,
,"
. .' '.
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CM 1995/J:12
Inernational Council for the
Exploration of the Sea
HERRING STOCK LENGTH-AT- AGE STRUCTURE
IN THE EASTERN BALTIC
by
M. Fetter and ANaglis
Latvian Fisheries Research Institute
•
6, Daugavgrivas st., Riga, LV-1 007, Latvia
Mean length-at-age of herring collected from commerclal catches in Subdivision
28 in 1964-1993 was analysed in the aspect of Hs annual and long-term
fluctuations. Annual changes of herring length showed the tendency tor decrease
of length-at-age from the beginning to the end of the year for all age groups except
age group 1. In tho most cases the minima of mean length-at-age were observed
in the quarters 3 and 4. Relationship betvJeen fluctuations of herring length-at-age
durlng the year and the share of gulf herring In the sea was revealed tor nge
groups 3-7. This relationship was espeeially pronouneed in those years when the
amount of gulf herring in the sea was quite large. Long-term fluctuations of herring
length-at-age were found to be elosely eonneeted with stock size of herring in U,e
Gulf of Rlga especially in the last years 'Nhen the intenslty of gulf herring migration
towards the sea was high due to large stock number and worsening of feeding .
conditions in the Gulf of Rlga.
Introducti on
Changes of Saltie herring growth were analysed and generalized in frames of the
proJect Growth Changes of Herring In the Saltle (TemaNord 1994:5:32) headed by
H. Sparholt. In that Report the results of long-term observations concerning
changes of herring size-at-age from the end of 19703 till the beginning of 19903
were presented and diseussed in detail tor all Saltic regions. Different hypotheses
dealing wlth the eauses of herring slze-at-age changes were dlscussed in that
paper, Le., migration ehanges, influenee of hydrologieal eonditions, ehanges in
feeding eonditions, the efteet of herring stock size on its growth, the influence of
cod stock and some other faetors that could lead. to strong changes in herring
size-at-age. Summlng up the main eonelusions of that Projeet it can be sald that
the observed decrease in herring size-at-age may be explained most adequately
by the changes in the size selective mortality of herring caused by the reduced cod
stock.
..'
Our paper presents a more detailed conslderatlon, than It has been made earlier,
of seasonal and long-term fluctuations of herring length-at-age in Subdivision 28
as regards one aspect, i.e., migration of smaJler herring from the Gulf of Riga to
the open sea. Identification of open sea herring and gulf herring by otoliths is
rather dUficult slnce dlstlnctive features of otoliths overlap to a certaln extent.
However using a large series of observations the general tendencies can be
exposed.
The main aim of our report is to deiTlonstrate seasonal and long-term fluctuations
in herring length-at-age as weil as to reveal the significance of possible influence
on them by gulf herring migrations because gulf herring is an inherent component
of herring catches in Subdivisions 28.
Material and methods
Data on herring for the period of 1964-1993 were used in this paper. Sampies of
herring were collected from commerclal catches in Subdivision 28. Every month,
with few exclusions in some years, 200-400 fishes were taken from commercial
catches belng then examlned by standard methods. Herring age and Its
populational identification were determined by otoliths. Herring length was
measured by Smith. One month was chosen as the minimum of an averaged unit.
Data on stock size of herring in the Gulf of Riga were taken from the Report of the
Working Group on the Assessment of demersal and pelaglo stocks in the Baltic
(ICES CM 1995!Assess:18).
•
Results and discussion
Annual changes of herring length-at-age.
Multi-annual fluctuations of monthly mean values of herring length and the range
of its changes for all herring age groups are presented in Fig. 1 and 2. r..~ean
values of length and its minima and maxima are shown in Table 1. Characterizing
time-series of herring length fluctuations as a whole (Flg 1, 2) It can be noted that
the least minima occurred beginning trom 1987 tor age groups 3 and older. For
age groups 1 and 2 the least minima were observed in the period before 1970.
High maxima tor practically all age groups were more oHen recorded during 19781985 und 1966-1971.
Yearly length ranges tor tish of the same age changed from year to year. It can be
noted that high values of yearly length ranges for age groups 3-10 were found
more frequently after 1978 than before that year.
Seasonal fluctuatlons of herring length-at-age are characterized by great variety.
Fig. 3 shows yearly distribution of length maxima for age groups 1-9. Natural
•
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scheme of herring growth implles that fish rihould have Its maximum length in the
end of the year at least in .quar.ter 4. However, as Fig. 3 demonstrateo it io
impossible at all to determine nny definite month for the length maxima of age
group 2. Insignificant concentration of the length maxima for age group1 can be
noted by the end of year. Yearly distribution of length maxima tor age group 3 cti!1
has a weak tendency to grouping fn quarters 1 and 2. As regardo age groupo 4-9
their length maxima were mainly reglotered in the first halt of the year. However~
the curves of herring length changes have several peaks during the year (Fig 4,
A). Mean length of age group 5 showed its maxima in April and July in 1970 und
1971, unlike 1991 and 1992 when length changes were of a smooth character but
mnxlma were recorded In February - March, and In August (1992). A tendency for
yearly deerease in mean length of age groups 2-7 was espeeially distinctly
pronounced when averaged data were considered (Fig 4,8). Natural annual
increment of length was observed only for age 1.The rest of age groups
demonstrated decrement of thelr length in quarter 4 as compared with quarters 1
and 2 (Fig. 4, B). As It was ShO\Vll earlier (Naglis, 1981), during 1972-1977
maximum mean length was observed in February-March, then it decreased in
April and was undergoing small changes till December. Mean values of herring
length for a whole period of observations (1964-1993) demonstrate that there is a
real tendency towards decrease of length from its maximum values in February April to minimum ones in August-Oetober tollowed by insignifieant inerease ot
length in Deeember (Fig. 5A). Suffieient differences in the pattern of length-at-age
annual fluctuatlone are evidently obeerved between eeparate age groupe and
years. Fig. 5 presente annual f1uctuations of length averaged by three-year
periode bet.. veen whieh there were differenees both in annuallength ranges, and in
mean values of length. Winter or spring maxima of herring mean length-at-age
cnn be explained by the tact thnt concentrations of fishes are formed necording to
tt1e degree of their preparation for spawn during pre-spmvning period. The bigge~t
individuals are as a rule the first to spawn in each age group. They form
concentrations of greater density that are the object of fishery in February - April.
The lessening of length-at-age in summer can be explruned by the fact that herring
starts to feed actively after spawn, and big individuals do not gather in separate
choals but dieslpate over teeding areas among the rest of fishes.
However, in Subdivision 28 smaller gulf herring appears in catches beginning from
June-July. It leaves the Gulf of Riga after spawn and stays in the open sea
sometimes till the end of the year. Summer migration of gulf herring from the Gulf
by tagging. Still our
of Riga to the open sea
. was unfortunately not investigated
.
data as regards populational composition of herring in Subdivision 28 testify to the
fact that herring wlth low growth rate and otoliths typlcal tor gulf herring
(Ojaveer 1981) was encountered in .catehes almost permanently though its share
J
changed conciderably from year to year and from month to month during one year
(Flg. 7, 8). In the period of 1964-1993 the mean share of gulf herring did not
exeeed 10 % in eatehes, durir;Jg 1976-1978 it was less than 5%, in 1979-1981 it~
share was negligibly small but during 1991-1993 the share of gulf herring reached
40% (Oetober 1991-1993, Fig 8). In 1981-1986 its share was about 1 % per year
(Fig 7). Oecurrence of smaller gulf,herring in mixed eatches obviously affected the
mean length-at-age making it less._ However, this effeet was not revealed equally
tor different age groups. To describe thls fact quantitatively the eorrelation
eoefficients were calculated betwe~n monthly mean length of different age groups
and the share of gulf herring in the open sea for three periods: 1979-1981
eharaeterized by smaJl amount of gulf herring in the sea; 1991-1993 charaeterized
by large amount of gUlt herring in the sea, and 1964-1993, I.e. the whole period of
observations. Despite quite large values of eorrelation eoeffieients (Table 2)
bet\veen herring mean length and the share of gulf herrjng we still consjder annual
length f1uetuations to be determined at least by 1'.\'o taetors. Coneentration of
larger flshes in pre-spawning period causes the length maximum in February' April, sometimes in May. By July - August after spmvning big fishes do not already
form dense concentratlons and therefore their mean length decreases in catches
even in years when the share of gult herring in the sea is very small (Fig. 5 and 6,
period of 1979-1981). In years of large amount of gult herring in the sea this
lessening of mean length js more pronouneed (1987-1989,1991-1993). In the
period of small amount of gulf herring in the sea the eorrelation coefficient3 ure
suffieiently tower, espeeially for age groups 2 and 3. Analogous annual ehanges in
herring length-at-age were described tor western part of the Gulf of Finlund und
neighbouring open sea regions (Ojaveer, 1988).
Hence one of the reasons tor periodic nnnual tluctuntlons of herring length-at-age
in the sea are undoubtedly the migrations of open sea herring to the Gulf of Riga
tor spawning and opposltely directed migrations ot gult herring in the second half
ot the year.
Due to the decrease of herring length-at-age by the end of the yenr annual
absolute increments (em) caleulated as the difference betl,'veen length in quarter 4
and that In quarter 1 of the same year have negative values (Flg. 9A). More cr lees
oatiofactory resulto ....i ere obtained when ineremento were ealculated as differences
of herring length be1'.veen quarter 1 of the eurrent yeur and quarter 1 of the next
year, taking into neeount that the influenee of gulf herring was the least in thooe
quarters. But even In thls case negative values of length Inerements were gained
for soma age groups in several years (Fig. 9B).
Leng-term f1uctuatiena ef herring length-at-age.
Long-term fluctuations of herring length-at-age for the period of 1964-1993 are
presented in Fig.10 nnd 11 as deviations from the mean valueo (1964-1993). A
clearly pronounced synchronous character of deviations is noted for age groups
•
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- - - -
,
I
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:
I
,
,
,
',.J
3·7. Table 3 shows the results of couple comparlsons of quarterly mean length
deviations for age groups 1-7. ·during the whole period of observations (19641993). Commentlng on the resultG of comparlsons It should be polnted out that
synchronism of length deviations is most pronounced for fishes of two sequent
age groups and It weakens with the lncrease of age difference. Age group 1 ~md
partly age group 2 (quarter 1) are the exclusion. Their length fluctuationo correlate
very weakly wlth those of other'age groups. Age group 1 10 not representative
and therefore in this case it is sem~eless to discuss its size characteristics.
•
•
As regards age group 2 fluctuatlons of its length In quarter 1 evldently reflect the
eHect of some other factors differing from those for older flshes. As it w~m above
consldered the Influence of gulf herring on seasonal tluctuatlons of herring lengthat-age in the sea is revealed quite noticeably, especially in the years whcn Ho
share Is growing. Long-term changes In herring length-at-age are also expected to
be dependent on the amount of gulf herring migrated to the open sea. Quarter 1
demonstrates the least degree o! this Influence, but, quarters 3 and 4 show the
greatest degree (Table 4). In its turn the amount of gulf herring in the open sea
during 1970-1993 happened to be'Closely related with herring stock size In the Gulf
of Riga. Correlation coefficients between the share of gulf herring in the sea and
Ito stock olze In the Gulf of Riga are r=O.70 tor quarter 1; raO.SO for quarter 2;
r=O.S8 for quarter 3 and r=O.S9 for quarter 4. It was expected to be evident that
herring lengt~at-age f1uctuations In the sea ought to be also strongly connected
with stock size in the Gulf of Riga (Table 4). In reality we may speak on strong
correlatlon only for age groups 3-6 conslderlng thelr length for quarters 2-4 (Table
5). Fig. 12 demonstrates long-term dynamics of herring length-at-age in the sea,
the amount of the gulf herring In the sea and Its stock number In the Gulf of Riga in
the period of 1970-1993. The possible reasons for intensive migration of gulf
herrlng from the gulf to the open sea are the Increace of Itc stock and related with
it worsening of food supply tor o/der age groups in the Gult of Riga during last
years (1987-1993). Thls can be testifled by the decrease of Its length and welght
(Kornilovsi 1994) and by quite sharp decrease of Limnocalanus grimaldii
abundance that Is a very Important food Item for older age groups of gulf herring In
the Gulf of Riga (Kornilovs, 1992; Sidrevics et al. 1993).
Thus basing on oUr data we assume that herring length fluctuations in Subdivisio~
28 are determlned to a high extent by herring stock size in the Gulf of Riga, food
supply In the gulf and connected wlth thls Intemslty of migration to the open sen.
The above said is referred mainly to f1shes of older age groups (3-6) in the second
half of the year, when their feeding condltlons worsen due to consumption.
Summing up the considered facts we may conclude the following:
1. Seasonal and long-term fluctuations of herring length-at-age In Subdivision 28
have a clearly pronounced synchronous character
tor age groups 3-7. As long as
gulf herring Is a permanent component In mixed catches the Intenslty of its
migration from the Gult of Riga.~o the open sea affects quite strongly the length-atage fluctuatlons ot herring In Subdivision 28.
2. The least influence of gulf herring was revealed for quarter 1, the most one
occurred tor quarters 3 and 4.
i
3. In resent years (beginning from 19B7) the observed intensive migrations of gulf
herrlng to the open sea are evldently caused by the worsenlng of Its feeding
conditiono in the Gulf of Riga due to ito high stock number.
4. Despite certnln sUbjectivity and difficultles of npplico.tion, the method of herring
populational identification uoing otoliths fits in this case and is quite useful for
studles ot herring stock structure. Furthermore, by our oplnlon, It Is deslrable to
develop this method in future.
References
1. Anon. Growth Changes of Herring In the Baltic. Final Report. TemaNord
199.4:532.
•
2. Anon. Report of the Working Group on the Assessment of demersal and pelagio
stocks in the Saltic. ICES C.M. 1995/Assess:18.
3. G. KornlJovs, V. Serzlnsh, L. Sidrevlos. The analysis of mean welght-at-age
changes of Saltic Herrlng in the Gulf of Riga. ICES C.M. 1992/J:24.
4. G. Kornllovs. Yeo.rly length distribution of herring In the Gulf of Rigo. in relation to
populational structure of the stock. ICES C.M. 1994/J:9, Ref.H
5. Naglls A.K. FIshing and blologlcal characteristlc of the Baltic herring in Ventspils
zone of the Saltic Sea'. - In: Rybokhozyaistvennye issledovaniya (8aItNIIRKH),
Rlga: Avots, 1981, Issue 16, pp.64-70. (In Russ.)
6. Ojaveer, E.,Ev~ukhova, B., Rechlin, 0., Strzyzewska, K.. Results of
investlgatlon of population structure and otoliths of 8a1tlc spring spawnlng herring.
ICES, C.M. 1981/J:19, B.F.C.
7. Ojaveer, E.. 8o.ltlo Herring
HAgropromizdo.t 1988.(in Ruos.}
(biology
o.nd
fishery).
Moskva
VO
H
,
8. L. Sidrevlcs, R. Une, V. Berzlnsh and G. Kornilovs. Long-term changes of
zooplankton abundance in the Gulf of Rlga. ICES C.M. 1993/L:15, 61010glco.l
Ooeanography Committee.
•
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Tabla 1
'.
Minima, maxima and mean values.of hen1ng length-at-age In Subdlvlslon 28 for
period 1964-1993 by quarters
Age
Quarter
1
•
2
3
4
groups
1
2
3
4
5
6
7
10.6
12.9
15.4
16.2
16.7
17.0
17.9
14.4
16.7
18.7
20.2
21.3
22.1
22.7
13.0
-15.4
17.1
18.1
18.7
19.5
20.0
Min
Max
Mean
Min
Max
11.4
14.0
14.7
15.5
15.9
16.9
17.4
,14.7
17.3
17.9
19.6
20.6
22.0
23.0
Mean
"13.2
15.2
16.5
17.6
,18.4
19.5
20.0
Min
Max
Mean
Mln
Max
Mean
12.3
13.7
14.5
15.2
15.8
16.6
17.7
14.6
16.4
17.6
19.3
20.2
20.6
22.2
13.4
16.1
16.3
17.2
17.9
18.6
19.4
.13.1
.13.8
14.2
15.1
15.4
15.6
17.4
15.8
'16.8
18.1
18.9
20.1
21.7
21.2
14.0
15.5
16.6
17.5
18.1
18.7
19.2
Table2
Correlatlon coefflclents between monthly mean values of hen1ng length and the
share of gulf herring
,' "
',I
Agegroup
Per10d
I
..
1979-1981·
"
1991·1993**
2
-o.Z7
3
-0.58
-0.89
',: -0.82
4
-0.69
' "-0.90
'5
6
..
-0.70
' -0.75
~
'.
.
~.,
"
1964-1993
:
-.
.,. -0.81
"
''''-0.65
-0.72
-0.80
, ..,
\
". the share o' gulf hening was small, **. the share o' gulf hening was !arge.
-0.90
-0.94
-0.85
Table3
Correlation coefficients between mean length deviations of different age groups
wlthln one quarter 10r period 1964-1993
Quarter
Age
Agil
group
group
1
1
2
3
4
2
3
4
5
2
0.34
3
-0.18
0.50
4
0.11
0.55
0.84
5
0.13
0.42
0.73
0.93
6
0.14
0.46
0.64
0.85
0.86
7
0.16
0.34
0.51
0.72
0.79
2
0.25
3
0.23
0.82
4
0.22
0.76
0.93
5
0.22
0.82
0.85
0.92
8
0.30
0.59
0.72
0.78
0.83
7
-0.04
0.45
0.60
0.67
0.66
2
-0.14
3
-0.02
0.80
4
0.08
0.76
0.89
5
0.24
0.71
0.84
0.91
6
027
0.58
0.75
0.83
0.86
7
0.23
0.33
0.59
0.71
0.70
2
0.45
3
0.19
0.76
4
0.15
0.63
0.89
5
0.03
0.54
0.86
0.94
6
0.19
0.45
·0.70
0.81
0.86
7
0.29
0.46
0.59
0.88
0.88
for r> 0.47 p:O.01
6
•
0.92
0.71
•
0.78
o.n
----------------1
Table4
Correlation coefficients between herring mean length-at-age and the share of gulf
herring (for perlod 1964-1993) and stock slze of the gulf herrIng In the Gulf of Riga
(for period 1970-1993) by quarters
Aga
•
The share of gulf herrlng In the sea
Stock slze In the Gutf of Riga
lJ'OUP
q1
q2
q3
q4
q1
q2
q3
q4
1
0.30
-0.30
-0.08
-0.04
-0.64
-0.06
-0.22
-0.04
2
-0.36
-0.53
-0.64
-0.49
-0.50
-0.62
-0.60
-0.55
3
-0.64
-0.76
-0.78
-0.70
-0.53
-0.73
-0.75
-0.72
4
-0.61
-0.75
-0.80
. -0.84
-0.65
-0.80
-0.83
-0.80
5
-0.59
-0.70
-0.79
-0.85
-0.58
-0.79
-0.78
-0.84
6
-0.63
-0.65
-0.78
-0.82
-0.63
-0.72
-0.69
-0.74
7
-0.53
-0.50
-0.51
-0.71
-0.59
-0.62
-0.51
-0.54
8
-0.49
-0.65
-0.32
-0.54
-0.51
-0.60
-0.63
-0.39
9
-0.65
-0.70
-0.22
-0.58
-0.52
-0.50
-0.17
-0.55
I
I
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I
I
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I
I
,
I
I
I
I
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I
I
•
•
• min
0 max
:;1.:
mean
Fig 1 Herring length-at-age fluctuations in the period of 1964-1993
. (age groups 1-5)
cm
;;1
r age~ ~ l' 9 'I
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1
90
92
age 10
68
70
72
74
76
• min
78
0 max
80
82
84
86
88
~ mean
Fig 2. Herring length-at-age f1uctuations in period 1964-1993
(aga groups 6-10)
t
•
12
10
.r:: 8
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0
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00.
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•• •
0.
0
63
x
65
~
69
•
0
•
X
85
ff1
89
91
93
• ••0
•
•
::t::
::t::
••• •
XDO
OX
00 •
• •
0
X
• :t::
71
83
• age6
0
•
x
81
79
••
0
0
)()(
x age4 o age5
12
10
•
•
73
1:1
0
• •
X
0age8
0
••
0
75 77 79
::t::SIiJ87
•
::t::
.00
81
83
x
DX
85
87
89
91
93
.age9
Fig 3. Yearfy distribution of length maxima for herring age groups 1-9
•
.
.'
A
.
om
21
"
T
20
19
18
.......
17
16 )~.
• •
• •••• j," • • • • •••
'~""""~(""";J(..
• •• :l:'
.~.
.~.
: : : ... ' .
a.
•
,
• •
,
....
..•.
'v
• •••• " •• 'J:: •• " .• ~( .'
::1
1
':*:" •• " ....... , . '
.....
2
4
3
5
6
7
9
8
10
12
11
month
•
•
79 -----80 ---tr-70 - 0 - - 7 1
'91
;r... "92
B
cm
20 T
I
19
t
18
17
16
15
14
13
12
1
2
3
5
'--_---lI q 2 - - - q 3
6
7
age groups
- - ., - • q 4
Fig 4 Annual changes of herring length (A is mean length of age group 5
in different years. B is mean Jength of age groups 1-7
averaged by quarters tor period 1964-1993)
om
20
19
T __
r
age6
~5
1ej== ~4
17 } - - - 16
-----====--
age 3
tI
__----
~
---------
_--
-------------____
age 2
I
15
A
_~ "
t
__-'
----
-------
...-----
----
-----
14 l - - + - - - - t - - - ' - + - - - - t - - + - - - - t - - - - + - - - - t - - + - - - - t - - - - i
•
•
Jan
Feb
Mar
Apr
May
Jun
-',i:-76-78 -.,.k---79-81 ••
0 •
Jul
'87-89
Aug
Spt
Oet
-~·t--91-93
Nov
Dec
---64-93
Fig.5 Annual changes of herring mean length for age groups 2 and 3
averaged by t11ree-year periods (A - mean for 1964-1993)
21
•
20
1
~5
/"-
1
::P~6?Y~
I
,
17 I
1---
•
•
~
16+
I
15 I
14
~---1
'0- • - 00 •
.P
0- - -0.
- .. 0
~
~..
...._ _..---.----.."~./'"
~.
-.
J----+---....--_I----+-----+---+----+---t----+----+---I
cm
-:t-76-78
Fig.6
--.*k----7~81
- - 0 - • 87-89
-~.I--- 91-93
----64-93
Annual changes of herring mean length tor age groups
4-6 averaged by three-year periods
0/0
35
30
25
20
15
j\
10
-/
5
o Q~~-t--t---t--1-1i~::;:S:~~~~~~~:=:'-t-1--1
~M~Q~~~~n~~~~nn~~~~~~~M~M~~~~~
-:t:-
1q
--0-
2q
•
3q
4q
Ito
•
Fig. 7 The share of gulf hening (Oft» during the period 1964-1993
0/0
40
35
... .....
t)
30
.
25
. .
.
....
20
15
.
....
10
•
• • 1::>.
(> ••
..
.
\
\~
..... ~
-----=---::--~-_\
.
(>
o~~-":;:==~---Ak--'=====+:===*===~~==~==~~
Jan
Feb
Mar
-X-76-78
Apr
Ito
May
Jun
Jul
79-81" .. o· ... 87-89
Aug
Spt
•
Oet
Nov
Dee
91-93 - - - 64-93
Ag. 8 AnnuaJ changes of the share of gulf herrfng by three-year periods
•
A
,
"
ID~I
1I
•
age3
."
1
age6
L
0""
I
t
·2 t
-1
·3
64
66
68
70
72
74
76
78
80
82
84
86
88
90
92 93
Fig. 9 Yearly inerements (em) from quarter 1 to quarter 4 of the same year
(to beoonl)
8
(contlnuatlon of Ag 9)
age2
4
;I
O~,L:.;;J..,t:o;J..,,t:o;J..,.&;;;;:,41:;;;;1.,.1;;;;;;I..JL:;;;1.J=.,I;;:;;;J,.1~:::.:J.tLl;;;41:::::1.,J:o~~~~~~~~;&'+=~4=~~"="
4
•
age3
3
2
1
O~.w..~~=...L~J..l;i;;~L.J;:;;J...w..,.l.;.i;J.JI;;;;;;4I;;;;;4I;~-.J,;.;;;~~4J;;;;;;L.w.J..,.t:;;;;4L::;J..,1~~~~r-
-1
4
3
2
age4
1
O~.,J:.;;;;L~,J.;;;;;1,~~I;;;;o;4J=oI;;:;;;J,.1~=-,,;;;J..,I:::il.,L~~:T-~J.-~4=~"=
'=''U:l:>:J'O~='
-1
-2
4
3
age5
2
1
OFL,J:;:.:.I,..-,J.:.:.1,.~~"""4I.:;;.l,.1.:;;J,..L;:;J,...-.l.:~~~~~~~L,.L:;;J~_~=,J:..:.J.,="'_-.'=a,
-1
-2
4
age6
3
2
1
0
-1
-2
64
66
68
70
72
74
76
78
80
82
84
86
88
90
Fig. 9 Year1y Increments (cm) from quarter 1 to quarter 1 the next year
92
•
Quarter 1
I
,
,
I
,
,
,
,
I
t
I
I
I
t
I
I
I
I
1.21
1.11
Quarter 3
1~:/?"~~~~~~~
*
0.91
I
0.8
-,o--+-.--+-,-...--+-,-+1-+-1-+I-+-'-+.--+-._.-+-._,>--+-1
4-1-+.-+-.-+.-+-.-+.--+-.-+'-+-1_,>--+-l
, -+'--0---<1,
64
1.2
1
I
1.1~
1~
0.9
O~8
Quarter 4
I
1
II
~
I
I
~
J
,
~
J
I
J
J
m n
I
I
~
- D - - age2
I
I
n
•
I
J
I
~
J
00
age3
I
I
I
~
I
~
•
I
I
,
M
I
M
I
I
00
I
I
f
~
age4
Fig 10.Deviation tram mean length (1964-1993) for age groups 2-4.
1.2
T
I
1; /~.~~
,.~
1.1t·
k'/-':--- '
0.9
Quarter 1
+I
0.81
I
I
~
I
00
I
I
I
I
I
I
ro n
00
I
!
i
I
~
~
I
i
I
~
I
I
i
~
00
I
i
~
i
j
i
I
~
00
I
I
;.
~
00
Quarter 2
•
I
I
I
I
I
I
I
I
I
I
I
I
I
I
,
I
I
,
•
I
I
I
I
•
,
I
I
I
1.2 T
i
!
1.1
Quarter 3
h./'::~f=:a::U~.d~
1
0.9 t
I
0.8
J
,
~
1.2
I
I
I
~
00
I
,
I
I
I
ro n
t
I
I
I
n
~
,
I
n
I
I
I
I
~
00
T
I
I
i
i
~
M
I
,
I
I
I
~
00
M
t\
I
~"-cl
!
~
,A",',
D-..
•
~I
...................
a=:.
D-o.\.
1 ~~.
-r;
'"
. .(
~ ...----.~._
,
'
...- - - " . .
•
u'- --
1.11
Quarter 4
!
0.9
0.8
tI
•
-
'.
"
',Ä,
"
\...~
'
I
I
~
I
I
I
00
I
00
fIt
I
ro n
--{]--- age 5
•
I
I
~
I
~
•
I
I
n
I
I
00
I
I
~
I
I
I
~
I
00
I
I
~
I
I
00
~-*
,
I
J
~
age6 -41.""'-- age 7
Fig 11 . Deviation tram mean length (1964-1993) tor age groups 5-7
•
..
....
1::
Ci)
c:
~
c:
1IS
G
E
70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93
1 - mean length of age groups 3-6 in quarter 4
2 - stock number (age groups 2-10) of gulf herring in the Gulf of Riga
3 - the share of gulf herring in the sea in quarter 4
Fig. 12 Long-term dynamics (deviations from mean values) of herring length-at-age,
the amount of gulf herring in tlle sea and stock size of Ilerring in U,e Gulf cf Riga