CM_1985_H_38.pdf (1.681Mb)

I I , International Council for the Exploration of the Sea

-.f.:.!i- 1985/H: 38

Pelagic Fish Cttee

THE EGG PRODUCTION AND SPAWNING STOCK SIZE OF THE NORTH SEA MACKEREL STOCK IN 1984

Abstract

by

S.A. Iversen (1). T. Westgard (1), E. Kirkegaard (2), A. Eltink (3), P. Hopkins (4) and M. Walsh (4)

During the period 22 May to 17 July the spawning area of North Sea mackerel was investigated by research vessels from Scotland, the Netherlands, Denmark and Norway. Based on the 629 plankton samples and temperature observations obtained during this period the egg production and spawning stock size were estimated.

1) Institute of Marine Research, Bergen. Norway

2) Danish Institute for Fisheries and Marine Research, Copenhagen. Denmark

3) Netherlands Institute for Fishery Investigations.

Ijmuiden, the Netherlands

4) OAFS Marine Laboratory, Aberdeen. U.K.

Introduction

Since 1980 the mackerel spawning area in the North Sea has been covered several times each year during the spawning season to estimate total egg produciton and thereby the spawning stock size. Prior to 1980 the spawning area or parts of it was covered once during June/July (Iversen 1977, 1981).

In 1982 (lversen and Eltink. 1983. Hopkins and Walsh, 1983) and in 1984 these investigations have been carried out as a joint venture by several countries.

Material and methods

During the period 22 May to 17 July 1984 the spawning area in the North Sea was covered six times by the Dutch ("Tridens"). Danish ("Dana"), Scottish ("Scotia") and the two Norwegian vessels ("Michael Sars" and

"Eldjarn"). Table 1 shows the periods the different vessels worked t area.

The stations for each of the six coverages ore shown in Figs.1-6. During May. "Tridens" worked the area south of 57 N rith the main sampling effort in the area between 55°30'N and 57 N. The second and third coverages were made by three vessels in six and four days respectively.

The fourth coverage was carried out by two vessels during ten days and the two last coverages were done by one vessel in 14 and 6 days respectively. The total effort of the five vessels during the period of the investigations was 75 survey days. The different vessels worked the area with different plankton samplers (Table 2). The samplers worked stepwise for five minutes at each of the depths 20. 15. 10. 5 m and just below the sea surface,and were assumed to sample representatively a water column depth of 22.5 m . A mounted sounder on the sampler continously measured the sampling depth. This was monitored aboard the ship and the sampling depth adjusted as necessary. To calculate the water volume sampled each sampler was equipped with a flowmeter.

The plankton samples were preserved in 4Z formaldehyde. On some of t. ~

vessels the plankton samples were analysed a couple of hours after collecting while in others they were stored and analysed after the survey. The mackerel eggs were aged according to the state of development and the larvae were measured to the nearest mm below.

In this investigation et.al., 1981), including to estimate the daily egg and 18 egg were estimated et.al. (1981)

the mackerel eggs in stage 1A and 18 (Lockwood the stage of the blastodisc formation. are used production at each station. The age of stage 1A according to the formula given by Lockwood

ln Time= -1.61 ln Temperature+ 7.76

Where Time is the age of the egg at the end of stage 18 in days and Temperature is the temperature of the sea surface where the eggs were sampled in degrees celsius. Temperature was measured on each plankton station.

Daily egg production was estimated in two ways as described by Iversen and Westgard (1984) and Pope and Woolner (1984). In the first method the egg production per day per station are interpolated into a finer grid of rectangles over the survey area. and confidence limits estimated empirically. In the latter method the mean number of eggs spawned per day

per m are estimated by rectangle and raised to the area of the rectangle 2

to give a total production estimate. The variance is estimated using the assumption of constant coefficient of variation as suggested by Pope ^an~

Woolner (1984).

Spawning area

During the first coverage relatively few eggs were observed (Fig.1).

Fig.2 shows the distribution of the eggs during the second survey. Eggs were observed over a much wider area than during the first survey. The Skagerrak was also partly covered during this survey. It seems from the egg distribution obtained during this coverage that the total spawning area in the North Sea was covered. The main concenffrations

ot

eggs were

o~served i& the area delinated by the coordinates 57 30' to 56 North and 2 30' to 5 East.

During the third coverage (Fig.3) the main egg concentrations were observed in the same area as during the former survey. However, the distribution pattern indicates that the total area was not covered during this survey. In the next survey (Fig.4) the main concentrations of eggs were observed to the north and south of those observed in preceding surveys. Skagerrak was not covered duri&g this survey. The main egg concentration was observed north of 57 north during the fifth coverage (Fig.5). It seems that the investigated area .especially to the north did not cover the total spawning area. A few samples were collected in the entrance of the Skagerrak, but no eggs were observed. During the last survey very few eggs were observed (Fig.6). Due to lack of time only the most important part of the spawning area was covered. By this time it seems that the spawning has more or less ended. Therefore during this survey the spawning area was probably fully covered.

Total egg production

The estimated average daily egg production based on each of the six surveys is shown in Table 3. As in earlier years the production in late May and in July was observed to be rather low. Peak egg production took place in June. Therefore the seasonal pattern of spawning in 1984 seems to be similar to that of recent years. Since 1982 the spawning period in the North Sea has been defined as the period 17 May to 25 July (lversen and Eltink. 1983, Iversen and Westgard, 1984). This is based both on the surveys and daily plankton samples collected at fixed positions in the Cod - Ekofish area in May - August during the years 1976-1983.

Egg production estimates using the method of Iversen and Westgard (1984) are given for each survey and for all surveys combined in Table 3a together with data on area coverage. As mentioned earlier the first, second and last surveys are supposed to cover the spawning area properly.

The second survey covers the largest area. The third. fourth and fifth surveys covered areas which were 22

Z.

8

Z

and 28

Z

lesser than the area investigated during the second survey.

If the following two conditions are fullfiled:

1. There were no major changes in total spawning area during the period 5 June- 11 July (coverages: 2,3,4 and 5).

2. The average distribution pattern of eggs over the sampled and unsampled parts of the spawning area were the same.

Then the egg production estimates based on the third, fourth and fifth

I ,J

I

survey can be adjusted by the above mentioned percentages.

such adjustments the total egg production is estimated at 78 The Skagerrak and Kattegat are usually not included in these Earlier investigations indicate that the contribution from may be approximately 10 1. {Iversen, 1973).

By ffplying x 10 ^eggs·~

estimates.

these areas

By applying the fecundity weight relation given by Iversen and Adoff {1983) and a sex ratio of 1:1, the spawning stock is estimated to about 120 000 tonnes {Table 3a). This is a reduction of about 50 X compared to the 1983 estimate {Iversen & Westgard, 1904). According to that paper the 95

Z

confidence limits of the egg production estimate is ^about~ 30

X.

In addition variance on the fecundity is unknown and thereby the 95 X confidence limits of the spawning stock estimate is greater ^than~ 30 /..

When the method of Pope and Woolner {1984) was applied the data from all the cruises were pooled and the spawning season arbitrarily divided into 6 x

io

^{day periods.} Within each period the arithmetic mean number of eggB/m /day was calculated for each statistical rectangle ^{1° longitude x 1/2 latidude), and raised to the area of the rectangle and pe od duration {Fig.7). These raised production estimates were summed over all rectangles and periods to estimate egg production over the whole spawning season, see Table 3b. The 951. confidence limits of the production estimates in sampled rectangles in each survey are also given.

Production in unsampled rectangles was estimated by interpolation. One possibility was to interpolate from adjacent rectangles but since there does seem to be some spatial structure in the distribution of spawning this might have been misleading, particularly since priority was given to those rectangles where production was expected to be high. It wa:s thought preferable to consider production in any one rectangle through time, and to linearly interpolate missing values from hauls made in the same rectangle in adjacent time periods. In effect, production curves were calculated for each rectangle, zero values being assumed prior to the first period and subsequent to the last period. Where a rectangle was unsampled throughout the spawning season,no interpolation was possible and egg production in that rectangle was presumed zero. This should not present too many problems if high production rectangles were given priority in sampling, because those which are never sampled ^L Jht to be those which contribute relatively little to the production anyway, and are likely to be on the periphery of the 1ipawning area. The method gives a total production estimate of 80 x 10 eggs and includes the interpolated values for the first and last periods. It is probably wrong to include the interpolated values for these two periods due to the observed egg distribution during the first and last coverages. If these are subtracted the total estimate is 72 x 10^11. eggs. The estimate is based on the upper 20m of the water column, while the · other method ^~s based on the upper 22. 5m . The adjusted estimate is then 81 x1

o'l.

eggs.

1This demonstrates that the two applied methods for estimating the total egg production give rather close resuls.

4

Reference&

IVERSEN.S.A. 1973. Utbredelse og mengde av makrellegg (Scomber scombrus L.) og zooplankton i Skagerrak og nordlige del av Nordsj0en i arene 1968-1972. Thesis, Univ. Bergen. 77pp.

[Mimeo.]

1977.Spawning. egg production and stock size of mackerel (Scomber scombrus

L.)

in the North Sea 1968-1975. ICES. C.M.

1i11/H:17 [Mimeo.]

1981. Spawning and trends in spawning stock size of North Sea mackerel during the period 1913-1980. ICES. C.M.1981/H:16 [Mimeo].

IVERSEN.S.A. and ADOFF.G.R. 1983. Fecundity observations on mackerel from the Norwegian coast. ICES. C.M.1983/H:45 [Mimeo.]

IVERSEN.S.A. and ELTINK.A. 1983. Spawning, egg production and stock size of North Sea mackerel in 1982. ICES. C.M.1983/H:46 [Mimeo.]

IVERSEN.S.A. and WESTGARD.T. 1984. Mackerel egg investigations in the North Sea. ICES. C.H. 1984/H:38 [Himeo.]

LOCKWOOD,S.J., NICHOLS,J.H. and DAWSON.W.A. 1981. The estimation of a mackerel (Scomber scombrus L.) spawning stock size by plankton survey. J.Plankton Res. 3(2). 217-233.

POPE.J.G. and WOOLNER, L. 1984. An investigation of the prec1s1on of the 1983 Western mackerel egg survey. ICES. C.M.1984/H:70 [Mimeo.]

WALSH,M .. HOPKINS,P. and RANKINE.P. 1983. Results of the North Sea mackerel egg surveys in 1982. ~ C.M.1983/H:49 [Mimeo.]

Table 1 Time and coverages of the spawning area by the different research vessels.

Coverage Research

I I I

vessel ¹ 2 3 4

Tridens 22.-31.5 5.- 6.6 13.-14.6

Dana 4.- 9.6 10.-14.6

Scotia 4.-10.6 10.-13.6 16.-21.6

Michael Sars 15.-25.6

Eldjarn

I

⁵

I

⁶

27.6-11.7 11-17.7

Table 2. Type of plankton samplers applied by the different vessels.

Applied

Research Mesh filtration

vessel Type of sampler size efficiencies

Tridens Gulf I I I 500 _IJ 1

Dana Gulf I I I 500 _IJ 0.83

Scotia High Speed Lock Ewe. 500 IJ 1 Michael Sars Bongo, 20 cm 500 _IJ 1 Eldjarn Bongo, 20 cm 500 _IJ 1

Table 3. Production estimate 1984

a) The Iversen and Westglrd ⁽¹⁹⁸⁴⁾ method:

Time Egg_~foduction C~ver:g area Adjusted egg

- 1 2

x10 m x10 production x10

22-31.5 0.56 94 0.56

4.6-10.6 2.58 200 2.58

10-14.6 2.25 164 2.75

15-25.6 1 . 8 0 185 1 . 9 4

27.6-11.7 0.56 156 0.72

11-17.7 0 . 11 36 0. 11

Total egg 72

-

⁷⁸

production Spawning

stock 1 0 0 0 tons 109

-

^{11 8}

b) The Pope and Woolner (1984) method:

For sampled rectangles:

Pro~~ttion No.replic. Coefficient No.of 95~ conf.

Period x10 rects of variation rects limit(+/-)

20-29.5 4 . 21 1 4 0.69 2 1 001.

30.5-8.6 1 6. 2 7 33 1 . 9 3 52 851.

9-10.6 18.37 47 1 . 4 2 6 1 581.

19-28.6 7.62 29 1.26 33 751.

29.6-8.7 6.64 39 0.85 48 64~

9-18.7 0.74 1 4 0.49 1 8 31~

Overall 53.85 1 7 6 1.22 ^30~

Interpolated component:

Period

Pro9~~tion

x10

20-29.5 5.87

30.5-0.6 4. 9 0

9-18.6 3.24

19-28.6 8.08

29.6-8.7 2.03

9-18.7 2.33

Overall 26.44

Total production estimate = 80.2832 x 10 ^{1 2} eggs

02oo o 1oo oooo 0 1oo 02oo 03oo 04oo 05oo 06oo oroo 08oo 09oo 10oo

60oo+---~-L----~----~L---~----~----~----~--~r-T---~----~----~---+6000

8

5900 5900

5800 5800

0 0 0 IS 0 0 0 0 0 0 0 0 0 0

5roo + + + + +++ + + + + + + + ^5roo

2 I 2 s -f'$1!~1 ~ ^I

+ + + + +

0 0 0 06 10 ol i54 0 \40 0 0 0 I

5600 + + + ++ + + + +++ + + + + ⁵⁶⁰⁰

0 0 0 I 0 0 0 0 0 I 0 I

+ + + + + + + + + + + +

5500 5500

0 I I I 0 I

+ ++ + + +

5400 5400

0200 0100 oooo 0 1oo 0200 0300 0400 0500 0600 Oroo 0800 0900 IOoo

Fig.1. The stations grid and distribution of stage ^1A and 18

mackerel eggs produced per square metre per day during the first survey, + = Tridens.

02oo o loo oooo o 1oo 02oo 03oo 04oo 05oo 06oo oroo o8oo 09oo 10oo

60oo+---~-L----~---L---~---L----~---~~r-+---L----~---L---+60oo

5900 5900

I 2

* *

2 9 14 2 0 i5

* * * * * * ₁₀

4 0 0

2i5 *

5800

* * * 2 6 liS * i5 i50 ⁵⁸⁰⁰

* * ^{* * **}

I 1Si5 0 0 0 12 i5 2 16 i5 I 0 I 412 22

* * * ** * * * ^{* *} * * * ^* * *

0 0 0 0 0 0 s s 2 112 0 0 ₀ 0 0 0 0 5 12 I liS

5roo t. ₀ t. ₀ t. t.t. t. _{0 0} t. _2S t. t. t. t. t. t. t. t. t. t. t. * * ^{** 5roo}

I So IS I 146 10 29 24 I 9

t. t. t. ^!; t. t. t. t. t. t. t. t. t. t.

0 0 0 I 2 6 IS n2i5 1n 11 liS i56 IS

t. t. A t. t. t. t. t. t. t. t. t. t. t.

ID 0 I I 2 2 0 i5 ISSI 4 4 0 i5 IS

A t. t. t. t. t. t. t. t. t. t. t. t. t. t.

i5 '16 161 so s 10 Si5 i5o n2 0 0 0 i5 i5

5600 t. ~ t. t.t. t. t. t. t. t. t. t. t. t. t. t. 5600

6 0 2i5 0 0 0 I i5i5 s s 110 i5

+ + + + + + + + + + + + +

5500 5500

0 0 0 2 i5 4 i5 I ol I

+ + + ++ + + + + +

5400 5400

0200 0100 oooo 0 lOO 0200 03oo 0400 0500 0600 Oroo 0800 0900 1000

Fig.2. The stations grid and distribution of stage 1A and 18

mackerel eggs produced per square metre per day during the second survey, + = Tridens,

*

^{= Dana,}

" ⁼

^Scotia.

5900

5800

5(00

5600

5500

6 4

* *

55 165

* *

10

*

5 I 4 7

* * * *

;a ^~s i ~⁰~o i5;

12 27

* *

10 -,

* *

0 0 0 0 I 10 B B 201448 11 I 0 I I 57 2 7 I 5

'* * *** * * *** ** * *** ****·* -, 127 29

0 0

A A

0

A A

5 11!8 5

+ + +

0 I 2

A A A

I I 6

A A A

I 5 .of 2

++ .,f, A

o I

A A

102 &i 2 19

+ + + + + ⁰

+ 0

A A A A

I 6 5 19 4 58 29 75

A A A A A A A A

I 5 6 9 2

A A A A A

2 251 0 0 0 I 2

A A A A A A A A

I

+

2 I

++

14 I 7 I

+ + + +

+ 5 + ²

21 A

10 A

19 25

* * * ^{2 6}

*

5900

5800

5(00

5600

55oo

54oo+---~--~-.---.---.---r---.---.---.---.---.----~---.----~54oo o2oo o ; oo oooo o ; oo o2oo o3oo o4oo 05oo 06oo oroo 08oo 09oo 1 ooo 1 1 oo

Fig.J. The stations grid and distribution of stage 1A and 18 mackerel eggs produced per square m~tre per day during

the third survey, +

=

Tridens,

* =

Dana, A= Scotia.

02oO 0 loo OOoq 0 loo 02oo 03oo 04oo 0500 0600 Orae 0800 0900 lOco

60oo+----n~---L---L----~---~---L---~~r-~----~---~---L---t60oo

5900

58oo

5100

5600

5500

0 A

15 10

A A

15 40

A A 5 6 15

A A A

59 20

A A

9 A

I

A 0

A 0 A 0 5 9 IB 1229 46 56 22 2 !I B 0 5

A A A A AA A A A A A A A A

I :S

A A

0 0 I 0 0 0 I :S :S :S 0 0 I I I I I 2 2 1:5 6 I I B

A A A A A A A A A A A A. A A A A A A A A A A

I A

2 A

6 A :S 4 I

A A A 0

A 0

A 2

A I A

:s

A 2 2 6 59 12 11 I 2

A A A A A A.A A

:S I I 4

A A A A

1 5 15 1rn -, 4 n :ss:s 4 n :s 2 5 0 0 0 00 0 0 0 0 0 0 0 0 0 0

n 1 2s :s 2 0 0 0 00

0 0

B 0 12

0 55 0 0

5 0 2 1:526 2 0 0 0 0 0 0 0 0

I 0 0

0

0 0 0 I 2

0 0 0 0 0

0 0

0 g 6' 0 ⁶ 0 ^I 2 0

I 0

I 0

0 0

6 0

4 5 0 0

51 2

0 0

0 25

I 0

0 05 0

0 !Oi!O

I 0 ~ I

0 0 0 0 6 A

5900

5800

Sloe

5600

5500

54oo+---.----~.---,---,---.---.---,---,---,r---.---~---+54oo

Fig.4.

02oo 0 loo OOoo 0 loo 02oo 03oo 04oo 0500 06oo Orae 08oo 09oo lOco

The stations grid and distribution of stage 1A and 18 mackerel eggs produced per square metre per day during the fourth survey, A = Scotia. = Michael .Sars.

02oo 0 1 oo oooo 0 1 oo 02oo 03oo 04oo 05oo 0600 Oloo 08oo 09oo 1 ooo

60oo+----n~---L---__ _ L _ _ _ _ _J ______ ~ ____ _ L _ _ _ _ _ _ L_~r-~----_J---~---L---+60oo

5900 5900

16 "" ^{;s ;s 0}

0 0 0 0 0

I 0 "" ¹⁸ ' ^{;s 21 ;s I 0}

5800 ^{0 0 0 0 0 0 0 0 0 0} 5800

"" ⁰

0 _SJ "" ₀ ^{I 0} 0

0 0

00 0 ^{I 9 91 19 ;s I I} 0 0 0

DO 0 0 0 0 0 0 0 0 0 0 0

2 2 2 0

0 0 0 0

0 0 0 0 I 2

' ^{2 2 2 2 I I I I I 0 0 0"1 2}

5(00 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 OD 0 5(00

0 0

0 2 2 I 2 0 ^{;s 2} "" ^{9 0 0 2}

0 0 0 0 0 0 0 0 0 0 0 0 0

0 0 I 0 0 0 I ;s 18 5 I "15 ^I 0 0

5600 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 5600

2 0

I"' I I 0 0 0 0 I 0

DO 0 0 0 0 0 0 0 0

0 0

0 0 0 0 0 0 0 I I 0

55oo ^{0 0 0 0 0 0 0 0 0 0} 5500

5 4 o o + - - - . - - - - i - . - - - . - - - . - - - . - - - . - - - . - - - . - - - . - - - . - - - £ T - - - t 5 4 o o

02oo 0 loo OOoo 0 loo 02oo 03oo 04oo 05oo 06oo Q(oo 08oo 09oo 10oo

Fig.S.

5900

5800

5(00

The stations grid and distribution of stage 1A and 18 mackerel eggs produced per square metre per day during

the fifth survey, o = Eldjarn.

0 0 0 0 0 0 I 0

0 0 0 0 0 0 0 0

5900

0 ;s 6 0 ^{;s I} 0

0 0 0 0 0 0 0

0 0 0 2 2 2 ;s 2

0 0 0 0 0 0 0 0 5800

0 I 0

"" ^{I 0 2 I}

0 0 0 0 0 0 0 0

0 0 ² 0 I I I I

0 0 0 0 0 0 0 0 5(00

~~---.---.---.---.---,---.---.---+5600

03oo 0200 0 lOO OOoo 0 lOO 0200 0300 0400 0500 06oo

Fig.6. The stations grid and distribution of stage 1A and 18 mackerel eggs produced per square metre per day during the sixth survey, o

=

Eldjarn.

1L

0 2 00

6 0 00

JO

59 00

30

58 00

h

JO l)

57 00

JO

56 00

0 1 00

l!

I>

30

I\

\ \

55 00

-

0 0 00

0.0 0.0

0. I 0.2

1 .2 0.5

0.3 41.5

0.8 4 0. 6

2.0 2 6. I

!i.O 2 6. 1

129.3 6 2. 8

0 1 00 0 2 00 0 3 00

0.0 0.0 1. 9

0.0 0.0 3.7

32.7 44.7

305.6 55. 1

1 4 7.1 25. 1

3.4 4 7. 7 130.3

4. 1 44.6 1 48.6

1. 8 8 9. 4 2 2 8. 6 2.7 1 I 9. 7 20.8

9.6 12 5. 8 I 6 4. 5

1.1 41.3 26. J

14 4. 3 8 5. 9 J 7 9. 9

5.9 2.7 3. I

3 3. 2 1 8 9. 9 21 6. 4 2 9. 3 I 0. 9 3 4. I

1 41.5 11.7 4 7. 0

1.3 0.6 0.3

4 4. 7 0.3

I 11.7 0. I

0 4 00 0 5 00 0 6 00 0 7 00 0 8 00 0 9 00 1 0 00

\ ^{m :0 --- [}

?

·Yt(

.I

45.2 1 6. 8

!;t J

22.6 42.1 ~·V

1 3 8. 6 59.8 89.3 26.4

1 9 0. 2 2 9. 7 9 6. 3 25.8

I

57.1 2 4. 3 9.9 118.9 1 4. 9

J

2 7. I 1 9. 4 2.6 7 5. 2 7.

~~ " f-

2 9 9. 5 1 70.2 70.3 7 4.3

4 9. 8 2 4. I 8.3 20.9

-

f- 2 4. 7 32.9 7 4. 4

I 7, 7 24.6 3 8. 9

("'\ ,_

107.5 153.6 2 5. 9 2.8 "'

I 0 7. 4 1 53.0 3 7. 7 4.2

,(\.

..

-'\

I 0 0. 0 41 .3 4 9. 4 9.5

1. 8 21.6 40 .o 1 4. 3 1!-U/

f-

5.2 7.2 8.4 1 4 4. 8

(1>\

7.5 9.7 12 4. 7 21 4. 4

\

^{0.3 0.8 o.o 0.0 1 7 1 7. 5 .o 2 7. 2 44.3 3 6. 4 4.5 21.3 2.5 10.6 4.8 155. 1 1 6 1. 6}

^~

^{,.... ;::}

JO

54 00

'\

_,J;;l',.-

\'-.

~~

_"' - -

- )

_/)

^v?·~~

^f-

I I T -I I I

JO

53 00

0 2 00 0 1 00 0 0 00 0 1 00 0 2 00 0 4 00 0 5 00 0 6 00 0 7 00 0 8 00 0 9 00 1 0 00

Fig.7. Egg produ~}&on by ICES statistical rectangle.

eggs x 10 .

Upper figure: sampled component

lower figure: unsampled component (interpolated)