Abstract

Kazinga Channel with Lake Edward (formerly LakeIdi Amin), Lake George can be considered a self­sufficient ecosystem, given the restricted nature ofitsconnection with Lake Edward (Fig. 1).

Lake George has been studied rather extensively,both in terms ofits fish fauna (Greenwood 1973) andin the context ofthe International Biological Program(IBP). Burgis and Dunn (1978), Beadle (1981) andBurgis and Symoens (1988) present reviews of therelevant works, which are considered below.

Introduction

In this contribution, a trophic ecosystem model ofLake George. in Uganda is presented, based on anapproach alreadyusedto constructmodels ofa numberof other African lakes and ecosystems (see Degnbolthis vol., Kolding this vol., Moreau et al., this vol.).

This paper aims:1. to add Lake George, which has been well

studiedin terms ofits ecology and constituentfauna andflora, to the series oflakes thathavebeen describedusing the trophicmodellingapproach; and

2. to demonstratefurther the -utilityand versatility ofthe ECOPATH IIapproach andsoftware; and itsuse in integratingthe work of differ­ent researchers.

Lake George isrelatively small, 250 km2,

andhas a mean depth of2.4m, with a maximum of4 m. Fig. 1. Map of Lake George, showing its connection, via the Kazinga Channel, with Lake Edward,Although connected via the and their location in Africa.

A trophic model of Lake George, Uganda, Central Africa, was constructed using published quantitative andqualitative information on the various biotic components of the lake and the ECOPATH II approach and software. Itis shown that the available production and biomass estimates for the various groups in the system are consistent witheach other, and that it is possible to make a balanced model of the major trophic intera~ns in Lake George.

bInternational Center for LivingAquatic Resources ManagementMCPO Box 2631, 0718 Makati

Metro Manila, Philippines

J. MOREAUa, V. CHRISTENSENb and D. PAULybaDepartment ofInland Fisheries - INP /ENSAT

145 Avenue de Muret - 31076Toulouse Cedex, France

A Trophic Ecosystem Model of Lake George, Uganda

MOREAU, J., V. CHRISTENSEN and D. PAULY. 1993. A trophic ecosystem model of Lake George, Uganda, p.124-129. In V. Christensen and D. Pauly (eds.) Trophic models of aquatic ecosystems. ICLARM Conf. Proc.26,390 p.

Materials and Methods

The model of Lake George was constructed byapplying the ECOPATH II approach and software ofChristensenandPauly(1992a, 1992b)to datacollectedby various authors in Lake George, and standardizedby this paper's authors.

The basic equation ofECOPATH II expresses thatfor each group (i) in the model,

B. (PIB). EE. =Y. + L. (B. (QIB). DC..) •••1)1 1 1 1 J J J Jl

125

Production/Biomass Ratio (P/B)

As shown by Allen (1971), under an equilibriumassumption, when von Bertalanffy growth can beassumed (as is here the case), PIB is equal to Z asdefined in fisheries science. Hence we have estimatedthis parameter for the fishes from length-frequencydata as outlinedin Gayanilo et al. (1989). Forthe othergroups, literature values were taken mainly fromWinberg (1971) and Payne (1986). All values of PIBpresented here are annual.

Table 1. Basic information on elements ("boxes") of trophic model of Lake George.a

aNumbers in brackets refer to maximum length, in em.

Fisheries Catches (Y)

Catch estimates pertaining to the 1970s wereobtained for the fish groups in Table 1 from recordsof the Uganda Department of Fisheries (Gwahaba1973; Dunn 1973, 1975, 1989). They are expressedhere, like all other flows, in t·ww·km-2·year-1.

Ecotrophic Efficiency (EE)

The average composition of the food of eachconsumer organism is presented in Table 3. The tableis on a weightbasis, andwas assembledfrom publishedinformation.

Diet Composition (DC)

This parameter expresses the food consumption(Q) ofan age-structured population infishes relativeto its biomass (B), on an annual basis. Except for O.niloticus and H. nigripinnis, the estimate of QIBused here was obtained via the empirical model ofPalomares (1991) who also showed that freshwaterand marine fishes have similar QIB values whentheir shapes, size, food type and environmentaltemperature are equal, thus justifying the use of a

model based on both marine andfreshwater fishes.

The Q/B estimated for O.niloticus and H. nigripinniswere taken from Palomares(1991), who based hercomputations on stomachcontents data from Moriarty andMoriarty (1973).

Food Consumption (Q/B)

This is the fraction of theproduction of any group that isconsumed within the system, orcaught' by the fishery. Thisparameter is difficult to estimateand is usually assumed to rangefromlowvalues (inapexpredators)to 0.95 (Ricker 1969). Note that

ECOPATH II directs the fraction (1-EE) ofproductiontoward the detritus, a feature that is of relevancewhen attempts are made to equilibrate an ECOPATHIImodel. Note also that the EEvalues differfrom grossefficiency, GE = (PIB)/(QIB), used here to check theinputs in Table 2, but not further discussed.

Fishing eagles, kingfishers, cormorants,pelicansCatfish (85)Catfish (85)Lungfish (75)Predatory dwarf bream (20)Benthophagous dwarf bream (12)Phytoplanktophagous dwarf bream (10)Nile tilapia (40)Tilapia (35)Thermocyclops -hyalinus + Mesocyclops leuckartiChaoborus spp., Copepods, Oligochaetes,Ostracods (Cyprinotus spp.), Chironomus spp.Blue-green algae (Anabaena, Microcystis,Lingbya) (70% of biomass); Diatoms (Melosira,Nitzschia, Synedra); Chlorophytes (Pediastrumand Scenedesmus)

Fish-eating birds

Phytoplankton

Benthic producersDetritus

Bagrus docmacClarias gariepinusProtopterus aethiopicusHaplochromis squamipinnis :H. angustifronsH. nigripinnisOreochromis niloticusO. leucostictusZooplanktonZoobenthos

1.

12.

2.3.4.5.6.7.8.9.

10.11.

13.14.

where Bi is the biomass of i, (PIB)i its production!biomass ratio, EEiits ecotrophic efficiency, Yiits yield(= fisheries catch), Bj the biomass ofits k predators j,(QIB)j' the food consumption per unit biomass ofj andDCji the fraction of i in the diet of predator j.

This equation implies equilibrium, i.e., inputto a group is assumed to equal output from thegroup over the period considered. This assumptionappears unavoidable in view of the scatterednature of the dataset considered here. It isjustified, on the other hand, by the between­year consistency ofphytoplankton biomass reportedby Ganf and Viner (1973).

Table 1 presents the groups used to describe LakeGeorge, along with some of their characteristics.

Except for the birds and the phytoplankton, allbiomasses were estimated using ECOPATH II.·Estimates of parameters were provided as follows.

aSumba (1983).bMoreau et al. (this vol.).cGuessed values based on the maximum observed length for PIB (see Moreau et al., this vol.) and on the gross

efficiency for QIB.dComputed from an estimate of natural mortality M =2.9 year-l by Palomares (1991), assuming F =0.2 year-l in a

population which is lightly exploited.eMoriarty and Moriarty (1973).fGwahaba (1973). The observed biomass for O. niloticus pertains only to the inshore waters.gBurgis (1974).hPayne (1986), Winberg (1971).iGuessed values, based on the gross efficiency for these groups and estimates from Polovina (1984) and Polovina and

Ow (1985).jGanf(1972, 1974, 1975), Burgis and Dunn (1978).kDarlington (1977).

Table 2. Input values for the required parameters for ECOPATH modelling of Lake George ecosystem (see alsoTables 1 and 3). Computed and observed biomasses are also shown. Values ofEE are guesses based on the knownlevel ofexploitation and/or predation ofthe group under consideration. Catches comefrom several sources: Gwahaba(1973), Dunn (1973, 1989), Burgis (1978), G.W. Ssentongo (pers. comm.). They refer to the early 1970s.Gross efficiency is computed as (PIB)/(QIB) and is usually between 0.1 and 0.3.

Group Catches Biomass PIB QIB EE Computed Observed(t'km-2 (t'km-2) (year-l) (year-l) biomass biomassesyear-l) (t'km-2) (t'km-2)

1. Birds 0.0 0.022a 0.25a 58.00a (0.022)a2. B. docmac 0.3 0.90b 5.45b 0.95 0.50 (0.4-0.5)f3. C. gariepinus 0.8 0.90b 5.33b 0.95 1:16 (0.7-1.2)f4. P. aethiopicus 0.6 0.50b 4.85b 0.95 1.50 (1.4-1.6)£5. H. squamipinnis 0.8 1.70c 8.80C 0.95 0.62 (0~4-0.7)f

6. H. angustifrons 0.4 2.50c 16.00c 0.30 2.55 (2.1-2.9)f7. H. nigripinnis 0.5 3.10d 17.50e 0.25 6.61 (5.2-6.9)f8. O. niloticus 10.5 1.30f 12.80e 0.95 9.89 (8.5-12.1)f9. O. leucostictus 0.4 1.10f 12.50e 0.95 0.59 (0.4-0.6)f

10. Zooplankton 0.0 26.00g 140.00i 0.60 4.47 (2.7-5.8)g11. Zoobenthos 0.0 4.50h 26.00i 0.80 10.80 (9.8-11.4)k12. Phytoplankton 0.0 30.Qi 66.0Qi 0.00 (30Y13. Benthic producers 0.0 5.00h 0.00 0.95 19.81

126Balancing ofthe Model

The equilibrium assumption implicit to equation(1) is important in that it strongly constrains thepossible solution, i.e., the range of parameters thatwill satisfya setofsimultaneous equations such as (1).Thus, the solution accepted as realistic is that whichrequired the least modifications of the initial inputs(including the diet matrix), and yet generatedbiologically and thermodynamically possible outputs(i.e., all GE and EE < 1).

Results

Tables 2 and 3 present the key features of ourmodel ofLake George, which is also illustrated in Fig.2. The estimated biomasses are either within theranges, or close to the biomasses so far published and,therefore, Fig. 2 represents a "possible" Lake Georgesituation. Thefishbiomassis dominatedbyO. niloticus,an herbivore, whose central role in Lake George waspreviously emphasized by Gwahaba (1973), and byMoriarty and Moriarty (1973) and by H. nigripinnisand H. angustifrons, small phytoplanktivores(Moriarty and Moriarty 1973) and zoobenthivores,

respectively (Gwahaba 1975). The major predators inthe system are the lungfish P. aethiopicus and thecatfish C. gariepinus, with consumptions of 7.3 and6.2 t·km-2·year-1, respectively.

The predatory fishes are caught by fishe~sand bybirds (Sumba 1983) and their EE (0.95) was assumedto be high. It is noted that the total consumption bybirds (1.28 t·km-2·year-1) is far from negligible. Itamounts to 8.5% ofthe actual catch (14.3 t·km-2·year-1).

EE is also high for Oreochromis species whichconstitutes the bulk ofthe actual catch and ofthe foodof the birds. In contrast, EE values are considerablylower for H angustifrons and H. nigripinnis. Thesetwo groups areverypoorlyexploited anddo not appearto suffer any severe predation (Moriarty et ale 1973;Dunn 1975).

Amongthefood sources, e.g~, zooplankton, benthos,phytoplankton and benthic producers, only the lastone has been expected to be heavily predated upon.The huge primary production of Lake George is notfully exploited (EE=0.95) and, to some extent, this isalso true for zooplankton (Burgis and Dunn 1978). EE(=0.8)isquitehighforzoobenthoswhichisanimportantsource of food for several fish species even if itsbiomass (10.8 t·km-2) is low when compared to other

127Table 3. Diet composition (in % ofweight ofstomach contents) ofconsumers in the Lake George ECOPATH II model. Groups 12, 13and 14, respectively: phytoplankton, benthic producers and detritus. Estimates are from: Sumba (1983) for group 1; Moreau et al.(this vol.) for groups 2, 3,4; Dunn (1975) for groups 5,6, 7; Moriarty and Moriarty (1973) for groups 7, 8, 9; Trewavas (1983) for group9; Burgis and Dunn (1978), Moriarty et al. (1973) for group 10; Payne (1986) and Palomares (1991) for group 11.

Prey

Consumer 1 2 3 4 5 6 7 8 9 10 11 12 13 14

1. Fish-eating birds 6 9 1 76 82. B. docmac 0.5 0.5 0.5 0.5 4 20 3 1 5 50 2 133. C. gariepinus 0.5 1 0.5 5 10 3 0.5 5 48.5 1 5 204. P. aethiopicus 0.5 0.5 0.5 0.5 3 10 ·2 5 60 2 5 115. H. squamipinnis 2 16 50 6 1 10 10 56. H. angustifrons 10 50 10 10 207. H. nigripinnis 2 90 3 58. O. niloticus 2 90 4 49. O. leucostictus 1 80 5 14

10. Zooplankton 5 9511. Zoobenthos 10 5 5 30 50

,I

j•

African lakes (Beadle 1981; Payne 1986; Burgis andSymoens 1988).

To some extent, this ECOPATH II model ofLakeGeorge confirms thefrequentlymentioned assumption(Burgis 1978; Burgis and Dunn 1978; Beadle 1981)that this ecosystem has a low ecological efficiency ascompared to otherAfrican lakes such as Lake Victoria(Moreau et al., this vol.). The gross efficiency of thefisheries (actual catch/primary production) is 0.0057in Lake George, between that ofLake Victoria prior to

.(0.0016) and after the introduction of Nile perch(0.0082).

Discussion

Interactions between Organisms

The Lake George ecosystem is quite well studied,and it is comforting to see that ECOPATH II coulddescribeitproperlyinterms ofbiomasses andecologicalproduction.

For instance, the obsetved catch (14.3 t·km-2·year-1)

is realistic if extracted from an average total fishbiomassof23.4t·km-2.Thelatterfigureisinagreementwith the evaluations of Gwahaba (1975): 16.4 and 29t·km-2,. depending on how one raises to the whole lakearea figures initially estimated only for some biotasand/orstations. The differencebetweenthe twofiguresgiven by Gwahaba seems to stem mainly from themethod of taking into account the important inshorebiomass of exploited O. niloticus. Furthermore, thelow values of EE for food sources, and also for thehaplochromine cichlids, contribute to explain the lowecological efficiency of the system (Burgis and Dunn1978;Payne 1986).A significantamountofthe primaryproduction is sedimented and exported through theKazinga Channel, the main outflow to Lake Edward(Fig. 1).

The assumed low ecotrophic efficiencies for thetwo haplochromines (No.6 and 7) indicate that thesespecies are incompletely utilized. It is estimated thata production of around 20 t·km-2·year-1, or more thanthe total present catches is unutilized. It is howevernot clear if this is an artefact caused by erroneousassumptions in the model or if the fishery on thesegroups could in fact be increased considerably.

As already mentioned, the ECOPATH model wasdeveloped for static situations under generalequilibrium conditions. However, we know little onthe states of tropical fish communities. Also, little isknown ofthe sensitivity ofthe model to perturbationscaused by fishing or ecological stresses.

The mixedtrophic impacts (Fig. 3; see Christensenand Pauly 1992a and 1992b for description) suggestthat the fishing pressure that is operating now has a'negativeimpactonallfish groups exceptHaplochromisangustifrons and H. nigripinnis, which show slightlypositive impacts. This indicates that the fisheriespresently has, relative to predation and competition,limited impact on those two species.

Interaction among Scientists

During the IBP study ofLake George, specialistsof different groups were associated with a teamsupported by IBP which provided opportunities tointeractand to exchange informations on a qualitativebasis. Thishas made possiblethe publicationofseveralsynthesis papers (see Burgis and Symoens 1988 forreview). ECOPATH II shows how the quantitativedata on each group can be used to describe theecosystem as a whole. Thus, we could verify that theestimates of biomasses and production of each maingroupprovidedbythe IBPteamwere largelyconsistentwith each other. We could also show the gaps inknowledge of this lake, at the end of the IBP project.To some extent, these gaps have forced the authors of

128

3

2

Legend'

~_sq~~sn~ating birds---* Catches'T'

0.01 00 d....: -----f+- Other exports33 ~

~ I~ c. H

<~0.6~ . 0.3 0.04 8=0.02 1.0-::- ---+ Flow to detritus

~51 ::1':~ Q:"~;: ~~riep;e::~ 1.7 B.. 6 -

(5. -= dogmac 0.1 (1.3)~ Respiration

001 8=.5P. 0.04 ----...,...- -=- It! 8=2 6 26.3-:" (2.7)

aethiopicus (6.2) o·(7.2)

1

176.014.4t6I.o! 1.4-:' 20.4

.~~~~ Zoobenthos 0.6

0.JIO.51.0 2~T~~t .l\! __ v8=10.8

2.710.5

0.1 0.2

0.6 .7 .6<i 'd

eIf)~4§R 0. ~

- H. Zooplankton ,Q.L(266.7)

g d nigripinnis~?

niloticus1.5 '- 005 3~ B~.5 ~0.4 8=0.6~ 8=6.6 8=9.9

....- -:"' 0.(594.4)(74) leucostictus (115.8) 'K

T3.51005 84.2t 5.2 L 5.9 1104.2

5.1 0.3 I01 0.2 4.1 140 594.4

4.1

It\! ~I ~ ~ :g I~

~0.4 1.0 5.1 114.0 ~

8enthicproducers 0.8 Detritus

~Phytoplankton

8=20.6 8=10.08=30

to.464A'

Fig. 2. ECOPATH II model ofLake George, indicating the biomasses (B in t'km-2) of the groups used for description ofthe ecosystem andthe flows connecting these (t-km-2·year-1).

Impacted group

.~c:·s.g..~c:::t

C en

~ ~~ 15o 0o 0N N

en::J

.;;::Q)o

Zoobenthos

Detritus

Fig. 3. Mixed trophic impacts of the groupsincluded in the Lake George ecosystemmodel. The figure shows the direct andindirect trophic impacts on the groupsmentioned on top from the groups given onthe left. Positive impacts are shown abovethe baseline, negative below; the impactsare relative, butcomparablebetween groups.-

••

•

•

••

-

.-

••••

-•

-

-

-

•••

•••

-

_____.----------..JI~ ------.._~--

....-

I ~ ....----JI•••• ••O. leucostictus

Zooplankton

Phytoplankton

Harvest

Benthic producers

H. nigripinnis

Fish-eating birds

B. docmac

C. gariepinus

P. aethiopicus

H. squamipinnis

H. angustifrons

a.~

~ 0. niloticus

encga..E

synthesis papers to make arbitrary assumptions onthe relative importance of transfers of energy andbiomass between the successive trophic levels (Burgisand Dunn 1978).

Ashort, andnotexhaustivelistofgapsinknowledgeof Lake George includes:

• impact of predation by fish-eating birds;• dynamics and ecological production of

predatory fishes, haplochromines andzoobenthos;

• diet composition and food consumption ofzoobenthos and benthophagolls fishes;

• actual catch and its range of variations foreach group of fishes;

• identification ofthe reason(s)why a large partof the primary production is not channelledinto secondary production (as mentioned byseveral authors and confirmed by our low EEvalue for phytoplankton); and

• extent of the predation on zooplankton byyoungfishes (all species considered)ininshoreareas.

Conclusion

ECOPATH II has allowed the authors to balancethe biomass and production of several interactinggroups in Lake George, based on data from theliteratureonthelakeitself, oradaptedfrominformationfrom other lakes. The accuracy of several previousbiomass and production estimates for major groupswas demonstrated and the underutilization of somesources of food by fishes (especially phytoplankton)was confirmed. However, some gaps in our knowledgeof the transfers of biomass between the groups havealso been pointed out.

References

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Beadle, L.C. 1981. The inland waters of tropical Africa - anintroduction to tropical limnology. 2nd ed. Longman, Londonand New York. 475 p.

Burgis, M.J. 1974. Revised estimates for the biomass and productionof zooplankton in Lake George, Uganda. Freshwat. BioI.4:535-541.

Burgis, M.J. 1978. Case studies of lake ecosystems at differentlatitudes: the tropics. The Lake George ecosystem. Verh. intoVer. LimnoI. 20:1139-1152.

Burgis, M.J. and I.G. Dunn. 1978. Production in three contrastingecosystems, p. 137-158. In S.D. Gerking (ed.) Ecology offreshwater fish production. Blackwells ScientificPublications,Oxford.

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