CM_1989_H_16.pdf (1.099Mb)

International Council for

the Exploration of the Sea ICES C.M.1982/H:16 Pelagic Fish Committee

THE EGG PRODUCTION AND SPAWNING STOCK SIZE

OF THE NORTH SEA MACKEREL AND HORSE MACKEREL STOCKS IN 1988 by

S.A. Iversen (1), A. Eltink (3) , E. Kirkegaard (2), D.W. Skagen (l}

Abstract

During the period 5 April to 28 July 1988 the spawning· areas of North Sea mackerel and horse mackerel were investigated by research vessels from the Netherlands, Denmark and Norway. Based on the plankton samples and temperature observations obtained during this period the egg production and spawning st?ck size were estimated. The total egg production of mackerel (25 x 10 ² eggs) represents a spawning stock biomass of 37 000

tennes, which is about 20

%

less than the 1986 estimate. The total egg production of horse mackerel (87 x 10¹² eggs) represents·a spawning stock size of 110 QOO tennes.

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

'il.

2

. ~' ' . ···ru.-:·

~· IN'ffiODUCTION

During the period 1980-1984 the North Sea was suryeyed·. every .. summer to .~.~timate t~e· total mackerel egg··productioQ. · .+n 198~ ; ;it . was d~cided to car~y out. the egg surveys···?vei-~. second year. The~:~urveys :have in recent years been· carried ·out . ._. on- ·an· ICES pasis.:,, -wit.h,~.::. several .. nations

"'. participating, The egg survey · data from.19å8 ~{3-v~::been used·.-.for stock asse~~m~nt purposes:

bY

··the working ·g.~oups

on.

tila assessment .:. of mackerel

(Anm:1:. , .1:989a). and'· hot>se ;.ihacke:rei :-- (-1-\llori.., 198'9b).

'-./···· ' . '

~. ·-

2. M.ATERIAL AND METHODS '·

_., ... ,.

The egg surveys in the North Sea were carried out by Denmark, Norway and the Netherlands during the period 5 April - 28 .~uly 1988. The northern ,-.are&:··. (A) was surveyed by Norway. and D.~nmark .tq cover the ma.in spawning area of mackerel and the sout~ern. area (B) was suPy~yect·by the Netherlands

·to cover_. the main spawniilg . area Qf ho:rse mackerel and ·sole (Fig~ 1) . The u 'northern area. was. cov~red' fou~ ::times. and. the southern area five times' but

· both areas were :completely covered only three times . ~

The timing of the surveys are shown in Table 1 .

. · TP.e Norwegi'an. vessel used a 20 c·m Bongo net. The Danish and Dutch vessels used a Gulf III sampler. The,_ ;lllesh size for all .gears was 500 11m. An echo sounder on the sampler continously measured the sampling depth. This was monitored aboard the vessel and the sampling depth adjusted as. necessary.

To calculate the water voliline sampled each sampler was equipped with a flowmeter.

In the northern area (A) .the samplers were towed stepwise in the depths, 20, 15, 10, 5 m and just b~low the .. s~asurface {0.5m). The Bongo net was towed with a speed of about 3 knots for 5 minutes and the Gulf III sampler

at

^a speed of about 5 knots ·for 2 1/2 minutes. The plankton samplers were assumed to sample representatively the upper 22.5 m, in which all mackerel and horse mackerel eggs are assumed to occur after the establishment of a thermocline. The plankton samples were sorted onboard, and the sampling effort in the spawning area was adjusted according to the abundance of mackerel eggs.

... I_t is ".as;ffiun;ed ·:·,r:thåt sole eggs ~are. distributed in the whole water column.

:~ For, that reason i:.n··"the southe'rn area (B} the Gulf III sampler was towed at

;a, s.peed of B knots in oblique haul~ till about 5 m above the bottom in

··t·:,.

order to catch···the eggs of horse mackerel, sole and mackerel. The sampler

fished 3 minutes per 10 meter .depth, but in shallow waters where the haul duration was less than 10 minutes/ttt'Qre than ane haul was carried out.

Figs.2-7 show the stations sampled during each coverage.

The plankton samples were preserved_in

4%

formaldehyde. The mackerel eggs

".w~re.

·aged

·according to the state of development and the larvae in . . the.~ No.rwegian samples · were · measured to t):le neares t mm below.

The .data are worked up' 'to numbers of ... eggs produced per day using the formula .. for s tage l eg-gs (O I'". each of .. 't:Oe two species :

,..

Mackerel:-.

-·• ⁱ

Horse mackerel:

..

Ih' Time

ln Time

:::1.

6f Irl.' ':f~mperature + 7. 76 (Lockwood et al.,1981}

-1.6l ln ·~emperat~re +~7.71 (Pipe and Walker, 1987)

3 Where Time is the age of the eggs at the end of stage lB in hours and temperature is the temperature (° C) at 5 m depth at the station where the eggs were sampled.

'· D~ily egg production was'. e§timated as desc~ibed by Ivevsen and Westgård ,{ 1984) .. According to thi~ tile.tliod t}t~ ^c egg productipns per day' per" station ., . are . interpolated in te?

:-:-a

fine grid of rectangles o'iJer the surveyed area.

· -· .Usually the area int~gra~ed has been deline~~'~d :hY a polygon. Due to spawning close "to th'e Dut:ch coast it wa$ d;i.(ti~u~J~. ta·:·deli·ne:a"te this area by a polygon ... Th.e inte·grated area shou)..d ... the;e~9~e be delirieated by the coastline. To get as close

as

possible to the coastal line a very fine grid net was applied (100 x 100 rectangles i.e.- 4.8 x 6 natiticai miles), Fig.8. However, for presenting the distribution of egg$ a coarser grid net of 50 x 50 rectangles was applied. ~-

...: f-'

-,

The ·fecundity estimates used in 1988 were of potential fecundity rather than realised fecundlty .''Potential fecundity is the maximum·· number of oocytes which inight be spawned in th~.Gurrent sea,son, with no allowance for _·resorption (a tre-Sia) c:>r~· d~ nova forn{ation

<?f.

-oocytes. For potential fecundity it is assumed that the number of eggs destined to be spawned in a season is fixed as idehtifiable developing . oocytes prior to spawning

(Anon, 1988).

Fecundity estimates used to determine spawn:i,.pg stock si.z,e_,_ in 1988 were:

North Sea mackerel (Iversen and Adoff 1983) -:

Fecundity 560

(w~ight.(g))

¹

·

¹⁴

n=67

Fecundity = 1.35 x length (cm)3·6 n=67, Horse mackerel {Eltink and Vingerho~d, 1~~9):

~-. ':._::·p.

Fec-undity = 1653 eggs/gram female ·horse mackerel' in maturity stage 4

3. RESULTS

3.1 Distribution of mackerel eggs

The distribution of mackerel eggs ol;>served during. the .d-ifferent.coverages are shown in Figs.9-14. Only few eggs were observed ,;L.n the :two .first surveys (Figs.9-10). In late May- early Jtine the spawning was mor·e intense with the main concentrations ~b~er~ed ~ff the. Dutch coast (Fig.11). Later the center of spawning had shifted northwards and .westwards (Figs.12-13).

During the last coverage the spawnirig intensity was reduced and most eggs were observed east of Jutland (Fig.14(.-

C·.\

3.2 Distribution of horsemackerei eggs

The distribution of horse mackerel egg is shown in Figs .1.5-lS ... During the first coverage no spawning of horse inackerel occur.ed. The firs-t -.eggs were found in the second coverage in the- last week of April, but the egg production was still at a very low level (Fig.15). During' the third coverage the last two weeks of '• May, two main $pawning .,areas were observed, one along the coast from France to the Dutch estuary and the other off the Dutch Wadden isles. The temperature was.above 12°C in these areas.

During the fourth coverage the main spawning seems to have shifted to the German Bight. Ho.wever ane haul at the southern end of the station grid still contained a rather high number of eggs (Fig.17). During the last coverage at the end of July the egg production was low and spawning mainly located in the German Bight (Fig.18).

4 The main spawning area of. the North Sea horse mackerel..is located in the North Sea along the French, Belgian, Dutch and German coast. The spawning takes place in May-July.

3.3 ~stimates of egg production and.spawning stock size 3.3.1 Mackerel

The text tab le ." below. .·: ~hows the timing of the surveys.- and the applied reference dates for the e.stimated· daily egg production. The reference dates for the different surveys are not necessarily the mid date of the survey period. In coverage four and six the reference date given are based

Surve y Reference Egg . Raising Adjusted egg

12 12

Coverage Period date prod.xlO . ractor production 10

1 5.4-12.4 9.4 0.02 +

2 24.4- 5.5 30.4 0.03 ⁺

.3 16. 5.,..10. 6· 5.6 0.19 1.3 0.3

4 10.6-29 .6_;. 20.6 0.84 1.05 0.9

5 29;6- 9-7 4.7 0.43 1.05 0.5

6 9.7 ... 28.7. 12.7 0.02 1.15 O.l

on the timing of the Danish and Norwegian surveys respectively since they contributed the major part of the egg production. The first two Dutch .. survey$· _showed that a few mackerel eggs were observed as early as in April in the SOJ.lthern. part of·. the North Sea. Daily plankton samples collected from the central part of the northern North Sea, within the main spawning area in 1981-1983, indicated···that spawning in this area started late May (Iversen and ^LJØeA,' 1985). The surveys in May-July did not cover the total spawning area. To adjust for the production in the uncovered areas, the egg production estimates based on the four last coverages were raised by

the factors given in the text table above.

The egg production curve is shown in Fig.19. The curves for 1984 and 1986 are also give~. Skagerrak was not covered in 1984 and 1986 while the estimated. prqduction in the western Skagerrak was included in 1988. The main spawning ~riod has usually been defined as 17 May-27 July. During this period in:1988 the egg production, based on the curve {Fig. 19), was est'imated at .. 25 x 10¹² eggs which is about 20 % less than estimated in 1986. The egg production in April and early May is not included in this estimate. Howevert investigations in earlier years (for ex. Iversen, 1982) and the estimated egg production in area B during 1988 indicate that the , .. egg production ,Prior to mid Ma_y · is insignificant. The estimated egg

_production equåls a spawning stock of 37 000 tennes .

_The .: -decreas,ing . trend in the spawning s tock size over the last years is ..

shown in the text table on next page.

Table 2 shows the relative age composition, mean length and weight at age

fo~ mackerel caught•by Norwegian and Danish research vessels in the period 30 April to 7 ·July. Three different gears were used, gill net, hook and line and trawl. No adjustment for gear selectivity has been made.

5 .rotal egg production ·Total spawning biomass

19841 )' 78 X 1012 ^{' • ' •}

118 000 tonnes (Iversen et. al. , 1985) . 19-861

) 30· ^X 1012_ _-'~5 000 tonn es (Iversen et.al.~1987).

19882 ) 25 X 1012

37 000 tonn es

1) Skagerrak and southern North Sea·not included 2) Western Skagerrak and southern North Sea included

The age composition does not indicate any outstanding yearclasses. The proportion of fish 10 years and older, which is very high (41%), shows a decreasing trend during the sampled period. The observed age compositions may not, due to gear selection, represent the true age composition in the stock. This may particularly apply for the gill net catches.

3.3.2 Horse mackerel

The text table below shows the timi,ng of the surveys and the applied reference dates for the estimated daily egg productions. The reference dates for the surveys are not necessarily the ·mid-dates. For horse mackerel the reference date is the mid-date of the Dutch surveys in the southern area (B), since this area contributed·to the major production of horse mackerel eggs.

An egg production curve was constructed (:' .. Fig. 20 ) . The total production of stage 1 horse mackerel eggs was estmated at 87- x 10¹² eggs .

Cover age Per i od . Reference Egg~production

date x10¹²

1 5.4-12.4 9.4

2 24.4- 5.5 30.4 ⁺

3 16.5-10.6 24.5 1.4

4 10.6-29.6 24.6 1.5

5 29.6- 9.7 No estimate in

area B

6 9.7-28.7 23.7 0.2

r:-

~-' -

By applying 1653 eggs per gram female horse mackerel in ·maturity stage 4 and assuming a sex ratio of 1:1, the·. corresponding spawning · stock biomass is 105 thousand tonnes horse mackerel.: in maturity stage; 4. The increase in weigh t from ma tur i ty s tage 4 fish to spawning fish ^.i is abou t 5 % {El tink and Vingerhoed, 1989). Therefore, the spawning stock biomass of the North Sea horse mackerel is estimated·at 110 thousand tonnes horse macke:t>el.

4 DISCUSSION AND CONCLUSION

The area covered in 1988 was extended compared with the previous years.

For the first time the whole North Sea and · most of the Skagerrak were surveyed. The resul ts confirm tnat the main spawning of' mackerel is-. taking place in the central part of the North Sea. Less than 10

%

of the stock was observed to spawn outside the area covered in previous years.

It was the first time in 1988 that for horse mackerel a complete coverage by egg survey in the North Sea over the whole spawning season was carried out, which survey resulted in a total egg production of 87

*

10¹² eggs.

6 In earlier yea~s . Macer (1974) combined a number of surv~ys ·of different . years ·and calculated a total egg production of 323

*

10¹ eggs in the

·

1 English Channel and southern North Sea area based on combined egg surveys carried out in April-May 1962, May 1967, June 1962, July 1968 and

·~ S.e~tembep . 1~62. The ~gg. production in . the English · Channel west 1° E was .. : m~1nly below 100 eggs/m , while much higher concentrati.ons (over 100 to . . 5'·300 eggs/m²⁾ occured east of 1° E in the English Channel and southern North Sea. Therefore, the high concentrations of eggs found at the south:ern bo,rc,ier ( o,f. the 1988 egg survey were expected to decrease rapidly furth~r to the sout~w~st into the English Channel and the main spawning area of the North Sea·horse mackerel was expected to be covered in 1988.

It seems that the horse mackerel migrates northward along the French, Belgian, Dutch and German coast during spawning from the end of April till

the end of July. They probably migrate further north in August, where they are observed by the Danish acoustic surveys. The total estimated amount of horse mackerel over the whole area based on the Danish acoustic survey in 1988 was 153 thousand tennes (Anon., 1989b), which is higher than the estimated 110 thousand tennes from the egg survey in 1988.

The histometric fecundity estimate of 1653 eggs/gram female is used instead of the volumetric fecundity estimate of 1489 eggs/gram female (Eltink and Vingerhoed, 1989) because in future probably only the histo- metric fecundity will be ^est~mated and applied and because the two methods did not differ significantly.

5 . REFERENCES

Anon.,1988. Report of the m~ckerel egg and recruitment workshop. ICES C.M.1988/H:3 42 pp.

Anon.,1989a. Report of the Mackerel Working Group. ICES C.M.1989/Assess:11.

Ånon.,1989b. Report of the Working Group on the assessment of Pelagic stocks in Divisions VIIIc and IXa and Horse Mackerel. ICES C.M.

1989/Assess:19.

Eltink, A. and Vingerhoed, B., 1989. The total fecundity of Western horse mackerel (Trachurus trachurus L.). ICES C.M. 1989/H:44.

Iversen, S.A. 1982. Spawning stock size of North Sea mackerel in 1981.

ICES C.M.1982/H:7, 13 pp.

Iversen, S.A. and Adoff, G.R. 1983. Fecundity observations on mackerel from the Norwegian coast. ICES C.M.1983/H:45, 6 pp.

Iversen, S.A. and Westgård, T. 1984. Mackerel egg investigations in the North Sea. ICES C.M.1984/H:38, 20 pp.

Iversen, S.A. and Ljøen, R. 1985. The spawning and distribution of mackerel eggs in the North Sea related to the hydrography. ICES C.M.1985/H:37.

Iversen, S.A., Westgård, T., Kirkegaard, E., Eltink, A., Hopkins, P. and Walsh, M. 1985. The egg production and spawning stock size of the North Sea mackerel stock in 1984. ICES C.M.1985/H:38, 11pp.

I vers en, S. A. , Kirkegaard, E. , and Wes tgaard, T. , 1987 . Mackerel egg production and spawning stock size in the North Sea 1986. ICES C.M.1987/H:8, 12 pp.

Lockwood, S.J., Nichols, J .H. and Dawson, W.A. 1981~ The es·timation of a mackerel (Scomber scombrus L.) spawning stock size ··by plankton survey. J .Plank.Res· .•. 3:217-233.

Macer, C.T., 1974. The reproductive b~ology of horse mackerel (Trachurus trachurus L.). in the North Sea and English Chanliel. J.Fish.Biol.

(1974)6, 415-438.

Pipe, R.K. and Walker, P. 1987. The effect of temperature on development and hatching of scad, Trachurus trachurus L., eggs. J.Fish Biol.,31:675:682.

Table l. The timing of the egg surveys in the ~orth Sea in 1988

Cover age

Area Ship l 2 3 4 5 6

A Dana ( Denmark) 31.5-10.6 10.6-16.6

(north) Michael Sars (Norway) 16.6-29.6 29.6-9.7 9.7-14.7

Isis (Netherlands) 5.4-12.4 30.6-2.6 21.6-23.6

B KW34 (Netherlands) 24.4-5.5

(south) Tridens (Netherlands) . 16.5-26.5 20.6-29.6 18.7-28.7

Table 2. Relative age composition (%), mean length (cm} and mean weight (gram} at age for

mackerel caught by Norwegian and Danish research vessels in the period 30 April to 7 July.

Age

DATE AREA GEAR 2 3 4 5 6 7 8 9 lO 11 12 13 14 15+

30.04 48F3 gill net o o o o 3 9 4 13 lO 6 6 lO 3 3 33

03.05 49F3 ¹¹ o o o o 2 5 8 9 6 5 l 11 8 3 42

24.05 44F7 o o l 5 5 8 17 29 4 5 5 8 2 l lO

26.05 42F3 o o o l o o 3 4 3 3· 8 lO 13 8 47

27.05 43F4 hook o o o l l 7 ¹¹ 7 7 5 7 6 12 2 34

28.05 44F4 gill net o o 5 16 4 l 18 7 lO l 6 9 5 3 15

29.05 45F4 ¹¹ o o 3 12 5 7 8 11 5 15 11 4 5 l 15

02.06 44GO Trawl o ^{4 8 12 8 12 12 12 4 4} f3 ⁸ o o o

03.06 42F2 Gill net o o o 7 2 15 19 7 15 lO 2 2 7 7 8

10.06 41FO Trawl o l 3 15 7 5 6 7 9 6 lO lO 8 7 5

11.06 41F2 o ^{2 lO 11 lO 12 13} 7 5 3 5 7 2 4 8

12.06 43F2 o 5 15 12 11 22 15 lO 2 l l l 2 l l

13.06 42F2 o 4 7 8 16 30 20 7 3 o o ² o ^l 3

14.06 41F2 2 o ^l 3 6 4 4 5 9 17 lO 12 11 8 10

14.06 41F3 o 58 8 4 4 3 3 2 l 4 2 2 2 4

22.06 41F3 gill net o 3 14 23 .) ^':1. 14 14 9 3 o 3 3 3 3 6

01.07 45F2 o o 3 8 8 lO 27 lO 4 2 3 7 3 l 14

07.07 41F5 12 4 5 19 5 ¹¹ 6 12 5 7 l 2 4 l 7

Ave rage 5 5 '9 5 lO 12 9 6 5 5 6 7 3 15

Mean weight in gram 131 275 359 403 520 523 535 590 568 603 670 662 813 723 772 Mean length in cm 24.4 31.3 34.4 35.7 39.1 39.3 39.3 39.6 39.9 40.9 42.5 42.5 43.2 43.3 44.6

7

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... l - - - - r - 1 .

48

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41 1~

43 42

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30 29 28 27

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43

~2

41 40 39 38 37 36 35

34 33

J2 31 30 d9

28 27

Fig. 1 The area surveyed in 1988 by Denmark and Norway (A) and by the Netherlands (B).

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0^{-~~,;- -}---·--r---

0 ~o ^{o r}

o Jo o o l o o n 1 o o ^{()1 )11,) 1 1 ,,,,}

Fi.g. 5 The stations sampled by the Dutch, the Danish and the Norwegian vessels during coverage 4 (10.06-29.06).

o Jo o 6 o o o

59 o o

58 o o

57 o o

56 00

55 o o

54 o o

5 Jo o

52 o o

0 l Otl

l[

X X

o 1 o o o .l tl o _ _ L_

X )( X X

X

x x x x x x

X X X X

0:, Otl o 1 o o o 'l tl o l 1 Otl

w.; -' -

^~·---

i

60""

r~ J\

X X A X

ty- /

~"-~'-V ~/

~ 9 o o

c.~~-.._J--' _{58 o o}

: .. ~j""

2 ;1 ""

A N155oo

~~ 1

c

) o'

r<s=is)~

^{54 ~.l o o o o}

52 o o

/ - ·---~->

~ ^{/ ,~[}

^{~ -_./~}

~_::r ___

^{- r.J----}

r

5 l o o

r ()!)(l o

. ~) l o o

11 Il O V

o Jo o

Fig.6

o l o,, o 1 o tl 0 .\ O•J (} ~) !l ') (l i O"

The stations sampled by the Norwegian vessel during coverage 5 (29.06-09.07).

1 1 o o

o J u o 6 o o o -1

59 o o

58 u o

57 o o 56oo-u-

l

55 o o

5 4 o o

5 jo o

O l n n o 3 o o o 5o o o 7 o o o 9 o o l l o o

. . . "' " ^~ ' ' ~l""

}"v: '7}L.~ . ^{5 9 o o}

o l o o

X X X X X X X X X X

\_ y

~V./

58 o o

@f;( _ _'"'"

: -~ }'l .. ,

. . . ~ ^i0\1

. " . . : : :: ~:\ ~![

55""

xxxxxxx~:x~~

^{XX X}

~

. . . · .. -n .. """

) 3 o o

X: ^XX

:r· :o'?~"' .

4 •. ' ' .

· X X i< i< )(,..;

~ "V~

5JoH

r·

^{J X "'9}

52 o o ^{. 5:.! 00}

""".

'

O

1 - ) l o o

. - - -^{- r - · --~---}r··· - - -- . ~>000 Jo o

Fig.7

o ^{l o o} o ^l o o o .l o o ^{(J ~l} o Il o i l) (l o 'l o,, 1 1 ~ tl

The stations sampled by the Dutch and the Norwegian vessels during coverage 6 (09.07-28.07).

Fig.8

I)Jvu G l l ·• ,, ~-

l 'J 11

\;.li(

59 o o ~~.t-."

5 B c J

5 i 00 •

r

'" j(

_{'\_}

"" 1.

~ ~ 00

~jo o.

~ 2 V ·1

4

5 l ••

1

'·,J .. ) 5o o~ -

t111.' ^('l l {\ l) 0 .\ Il t1 (J ~~ 0 0 0 7 0 ll 0 9 0 0 l 1 f· l

---d' ---- ---'

t1'·

. J _ . - - - - - ----L (• G •"'

Jl •j . • .

J~ H · . . . . · )li - ~-~ ~ ;. , ,_)~~ ^:gl

. . ~l(!_

V

~' y (l))

5 .~ "J

l

-~

[ ' J.

11] ~ -~

. r> ~s~·l'

rf

-~:--2.f\. ~

l

ri -

^{r:m f..}

~~~

..

^~t

rr

j

J

... r- ^{:; t~}

\L

~~l

;~~Sl l

/1

^{~ .}

~ i o ll

!-t• (' .~

~ ~ r Cl

~ 4 tttl

'J ~ u ()

/c,

'"2r

~e~ """

'"-' -~ ~-

../

_/.1 [~1 ~ 1- ^{l ~ t.'}

)_

- i ) f l l o l 0•1 (J ¹1 o ·~ O i no o 9 )•J l i " '

The grid net used in the estimation of egg production per day per rectangel, exemplified by the data from coverage 4.

OJoo 01oo 01oo 03oo 05oo 07oo 09oo 11oo 60001¹---~~---~---L---~~~----~---~---L 6000

59 o o

56 o o _{56 o o}

55 o o _{55 o o}

54 o o

53 o o

52 o o

l

^{52 00}

5 1 o o 5 1 o o

·r---r---,-L---.---.---r---~---+, SOoo

o 3 o o o 1 o o

Fig.9

o ¹ o o o 3 o o o 5o o o ⁷ o o o ⁹ o o

Number of mackerel eggs produced per square m per day in each rectangle during coverage 1.

1 o o

o ^{.3 o o}

60oo~'---~~---~---~---L-~~r---~---J_---~

o 1 o o o 1 o o o .3 o o o 5 00 o 7 o o o 9 o o 1 1 o o

56 o o

55 o o

5 4 o o

53 o o

5 2 o o

5 1 o o

50oo-r---~---~~---.---r---.---.---+, 50oo

03oo 01oo Otoo 03oo 05oo 07oo 09oo 100

Fig.10 Number of mackerel eggs produced per square m per day in each rectangle during coverage 2.

o ^{Jo o}

6000~¹·---~~---~---~---~~~~--~---J_---+

o 1 o o o 1 o o o 3 o o o 5o o o 7 o o o ⁹ o o

5 9 o o

58 o o

57 o o

56 o o

55 o o

54 o o

5 1 o o

5 6 o o

5 5o o

5 4 o o

·~---~---~~---~---~---~---~---~ 50oo

o ³ o o o ¹ o o o 1 o o o 3 o o o 5o o o 7 o o o 9 o o

Fig.11 Number of mackerel eggs produced per square m per day in each rectangle during coverage 3.

1 o o

o Jo o

60001---~~---~---~---L~n-~----L---~---~·

o 1 o o o 1 o o o 3 o o o 5o o o ⁷ o o o ⁹ o o 1 1 o o

59 o o

58 o o ₅₈ o o

57 o o 57 o o

56 o o 56 o o

55 o o ₅ 5o o

54 o o

53 o o

52 o o

5 1 o o

5000-r---r---~~---~---.---~---.---+, 50oo

03oo 01oo 01oo 03oo 05oo 07oo 09oo 11oo

Fig.12 Number of mackerel eggs produced per square m per day in each rectangle during coverage 4.

o 3 o o o l o o o ^{1 o o} o ^{3 o o} o ^5o o o ^{7 o} o o ^{9 00 1 1 o o}

6 o ^{o o l}

5 9 o o

58 o o

ls s

^{o o}

57 o o 57 o o

56 o o ₅₆ o o

55 o o ₅₅ o o

54 o o

53 o o

52 o o

ls

^{2 oo}

5 l o o 5 1 o o

5000-·~---~---~~---r---~---~---~---~, 50oo

03oo O 1 oo O 1 oo 03oo 05oo 07oo 09oo 1 oo

Fig.13 Number of mackerel eggs produced per square m per day in each rectangle during coverage 5.

03oo 01oo 01oo 03oo 05oo 07oo 09oo 11oo

60oo~'---,rL---~---~---~~~~--~---L---~60oo

59 o o 59 o o

r - r - r - r--r-- r-- r -

·~~~

lo lo In lo lo li lo

,

11 h 'o 'o lo lo lo lo lo lo lo 11 !o lo lo ~

lo !o ⁱ o lo !o lo In lo lo lo lo lo li lo lo ~ ~ lo ~ o lo ^(l Il o o lo lo lo lo lo lo lo lo lo lo ^{~ ()}

re

11 11 ⁽⁾ .o o lo lo

la-la-

lO- _~ r--r-

~ !o ^iQ o o lo lo lo lo lo lo lo lo lo

~ ^ij !o !o ~ ^IQ lo lo lo lo lo lo lo lo lo lo lo ~ lo lo lo ~ lo

~ ~ o h v o 'o lo lo lo lo lo lo lo lo lo lo lo 10 la lo 10 !o lo lo

58 o o 58 o o

~ lo k1 lo lo b lo lo lo lo lo lo lo lo lo lo lo ~ 10 lo lo 10 lo lo lo

~ ~~ ~ ~ ~ ~~

L L

lo lo lo kl lo lo 11

lo lo lo lo i'l ~ ...- r--r-- ,...-r-- r - lo ID lo 10 lo ~

lo lo lo lo 10 11 lo lo lo lo 10 lO ~ lo

lo lo lo lo lo lo lo lo 11 lo lo lo lo 10 lo ~

57 o o lo la lo lo lo lo lo lo lo lo lo lo ID lo lo lo 10 57 o o

o o o o o lo 'O o o lo lO il ⁽⁾ o o

'O 'o ~ o lo lo lo o o o o 11 L lo :o !O :o 'O lo lo lo o o o lo lo 11 ⁱ O

IQ o ^lo o lo lo lo lo o o lo lo lo lo lo lo

r,

56 o o

L

!o_!o_ lo !o o o lo lo lo o o ^~ o 56 o o

o lo b b 'o ^{'O () '4}

11 1 lo lo ~ lo 'o ^]

lo 11 11 ~ ~ ~ ~ l

lo lo ~ Il 1 l_ o lo 0 ^It

55 o o lo ~ ^~ ,, _{!o lo lo ~) h 55 o o}

lo_

-

^{lo lo o lo k1 lo}

54 o o 54 o o

53 o o 53 o o

52 o o _{5 2} o o

5 1 o o _{5 1} o o

50oo-r---.---.~---.---.---~---~---+, SOoo

0300 o 100 o 100 0300 osoo 0700 0900 1100

Fig.14 Number of mackerel eggs produced per square m per day in each rectangle during coverage 6.

0300 0100 0100 0300 0500 0700 0900 1100

60oo~---~~---J---~---~~n-.~----~---~---T60oo

59 o o

58001--

57 00

l ?

)

5 6 o o

55 o o 55 o o

54 o o 54 o o

5 .3 o o

52 o o

5 1 o o

5000-·~---.---~~---.---,---r---r---T, 50oo

03oo 01oo 01oo 03oo 05oo 07oo 09oo 1oo

Fig.15 Number of horse mackerel eggs produced per square m per day in each rectangle during coverage 2.

OJoo 0100 01oo OJoo 05oo 07oo 09oo 1oo

60oo~··---~~---~---~---¹~~~----~---~---+ 6Qoo

5 9 o o 5 9 o o

58 o o

57 o o

5 6 o o 5 6 o o

5 5o o 5 5o o

5 4 o o 5 4 o o

:::l • •

5 o o o -·\-, ---..---.----iC...---,.---"T---.---..,---+-, 5 o 0 0

o Jo o o ^l o o o 1 o o o Jo o o ^{5o o} o 7 o o o 9 o o

Fig.16 Number of horse mackerel eggs produced per square m per day in each rectangle during coverage 3.

1 o o