This paper not to be cited without prior reference to the authors

International Council for C. M. 1992/ J: 7 the Exploration of the Sea Baltic Committee

Food Consumption of and Sprat in the Baltic Sea.

By

F. Arrhenius & S. Hansson

Department of Systems Ecology, Stockholm University, 5-10691 5tockholm Sweden

ABSTRACT

We used a bioenergetics model together with data from leES to estimate the food

consumption of the·two major zooplanktivores, sprat and herring, in th Baltic Sea.

The annual food consumption was 5.0*107 tonnes for herring and 4.7*107 tonnes for

sprat. For herring about 90% of the food consumed was and the rest

mysids and benthos and for sprat 100% zooplankton. Larvae and young of the year,

of sprat and herring made up almost 40% of the total consumption and the peak

occured in August-September. Due to high abundances of young fish, sprat

consumption was six times higher than earlier estimated. Growth of herring may be

sensitive to the energy density of their prey. Simulations on herring with a fixed

consumption of prey biomass, but with a diet of zooplankton only instead of a mixed . diet, decreased the growth rate by about 25% forolder age groups.

1 ------l

Introduction

Two of the most important commercial in the Baltic are herring (C/upea harengus L.) and sprat ( sprattus L.). They are mainly zooplanktivores and may consume a considerable proportion of the zooplankton produetion. Caleulations of this consumption in the Baltie have been made by Thurow (Thurow 1980, Thurow

1984) and Elmgren (Elmgren, 1984), who tried to estimate total biomass, production and yield in the Baltie from eateh statistics. More precise calculations for older age- classes were made by Aneer (Ancer, 1980) using total production from VPA analyses and mean monthly weight per age. Until now fish production and consumption estimates havc not taken the first two years into account except for Elmgrcn (1984) who made roughly estimate on the age group O. Young ofthe year fish may, however, • be of great importance to the total consumption of a fish population, as showed by the freshwater clupeids pseudoharengus. Hewett and Stewart (1989) estimated that almost 50 % of total populations consumption aceounted for by larvae and young-of- the year.

Direet measurements of food consumption by fish is difficult. Field estimates of food consumption, derived from stornach cornents and evaeuation rates are often highly variable and require extensive effort. Therefore, energy budgets and energetic models, in combination with field data on fish growth and water temperature are important tools to predict food consumption (Rice and Cochran, 1984). Combined with data on, population size, age composition and diets, the bioenergetics models yield food consumption by the populations.

In this article, we apply this approach by using a bioenergeticss model for herring, developed by Rudstam (1988), together with growth rates, diets and fish abundanees, far herring and sprat in different parts of the Baltic Sea.

!;

2

.;

I:'I \ Material & Methods

The model - The bioenergeticss of the average individual Baltic herring have

been modeledby Rudstam (1988), based on a approach described by Hewett and

Johnson (1987). We used this model to simulate seasonal growth and consumption for

metamorphosed fish of different herring and sprat populations during the first nine

respectively seven years of life. For larval stages, we applied a Kl-value or gross

conversion efficiency of30% (Checkley 1984, Ki~rboe and Munk, 1986). For

modeling we considered sprat to be a yearling herring (Rudstam et al., 1992).

Diet - Seasonal and length dcpendent changes in the diet from the Baltic Sea

composition for herring was then assumed for four different size-groups, and for each

size-group diets were specified for four seasons winter, spring, summer and winter

(Table 1).

The energy density of herring vary seasonally between 5120 and 9440 J/g wet

wt for herring (Aneer, 1975a). The zooplankton prey were assumed to have a constant . . energy density of 2850 J/g wet wt (Laurence, 1976, Rudstam, 1988)~ The energy

.. content of mysids increased from 2976 J/g wet wt in juveniles to 3720 J/g wet wt in

adults (\Viktor and Szaniawska, 1988), and for the amphipod POfltoporeia sp. we • used a standard value of 3980 J/g wet wt for the period Gctober-March according to Hill et aI. (1992).

Temperature - The water temperature is a major factor in determining the

food consumption rates of fish. In the bioenergetics models the effects of temperature

on fish is dome-shaped with a maxi~um at un optimum temperature. The temperature

used are the average top 20 m for four different stations (Figure 1) in the BaItic with

3 ~ ,... one series for adults maximum of 12 oe and one serie for the first one and a half years of life with a maximum in July of 16 oe (Figure 2).

Studyareas - Differentiation of Baltic herring into subpopulations, especially delimitation of areas occupied by different subpopulations, has always been problematic (Ojaveer, 1989). For this study we decided to divided the herring stocks into eight units according to major difference in growth rates of different populations

(Figure 3). The sub-divisions correspond to the lCES statistical rectangles (Figure 1).

Abundance were compiled from several reports (Anon., 1987, 1991, Hagström et al.,

1991) and are presented in Table 2-4.

In the lCES reports, the Baltic sprat stock is divided into three management units: leES sub-divisions 22-25, 26+28 and 27, 29-32 (Anon., 1991). In our analysis only one population of sprat was used, since the growth rates were similar. •

Abundance and Martality - The abundance estimates for age-classes ~ 2

(Table 2,4) were used to calculate survival rates, assuming a constant instantaneous or daily mortality between observations Nt=No*e-(Z*t), where Nt and NO are population numbers at time 1. and Q respectively, and Z gives the mortality rate.

All populations were assumed to mature at age 3 and spawning date was set to

1 lune, except two southernmost populations of herring, which were given 15 April.

Abundance estimetes for the first two years are very unpredictable, due to scarcety of information [rom these life stages. \Ve run several simulations with • different mortality rates, always starting each subpopulation with a specific number of fry (Table 2-4). Mortality rates were adjusted to give the average abundanccs of age 2 at 1 lanuary given by ICES (Table 2-4). First we ran simulations with different mortality rates for early life stages between 95 and 85 % per month of larvae and secondly between 2.5 and 7.5 % for age 1. The simulations were also run to adjust fish abundence of about 3000 fisheslha at the 1 October during the first year (S.

Hansson, unpubl.).

4 . . , • Modeling Results and Discussions

Our estimates of annual food consumptions were 5.0*107 tonnes for herring

and 4.6*107 tonnes for sprat (Table 4-5),this is about 6.2 g C m-2 yr-l and 5.8 gC

m-2 yrl respectively (carbon content is 5 % of ihe wet weight). This is at least twice

as high as other estimates for herring (Aneer, 1980, Thurow, 1980, 1984, Elmgren,

1984), and six times higher for sprat (Elmgren, 1984). A closer lock at the data by

Elmgren (1984) show that biomassvalues are similar for all populations except from

the BaItic proper. This difference explain to a large extent his lower values for food

consumption of herring. The much higher estimates ofconsumption by sprat is partly

explained by high abundances of young age-grops, which give high food

consumption. • For herring about 90 % of the total food consumption consists of zooplankton, mainly mesozooplankton (Figure 5). This value is much higher than that used by

Aneer (1980) and Elmgren (1984) and is explainedby our use of data from offshore

populations, while Aneer (1980) and Elmgren (1984) based their calculations on data

from coastal popu~ation, which Aneer (1975b) found more mysids and benthos in

their diet. There are few values for the average zooplankton production in the Baltic.

Elmgren (1984) used a value of23 gC m-2 yrl, but this is high compared to other

investigments (i.e. Henroth and Ackefors, 1979).Using Elmgren (1984) values, sprat

and herring consumes half the average zooplankton production. This is high • considering that there are also other species that feed on zooplankton. Other prey than zooplankton constitutes only about 10% of the food

consumption for herring. The proportion was small for youngcr ages of herring, but

constituted up to 50 % during shorter periods for older fish. The Pontoporeia sp.

made up 1.5*106 tonnes, and mysids for 4*106 tonnes annual food consumed (Figure 7).

5 , . • Almost 10% of the total annual zooplankton consumption by the Baltic Sea .

herring and sprat may be attributed to the larval fish (Figure 5). Larvae and young of

the year of OOth herring and sprat together ingested almost 40% of the total

consumption of zooplankton. Yearling herring accounted for 15% of the population's

zooplanktivory, whereas all adult year classes combined accounted for 45%. Yearling

sprat accounted for about 25% of the population's zooplanktivory, whereas adult

sprat accounted for 35%. This may explain to some extent the high food consumption

for sprat, but the sprat biomass are lower than the biomass for herring for age groups

~ 1. Since the size of young sprat is smaller than that of herring, the abundance ot the

fonner is higher although the biomass are similar.

The consumption of zooplankton by the herring and sprat populations was

seasonal and peaked during August-September (Figure 7). This peak for sprat was

later than what was found for in the southern Baltic by van Khanh et al. (1972). Their

data supported that most intensive feeding took place in April-July and that feeding

was moderate in August-Getober. However, in their investigation only the gut

fullness were taken into aecount and not the evacuation rates. Planktivory have been

stated to be responsible for declining the zooplankton biomass in late summer at least

in eoastal waters (Hansson et al., 1990, Rudstam et al., 1992)

The contribution of young of the year to the zooplanktivorywas very high

from September through Getober (Figure 6). This was mueh due to lower eonversion

efficiency, compared with larvae. This also explain the switeh in food consumption from August for sprat, by a high conversion effeeiency of larvae (30 %) to a lower • (16.0 %) for young of the year. This was not as clear for herring, due to different

spawning dates for different subpopulations. During late autumn and winter the food consumption decreases. This is the result of slow growth, low temperatures and partly a shift in diet to prey with higher energy density.

6 r-----~------

, ,

Different assumptions on the mortality rates had dramatic effects on

estimated food. consumption of herring and sprat, especially for yearlings (Figure 8).

The most drastic effect occured by .. shortening the larval stage period and thereby .

increasing the YOY period. This results in an increase in the food consumption of

abaut 70 % for the whcle Baltic populations. Lowering the mortality of larvae,

resulting in more individuals reaching metamorphosis, increased the food

consumption by 41 % for the whole Baltic. Consequently, estimates of food .

consumption of herring and sprat in the Baltic, are sensitive to assumptions of

abundance, growth rate and mortality rates for larvae and YOY, These valuesare still

not weIl known and need to be better investigated.

The growth rate of Baltic has declined drastically in the Baltic

(Hagström et al., 1991). This reduction is Iarger for older herring than for younger

(15% for age 2-4 and 25% cf age >4), and Kostrichkina and Oyaveyer (1982)

suggested that such decreases in growth could be the result of decreased proportions

of large prey organisms in the diet of herring. To test this, we estimated the volume of

prey eaten by herring when the diet followed Table 1. We then replaced all non­

zooplankton prey with the same volume of zooplankton, resulting in a decreased

energy intake due to the lower energy density of zooplankton. This resulted in a

significantly decreased growth rate (Figure 9), similar to that described by Hagström

et al. (1991). One possible explanation to the decreased growth of herring could thus

be that low oxygen concentrations in battom waters have reduced the proportion of

large benthic and epibenthic such as mysids and amphipods.

7 , . • Conclusions

Our calculations show that baltic herring and sprat exert a strong predation pressure on zooplankton. Furthennore, the results indicate that young of the year herring and sprat are very important as zooplanktivores. Therefore, the magnitude of zooplanktivory by herring and sprat are sensitive to alternative but equally reasonable assumptions on growth and survival of early life stages. Further studies on these life stages are thus needed to understand trophic interaction and production potential of clupcids in the Baltic Sea.

References

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1970-72. Contrib. Askö Lab. Univ. Stockholm No 8, 1-36.

Aneer, G. 1975b. Composition of food of the Baltic herring ( harengus v.

membras L.), fourhorn sculpin (Myoxocephalus quadricornis L.) and -pou

(Zoarces viviparus L.) from deep soft bottomtrawling in the Askö-Landsort area during two consecutive years. Merentutkimuslait.

Julk./Havsforskningsinst. Skr., 239: 146-154.

Aneer, G. 1980. Estimates of feeding pressure on pelagic and benthic organisms by

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Anon. 1987. Report of the working group on assessment of the pelagic stocks in the

Baltic. Part 1. ICES Assesment. 20,1-163.

Anon. 1991. Report of the working group on assessment of the pelagic stocks in the

Baltic. ICES Assesment 18. ICES C.M. 1991/assess: 18, 1-142.

Ara, E., A. Ditto, I. Vourinen, J. Rinkman. 1986. The food selection of Baltic herring

in late summer in the northern Baltic Sea. leES C.M. 1986/J:26 1-19.

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8 ·, Elmgren, R.1984. Trophic dynamies in the enclosed, brackish Ba1tic Sea. Rapp. P.-v. Reun. Cons. int. Explor. Mer. 183: 152-169.

Flinkman, J., I. Vourinen, E. Aro. 1992. Planktivorous Baltic herring (Clupea

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Hagstföm, 0.; L.-E. Palmen, N. Häkansson, D. Kästner, H.B. Rothbarth, W. Grygiel,

and M.Wyszynski. 1991. Acoustic estimates of the herring and sprat stocks in the Baltic proper aetober 1990. ICES. NTIS, C.M. 1989/J:34, 1-25 Hansson, S., U.Larsson and S. Johansson. 1990. Selective predation by hering and

mysids, and zooplankton community structure in a Baltic Sea eoastal area. J. Plankton Res. 12 (5) : 1099-1116.

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Hewett, S.W., and D.J. Stewart. 1989. Zooplanktivory by alewives in Lake Michigan:

Ontogenetic, Seasonal, and historical pattern. Trans. Amer. Fish Soc. 118 : 581-596.

Hill, C., M.A. Quigley, J.F. Cavaletto, W. Gordon. 1992. Seasonal changes in lipid

eontent and eomposition inthe benthic amphipods Monoporeia aJfiinis and Pontoporeia femorata. Manuseript.

Ki~rboe, T., and P. Munk. 1986. Feeding and growth of larval herring, Clupea

harengus, in relation to density of nauplii. Env. Bio!. Fish. 17 (2) :

133-139. i ! Kostrichkina, Ye.E., and E.A. Oyaveyer. 1982. Lang-term changes in zooplankton I I and growth rate of herring in the Gulf of Riga. J. Hydrobio!. 18 : 37-43. I I I

9 I I, I, !I" :1 , . •

Laurence, G.c., 1976. Caloric content ofsome North Atlantic calanoid .

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skarpsill och strömming. Univ. of Stockholm, Mimeo. 1-14 (In Swedish)

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Polish with English summery).

Rice, J.A., and P.A. Cochran. 1984. Independent evaluation of a bioenergetics model

for largemouth bass. Ecology 65 (3) : 732-739.

Rudstam, L.G. 1988. Exploring the dynamics of herring consumption in the Baltic: •

Applications of an energetics model of fish growth. Kieler Meeresforsch.,

Sonderh. 6: 312-322.

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planktivory in a coastal area of the northem Baltic Sea. Mar. Ecol. Prog. Sero

80: 159-173.

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eons. int. Explor. Mer. 183: 170-179. van Khanh, N., 1. Drzycimski, and J. Chojnacki. 1972. Feeding and food composition

of sprat from Bomholm depth. Acta Ichthyologica et Piscatoria 11 (2) : 55-66.

Wiktor, K., and A. Szaniawska. 1988. Energy content in relation to the population

dynamics of Mysis mixta (Liljenborg) from the Southem Baltic. Kieler

Meeresforsch" Sonderh. 6 : 384-356.

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and amount of food consum~ by sprat, herring and in the southem Baltic in the years 1971-1974. lCES C.M. 1976/P:23, 1-7.

10 . . •

Table 1. Seasonal proportions (wet weight) of different prey items in the diets of herring in the Baltic Sea.

Proportion of diet

Size-class Date Zooplankton .Mysis Pontoporeia Others

o- 100 mm (0 - 5.9 g) 1-jan 1.00 0.00 0.00 0.00 1-apr 1.00 0.00 0.00 0.00 1-jul 1.00 0.00 0.00 0.00 l-oct 1.00 0.00 0.00 0.00

100 - 150 mm (5.9 - 21.3 g) I-jan 0.90 0.10 0.00 0.00 1-apr 1.00 0.00 0.00 0.00 1-jul 0.90 0.10 0.00 0.00 l-oct 0.80 0.10 0.05 0.05

150 - 200 mm (21.3 - 51.8 g) 1-jan 0.70 0.15 0.10 0.05 1-apr 0.95 0.00 0.00 0.05 1-jul 0.80 0.15 0.00 0.05 l-oct 0.70 0.15 0.10 0.05

~ 250 rum (~ 51.8 g) 1-jan 0.50 0.30 0.15 0.05 I-apr 0.95 0.00 0.00 0.05 I-jul 0.65 0.30 0.00 0.05 I-oct 0.50 0.30 0.15 0.05 Table 2. Population numbers in millions for various age-groups of herring in the Baltic Sea. The vaIues are divided into the eight different populations according the leES rectang1e of fJshing areas. VaIues are based 1argely on average annua1 estimates for each age-group from Reports of working group on assessment of Pelagic stocks in the Baltic (Anon. 1987, 1991). The numbers in these reports are prescnted as age-group 1-9 which is equvivalent to ~ur age-group 2-10. The estimated numbers of younger age-groups was made by caIculations presented in fJgure 4.

Clupea harengu~ Fishing areas (millions)

bd bd bc bd Age-group Date 22-24 ad 25-27 28-29 bc 30 31 32 Gulf or bc Coastal ad 0 en Sea R1a

Est eggs 4.82E+07' 3.70E+07 7.86E+07 1.03E+08 4.33E+07 1.60E+07 1.83E+07 8.98E+06 3.54E+08 Eggs spawned 10-apr I I-jun 3.85E+07 2.96E+07 6.29E+07 8.26E+07 3.46E+07 1.28E+07 1.46E+07 7.18E+06 2.83E+08 yolk-sac fry 20-apr 110-jun 3.47E+07 2.67E+07 5.66E+07 7.43E+07 3.12E+07 1.15E+07 l.32E+07 6.47E+06 2.55E+08 larva 25-apr I 15-jun 3.13E+07 2.40E+07 5.09E+07 6.69E+07 2.81E+07 1.04E+07 1.19E+07 5.82E+06 2.29E+08 metamorphosed 4-jul/24-aug 137E+05 1.05E+05 2.23E+05 2.93E+05 1.23E+05 4.54E+04 5.20E+04 2.55E+04 1.01E+06 0 l-oct 3.09E+04 2.40E+04 1.62E+04 3.53E+04 1.83E+04 5.38E+03 9.53E+03 4.08E+03 1.44E+05 1 I-jan 7039.5 5514.2 5429.7 14482.9 8172.1 2189.9 4636.7 1879.7 49344.7 2 I-jan 3835.0 3004.0 2958.0 7890.0 4452.0 1193.0 2526.0 1024.0 26882.0 3 I-jan 2278.3 1729.3 2148.0 5924.3 3053.9 887.1 2192.0 855.0 19067.9 4 I-jan 1482.9 1196.1 1797.5 4230.9 1862.2 619.4 1406.0 622.0 13217.0 5 I-jan 599.8 676.0 1131.0 2583.1 1415.2 434.4 660.0 346.0 7845.5 6 I-jan 152.8 273.0 807.0 1814.4 1000.5 311.6 330.3 179.0 4868.6 7 I-jan 96.7 116.0 655.0 1244.9 690.0 249.8 178.8 93.0 3324.2 8 I-jan 20.2 50.0 514.0 92I.l 446.0 178.5 98.8 41.0 2269.6 9 I-jan 8.6 29.8 452.0 613.2 308.0 132.2 49.5 17.0 1610.3 10 I-jan 4.0 13.8 388.0 447.8 172.2 78.9 29.0 4.3 1138.0

L. 1-10 15517.8 12602.2 16280.2 40152.6 21572.1 6274.8 12107.1 5061 129567.8

a Polu1ation spawning time: 10 april b Polulation spawning time: 1 june c Values on age-groups 2-10 based on average (Anon, 1987) on year 1983-1986 d Values on age-groups 2-10 based on average (Anon, 1991) on year 1983-1990 Table 3. Population biomass for various age-groups ofhcrring in the Baltic Sea. The values are dividcd into eight population according the leES rectangle of fishing areas. Values are based largely on average annual estimates on numbers (Table 2) for each age-group andaverage weight (Figure 3) from Reports of working group on assessment of Pe1agic stocks in the Baltic (Anon, 1987, 1991). The numbers in these reports are presented as age-group 1-9 which is equvivalent 10 our age-group 2-10. The estimated Spavming stock biomass are based on average values from leES for the different areas and period.

Clupea harengus Fishing areas (10*3 tOrules)

bd bc Age-group Date 22.24 ad 25·27 28.29 bc 30 bd 31 32 GuU or bd Coastal ad o en Sea bc Rla

Eggs spawncd 10-apr / 1-jun 20.82 15.99 33.96 44.60· 18.71 6.90 7.91 3.88 152.76 yolk-sac fry 20-apr /10-jun 15.26 11.73 24.90 32.71 13.72 5.06 5.80 2.85 112.03 larva 25-apr /15-jun· 21.88 16.79 35.65 46.83 19.64 7.28 8.31 4.07 160.45 metamorphosed 4-jul/24-aug 17.79 13.66 29.00 38.09 15.98 5.09 6.76 331 129.68 1 1-jan 89.40 66.72 59.18 91.24 40.86 10.95 30.14 12.59 401.08 2 1-jan 91.66 85.91 87.26 110.46 53.42 11.93 35.87 12.49 489.01 3 1-jan 110.04 83.87 90.65 136.26 61.08 15.26 44.28 14.71 556.14 4 I-jan 120.71 81.33 97.42 142.16 56.24 13.38 37.82 13.44 ·562.50 5 1-jan 65.98 57.12 79.96 113.14 52.65 12.25 24.02 9.76 414.88 6 I-jan 2038 27.60 6133 97.43 43.22 10.84 14.96 6.12 281.90 7 1-jan 14.99 13.18 56.00 79.42 33.95 10.34 10.16 3.85 221.89 8 1-jan 3.96 5.95 48.73 6632 24.22 9.35 6.74 2.15 167.41 9 1-jan 1.89 3.81 46.51 50.65 18.39 7.68 3.77 0.99 133.70 10 1-jan 0.94 1.86 42.33 40.21 10.97 5.26 2.27 0.29 104.13

1: 1-10 519.94 427.37 669.38 927.30 394.99 107.25 210.03 76.38 3333

Spawnlng stock BIomass 284 218 463 608 255 94 108 59 2089

a Po1u1ation spawning time: ]0 april b Po1ulation spawning time: 1june c Values on age-groups 2-10 baSed on average (Anon, 1987) on year 1983-1986 d Values on age-groups 2-10 based on average (Anon, 1991) on ycar 1983-1990 Table 4. Population biomass, mean weights, numbers, mortalities, gross production, consumption garnete production, and biomass conversion efficiency for various age-groups of sprat in the Baltic Sea. The va1ues are based on onc Baltic population according the lCES rectangle of fishing areas. Values are based on average annual (1983-1990) estimates for each age-group from Reports of working group on assessment of Pelagic stocks in the Baltic (Anon, 1991). The numbers in these reports are presented as age-group 1-9 which is equvivalcnt to our age-group 2-10. The estimated numbcrs of younger age-groups was made by ca1culations presented in figure 4. The duration ofyoung-of·the-year period are 128 days.

Sprattus sprattus Population Mean Population Daily Population Population Garnete Conversion efficiency (%) biomass weight numbcrs mortality production consumption mass Population Individual Age-group Date (tonnes) (g) (millions) (10*3 tennes) (10*3 tennes) (tennes)

E.~t cggs 2.'10E+OH 0.223144 Eggs spawned 1-jun 34.0 0.000177 1.92E+08 0.010536 yolk-sac fry 10-jun 76.0 0.00044 l.73E+08 0.010536 1arva 15-jun 108.8 0.0007 1.55E+08 0.076750 4986.9 0 30.0 30.0 metamorphosed 24-aug 88.5 0.13 6.81E+05 0.010487 2180.0 13620.0 0 16.0 11.3 1 I-jan 764.6 4.3 177824.0 0.001659 460.9 13020.0 0 3.5 3.9 2 1-jan 82504 8.5 96875.0 0.002600 74.2 7770.0 0 1.0 2.4 3 1-jan 447.1 11.9 37508.0 0.002901 -6.8 3332.0 22230.0 -0.2 1.2 4 1-jan 180.9 13.9 13008.0 0.001713 -004 1590.0 10760.0 0.0 0.5 5 1-jan 102.6 14.7 6962.0 0.002041 -0.8 848.1 5873.8 -0.1 0.6 6 1-jan 52.7 15.9 3305.0 0.001659 -0.3 456.8 3202.5 -0.1 0.4 7 1-jan 30.1 16.7 1804.0 0.002456 -1.7 217.9 1624.4 -0.8 0.3 8 1-jan 12.7 17.3 736.0 0.000742 -004 10Ll 739.7 -004 0.3

I. a1l 2704.76 45942.8 I. 1-8 2416.1 103.3 338022.0 524.8 27335.9 44430.3

• • Tabell5. Cornparisons ofgross production, consurnption, garnete production, and average biomass conversion efficiency (production plus garnete production/consurnption) for various age-groups of Bultic Sea herring.

Clupea harellgus

Population Population Garnete rnass Conversion efficiency (%) consumption production (10*3 tonnes) (10*3 tonnes) (tannes) Population Individual

larvae 7378.2 0.0 30.0 30.0 yoy 15164.8 2827.0 0.0 18.6 13.6 1 7418.3 360.7 0.0 4.9 5.2 2 5201.9 270.6 0.0 5.2 4.0 3 4438.3 190.0 22230.0 4.3 3.0 4 3368.1 92.1 26405.6 2.7 2.6 5 2442.6 62.2 18197.6 2.5 1.8 6 1692.0 ·38.9 13199.7 2.3 1.7 7 1272.9 24.7 9087.2 1.9 1.7 8 886.6 19.0 6971.4 2.1 I.l 9 686.3 11.4 5386.7 1.7 0.9 10 549.3 9.8 4583.0 1.8 0.7

:E 50499.4 0.0 106061.3 :EI-la 27956.3 0.0 · '

Figure 1. Map of the Baltic Sea. The area is divided by the leES subdivisions 22-32. ·.

20-r------. -e- YOY+Yearlings -9- Adult

16 .. .

u o 8 . ..

4 . ~ . O-+--...,...-__,.--_-..,.--~-...,...-__,.--_-...,...-..... o 74 149 223 298 372 Day

Figure 2. Th6 average seasonal cycles of water temperature assumed to be occupied by

various age-groups of herring and sprat in Baltic Sea, based on three open sea and one

coastal stations (1-4 in.Figure 1). YOY is young ofyear and yearlings are age 1 fish. ..

Clupea lzarellgus 250-r------.., ... ·6..... Gulf of Riga .0 · .. 0 ... Gulf of Finland

200 ····································cr················ . ·.. -<> Bothnian l3ay . .. __ 130thnian Sea

....=:1 28-29 3 150 ...... Q . · .. ',4: 25-27 Open

~ tiJ .83 'ö • Hf" ····ffi 25-27 Coastal ~ • 83 .. ·EE' ::: ...-0 ... 22-24 <) ;.,._~.:.:.:.~ '"' 100 ...... ;'.e: B ..::.: : . ~ ,.' 0 .,83 Ä" •• ' ','0 . ,.1...' " '-:8.., ..0 '83' Je" ,.g" ...... _;~~<:""::~;~:::~:;:l.:;; :.::.!:.:'~ 50 •. . .:e( .g ;;j~~~::~,.,-a" .. g;;;lO,I:: .' o+-.-..::F---r--r--.----r--r--~--r--r--r-__1 2 3 456 7 8 9 10 Agc-group

Figure 3. The average weight at capture 1983-1990 by leES (Anon., 1987, 1991) für eight different subpüpulatiüns of herring in the Baltic Sea. •

Average SSB 1983 - (1986) 1990 calculated for each stock from ICES report on pelagic stocks (Anon, 1987, 1990) ~bersor biomass Daily Mortality The expected number of eggs calculated .....(examples) (fIshing and natural mortality) from SSB, based on50 % females, 20 % of .Spawning stock Biomass (SSB) 255 body weight is gonads and each gonads 11 11 ~ contains of 90 % eggsand each egg Est eggs 4.33E+07 0.02314 weight about 0.53 mg for herring and Egg 3,46E+07 0.10536 ~ 0.178 mg tor spra! Yolk-sac fry 3.l2E+07 0.10536 Melamorphosis after Larva 1.63E+07 0.07675 , 85 days at a weight of Meta morphosis 1.63E+05 0.01646-0.02903 I. 0.00167 \. 10 % mortalily 0.13 9 and lenght of 1 20689.5 I 30 mm 2 4762.1 0.00039-0.00151 10 % mortality during the first 10 days 3 3039.4 0.00049-0.00 136 4 2101.1 0.00075-0.00248 5 1481.8 0.00091-0.00375 90 % mortality per month during 70 days 6 1045.0 0.00061-0.00234 7 701.1 0.00066-0.00429 Different mortality per month tor each 8 479.9 0.00035-0.00241 stock depending on the abundance of the 9 331.7 0.00042-0.00377 stock at hatch and the abundance at 1 10 231.6 0.00056-0.00374 january, age 2, average value from from ICES report (Anon, 1987. 1990)

Average abundance 1983 - (1986) 1990 tor each stock calculated trom ICES report on pelagic stocks 5 % mortality per month (Anon, 1987, 1990)

Figure 4. Summery of the calculations ofbiomass and abundance for the different stocks ofherring. and sprat presented in tables 2-4. In the right column also the daily mortality used are presented. For age-group 2-10 and metamorphosis-group the values shows interval ofvaules used for different population depending on the abundance of data from leES rcport (Anon, 1987, 1991). ...

Clupea harengus 7 1.8 10 Others 7 o 1.5 10 ~ Pontoporeia 11 Mysis 7 1.3 10 Im Zooplankton

7 0 1.0 10 '"~ ~ 0 E-< 6 7.5 10

6 5.0 10

6 2.5 10

0 larvae yoy 2 3 4 5 6 7 8 9 10 Age-class

Sprattus sprattus 7 1.8 10

1.5 107.

7 1.3 10

7 0 1.0 10 '"~ ~ 0 E-< 6 7.5 10

6 5.0 10

6 2.5 10

o larvaeyoy 2 3 4 5 6 7 8 9 10 Age-class

Figure 5. Total predation by hening and sprat in Baltic Sea for all prey types by age- groups. The duration of age-groups: larvae, 70 d; metamorphosis 128 or 173 days for different stocks. I ...... , . L •

Clupea harcngus. 1.0 ~ 07 ...r-...... ;:.;...... ;.;:,,;,...;...;.;;;...... :....:...... ;.;..;...... """'-"""'---:'--,

6 m agc 3·10 7.510 0 agc 2 E9 agc 1 eil 0 yoy § 6 0 5.0 10 • f-< &1 larvae

6 2.5 10

o~::.:.;E~~F~~~r...... ,..~ jan Icb rnar apr may j~n jul aug scp OCl nov dcc Monlh

Sprattus sprattus 1.0 107 ..;,.;;...... """'-...... :.....;..;.,....:..-...... ~------...... _ ...... """'-"""'--,

6 Ei Age 3-10 7.5 10 0 age 2 ~ age 1

tIl u D yoy 6 § 5.0 10 0 lS1 larvae f-<

jan feb rnar apr may jun jul aug sep oct nov dcc Monlh

Figurc 6. Total predation by herring and sprat in Baltic Sea for all prey types by month

of year and age-groups (see legends),'The duration of age-groups: larvae. 70 d; metamorphosis 128 or 173 days for different stocks. ------~--~-

I!'J ase 3-10 Consumption - Zooplankton Consumption - lYlysis 8 ase S-10 0 ase 2 "d age 3-4 ~ 7 6 EJ ~..... 0'0 1.0 10 0 age 1 1.2 10 ::r. ~ ., age 1-2 0 =r,) Clupea },arengus yay Clupea harengus • CI> 6 ::s 6 • 1.0 10 '0 '1 8.0 10 8- ~ . [] larvae 0- () 0 8.0 lOS '< 6 $:.) a 6.0 10 C1Q '0 .. (1l , "c: :'l I ~. c: c: 6.0 laS O'~ 0 "0 CI> .... 6 .... 0 0 4.0 10 e:: , ::s 5 '0 0 4.0 10 Yl ...., 'F 6 2.010 5 § 0a 2.0 10 0..' ::s ..... ::~ /.~ ::s-' .. 0 >-" 50 '< 0 0 (1l '0 jan reb mat apr may jun jul i.ug sep DeI nov ,k:c jan reb mat apt may jun jul aug sep oet noy dcc "1 CI) '-' §. Month Month 0- $:.) '< C...... 0 :::J ::s (1l 0 tJj ;:J tj ase 5-10 Consumption - Pontoporeia [l" age 5·10 e.- ~ Consumption - Others ..... 0 age 3-4 . () g 0 age 3-4 CIJ 6.0 loS ase 1·2 3.5105 (1l e: age 1·2 i'» ::::l • (1l • ::s-' Clupea harengus (1l @ 5.0 1'; 3.0 loS Clupea harellgus a. ;:J..... ;:J 5 '0 2.510 CT':l @ 4.0 loS "cl '< 5 .g ..... :'l .. 2.010 .e:: ~ 3.0 loS " >-" :a 0 " ~ (1l .... "0 5 O. III .... 1.5 10 0 ;:J 2.0 1 eS ~ 1.0 lOS 0 '0 >-" 1.0 1eS 4 ~ 5.010 ...... :. )s.: 0 0 0 ? jan feb mit apt may jun jul aus sep oe[ noy dec Jan Feb Mat Apr May Jun Jul AuS Sep Oet Noy Dce Month Monlh.

~

~ a) Different lcnght ofJarvac stage

S~ort

Modium

Loog

6 3.010 ~ Q Q ~ 6 2.010

6 1.0 10

0 la.nac Age-group yoy Sho" Long I DiHereocc in WLat + 73.7 % ·24.2 % ) rood confumpUoD I

b) Different mortality during larvae stage

iW High: 95 9'0 o Modium: 90 9'0 () Low: 859'0

Difference in IOW !ood coruumpuoo

c) Different mortality ofagc-group 1

m Low: 2.59'0 o Medium: 5.0'10 o IIigh: 7.59'0

yoy 1+ • 2.5 9'0 7.59'0 Dif(crcnD: in total ·0.6 'J, +0.5 9'0 rood coarump

90-85 %), c) different monthly mortality during age 1 (2.5-5.0-7.5 %). The standard run is represented by the medium values., Clupea harengus 100------,

-+2-+--- Nonnal --1_'-- Lew 80

60

40

20

o~1'---r--~-T---r---r---r--...,.---,r---r__-I"'-"1 1 3 5 7 9 Age-group

Figure 9. Comparison of growth of herring in the northem Baltic subpopulation {ICES area 28-29) between" theo standard run with mixed met and a diet only by zooplankton.

The assumption was that the fish consume the same volume offood.