting was also recorded near and on the shelfq This part of the stock was a mixture of young- and older fish. Fig. 5 give the length distributions in
the samples from the shelf and from outside the shelf respectively.
The total mean length in the samples was 29.9 cm. The age-com- positions in samples from the ·same two areas, are shown in Fig, B.
The majority of the stock consisted of mostly 3 to 8 year old fish, with smaller contribution of all age-groups up to 16 yearsQ On the shelf, 1 to 3 year old fish predominated in the
recordings.
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No immature fish were found in the samples from outside the shelf (Table 1) and more than half of the stock was ripe or spent. On the shelf, 15.6 %males and 31.0 %females were found to be immature.
Table 2 gives the mean weight of blue whiting in different length
groups~ These are lower than the weights given by Buzeta and Nakken (1975) for the years 1972-74.
The abundance within the area surveyed was estimated to be 3.9
mill. tonnes with a mean density of 138 tonnes/sq. n.m. (Fig. 7).
This is a rather rough and preliminary estimate. Due to the time limit and bad weather conditions, only part of the planned area could be covered. In second half of April the stock had started to migrate northwards towards the Norwegian Sea, and the northern limit of the distribution was not located.
A probable ,density coefficient of 548 fish per mm per sq. n.mile was used. Based on the different ship to ship calibrations men~
tioned above, this is calculated from C = 540 fish pr. mm per sq. n.m. with a fish length of 28.63 cm, given by Blindheim and JakupsstovG (1977).
Recalculation of the abundance will be done when the necessary data become available. However, theoretical there is little difference in the acoustic systems of »G~D. Sars" and"Michael Sa rs" • They a re equipped with the sa me type of echo sounder with same source level. A new estimate therefore most probably will be in the same order, i.e. 4 mill.tonnes.
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References
Blindheim, J. og Jakupsstova, S.H~ 1976. Unders121kelser av kolmule og hydrografi i Norskehavet, 12. juli- 11. august 1976. [Investigations on blue
whiting and hydrography in the Norwegian Sea, 12 July - 11 August 1976} ~ Fisken Hav., 1976
(2): 29-41.
Buzsta, R~B., J~kupsstovQ, S.H., Midttun, L. and Vestnes, G.
1974 • . Preliminary results of the Norwegian acoustic survey of blue whiting (March-April 1974). Coun.Meet.int.Coun.Explor.Sea, 1974
(B: 13): l-4, 3 Tabp, 3 Fig~ [Mimeo~
Fa rb e s , S • T • an d N a k ken , 0 • 19 7 2 • Man u a 1 of met h o d s for f i s h e ri e s resource survey and appraisal, Part 2.
FAD Manual in Fisheries Science No.5. 1972.
Jakupsstovu, S.H. og Midttun, L. 1972. Kolmuleunders0kelser nord- vest for De britiske 0yer i februar-mars 1972.
[Blue whiting surveys northwest of the British Isles in February-March 1972}.. Fiskets Gang, 58: 428-433.
Jakupsstovu, s.H., Midttun, L. 1977. Kolmuleunders0kelser nord- vest av De britiske 0yer i mai 1975 og mars- april 1976, (Blue whiting investigations north- west of the British Isles in May 1975 and March- April 1976]. Fisken Hav." 1977(1): 15-20 ..
Jakupsstovu, S.H.i, Dlsen, K. og Midttun, L. 1973. Kolmule- undersl21kelser nordvest for De britiske 0yer i mars-april 1973w [Blue whiting investigations northwest of the British Isles in March-April 1973]. Fiskets Gan~~ 59: 784-789.
Table 2.
Cml 18 20 22
Table 1. Maturity in samples of blue whiting Trom A) the continental shelT and from 8) out- side the. shelf.
A B
Maturit . 00 % ~~ 0. ... 00 ~~
immature '15, 6 31,0 0 0
maturing 7,1 25,3 5,2 38,6
·ripe 10,2 9,9 35,4 22,7
spent 67,2 33,B 59,3 38,6
. Weight o-f blue whitin·g (g) .aTJd number (n) in different length- groups, April 1979.
24 26 28 30 32 34 36 38 40
g
I
27 41 49 45 58 68 75 80 86 101 107 120 131 140 150 158 174 183 193 249 250 280n 3 22 8 6 6 11 9 17 20 43 76 109 137 173 116 64 39 19 6 4 1 1
"-..1
42
62_!
61"
GO'"
z 1
1::.2
0 .J
I .
2~0z...:..····
z-~z ~·
-.:::::::;A-Z....-019
z
~I-
2~-z
""
z~~z~
rt:~
~b
CO:.z
56.,, ' I ' --..j se;=:='" u f (, rr r'> I ~ ,,.. , I~ . '
tr 10°
Fig .. 1.. Cruise track and statiOns=:,with:' RlV
"Michael Sars", 18 April. to 4 ·May 1979.. 1) Hydrographicar·· .• s.ta·:tions with CTD-sonde, 2) Pelagic trawl stations, 3) Bottom trawl stations.
62. -~
~
''
"~l~
~61° ~B
· ·· .Yo"
~ 4
~ ~;:!>
~b qj:·.z
~~
56., ' ' ' t v ( ( , rr ;...,. I 1:"1:::::> •' A I ,
10'" e· 6. 1.,.
12.
Fig. 2. Temperature, t°C
at
the surface.CO
62'" -~
~ .
>'(" ~·\ \ ,1;!0..
~
~
61"
60"
~B
~24vs.s
/ .
B.S/
8
~ 4
~~J!f'
~b CV:·z
. ~!
5 6 " ' \1 (!J.i rf f\, )?C" f":'? I
12" I I I I I I I
10" a• 6. .4" 20
Fig. 3. Temperature~ t°C at the·bottom.
62'"
61"
60"
~1a"'
-~~
2~~~
/ 50 ~\ 0 20
1 1
ioo
/#) --,
I , 0 300 '
I~
I 100
0
20
~--- 1
/ 50 100 . _,---300
0 .
f
~ 4
rt~~J!f'
~b CV:·z
56"~
1~2:.----~~----~·;---_;•==~lj~--~--~v~(~l~H~if~J,~>~--~Y?C~~;£2?~--~----__j
'< I ' I I10"' a· 6" 4" r
Fig. 4. Integrated echo intensity of blue whiting in cm pr. n.mile.
LD
- 10 ...
20 A
Q.l
0110
E
1::
QJ u
~~:j
n:700·.~ B · ..~ ~
30
20
10
18 22 26 30 34
length in cm
I
38
I
1.2
Fig. 5. Lengthfrequencies of blue whiting from A) the continental shelf and B) outside the shelf.
A n: 100
2 4 6 8 10 12 14 16 age in years
Fig, 6.Age-composition of blue whiting
.
from A) the continental shelf and B) outside the shelf •.
"" 11 -
62
•
..
~~~~ .
~ ~
~
~
0,3 102.7 95.3 1.1 61 •
6,8 145.9 122. E 3.8 117.6 124 •. 9
~~
.. r1
'24' 4 169,3 154. 4 104.5 124,9 131.3 2.0
• 11. '
c:..
60
31,0 231.3 132, 134.4 55.0 13. 1 142. 1 180,0 178,4 37. 1 ~;J
~~
•
,rj 59
11. 1 567.0
ll6.~_· f/
62' 1 242.2 10.9
• - 59,1 123, 3 0.8
~() ;E~.
...·~ ~
0.6 92.0 6.9
~ ~ ):;!~
.!..
·:.5 £'w~ t
1.1 6. 3 32.5
{!' ~ f
~
0. 1 3.6 13. 1 0,2 ,1~
d;/1(
0
12° 1 f' a• 6· 2p 2·
58
57
56
Fig. 7, Abundance of blue whiting in thousand
tonnes~
