SPONSORS

Hydrobiologists from East, Central and West Africa with substantial support from otherAfrican countries, Fishery Scientists in the United States, Canada, U. K., Europe, SovietUnion and Israel.

EDITOR

Dr. 1. Okedi, Director, E.A.F.F.R.O., Jinja, Uganda.

EDITORIAL BOARD

AN INSTANCE OF

SOUTHERN OFFSH

G. E. B. KITAKA

Mr. M. Abolarin, Co-Manager, Kainji LakeProject, Lagos, Nigeria.Mr. J. Kambona, Chief Fisheries Officer,Dar es Salaam, Tanzania.Mr. J. Mubanga, Director, Fisheries Division,Chilanga, Zambia.Dr. L. Obeng, Director, Institute of AquaticBiology, Achimota, Ghana.Mr. N. Odero, Director of Fisheries, Nairobi,Kenya.Mr. S.N. Semakula, Chief Fisheries, Officer,Entebbe, Uganda.

PROGRAMME

Professor W.B. Banage, Makelere Univer­sity, Kampala, Uganda.Mr. R.E. Morris, Director, E.A.M.F.R.O.,Zanzibar.Dr. T. Petr, Senior Lecturer in Hydrobiology,Makerere University, Kampala, Uganda.PlOfessor Mohamed Hyder, University ofNairobi, Kenya.Professor, A. F. De Bont, Universite deKinshasa, Kinshasa XI, Republique Demo­cratique du Zaire

E~

W~

m;thmiVile(

G.Jil

The African Journal of Tropical Hydrobiology and Fisheries will only accept original andwell suppOIted ideas on techniques, methodology and research findings from scientists, fishery

fficers, fishery economists and sociologists.The Journal will therefore strengthen the African lesearch scientist by making researchmaterial available and also increasil1g the awareness and utility of aquatic resources.[ts quality will conform to International standards, and will be published in English andFrench.

MANUSCRIPT ADDRESSManuscripts should be addressed to E.A.F.F.R.O.,EastAfrican freshwater Fisheries, ResearchOrganisation, East African Community, Box 343, Jinja, Uganda.

REPRINTSAuthors will receive 60 reprints free of charge. Extra reprints may be procured on cost.

PUBLISHEREast African Literature Bureau, P.O. Box 30022, Nairobi, Kenya.

ISSUESThe Journal consists of one volume n year, consisting of two issues with approximatelyeighty pages each.

SUBSCRIPTIONAnnual subscription within East Africa Sh. 35. Outside East Africa, East African Sh. 70,

$ 10.00

Upwelling of deep watsurface is of great biolcespecially in stratified lakespreviously locked up inwater are brought up into I

zone; but the process mayphic mortalities of animfish, if this water shoulecontain appreciable amausuch as hydrogen sulphiddioxide (WOR THHGRAHAM 1929; FISH UTER 1963, 1968; AeRIE1966).

Upwelling is normally ilcurrents moving away frby a downward leewardmocline in response to thepilimnetic water, and ofcurrents associated with tlthe thermocline during ,movement after cessatior

A

..,

SPONSORS

Hydrobiologist from East, Central and West Africa with substantial support from otherAfrican countries, Fishery Scientists in the United States, Canada, U. K., Europe, SovietUnion and Israel.

EDITOR

Dr. J. Okedi, Director, E.A.F.F.R.O., Jinja, Uganda.

DITORIAL BOARD

Mr. M. Abolarin, Co-Manager, Kainji LakeProject, Lag s, Nigeria.Mr. J. Kambona, Chief Fisheries Officer,Dar e Salaam, Tanzania.Mr. J. Mubanga, Director, Fi heries Division,Chilanga, Zambia.Dr. L. Obeng, Director, Institute of AquaticBiology, Achimota, Ghana.Mr.. Odero, Director of Fisheries, Nairobi,Kenya.Mr. S.N. Semakula, Chief Fisheries, Officer,Entebbe, Uganda.

PROGRAMME

Professor W.B. Banage, Makelere Univer­sity, Kampala, Uganda.Mr. R.E. Morris, Director, E.A.M.F.R.O.,Zanzibar.Dr. T. Petr, Senior Lecturer in Hydrobiology,Makerere University, Kampala, Uganda.PlOfessor Mohamed Hyder, Univer ity ofNairobi, Kenya.Professor, A. F. De Bont, Universite deKinshasa, Kinshasa XI, Republique Demo­cratique du Zaire

The African Journal of Tropical Hydrobiology and Fisheries will only accept original andwell support d ideas 011 techniques, methodology and research findings from scientists, fisheryofficers, fishery economists and sociologists.Th Journal will therefore strengthen the African lesearch scientist by making researchmaterial available and also increasing the awareness and utility of aquatic resources.It quality ill conform to International standards, and will be pubh h d ill English andFren h.

MANUSCRIPT ADDRESSManuscripts should be addressed to E.A. F.F. R.O., East African Freshwater Fi, heries, ResearchOrgal isation, East African Community, Box 343, Jinja, Uganda.

REPRiNTSAuthors will receive 60 reprints free of charge. Extra reprints may be procureJ on cos1.

PUBLISHEREast African Literature Bureau, P.O. Box 30022, Nairobi, Kenya.

ISSUESThe Journal consists or one volum [l year, consisting of two issues with approximatelyeighty pages each.

SUBSCRIPTIONAnnual subscriptIon within ast Africa Sh. 35. Outside East Aflica, East African Sh. 70,

S $ 10.00

THE ROLE OF ECOLOGICAL STUDIES IN THE RATIONAL

MANAGEMENT OF FISH STOCKS

A. B. E. MABAYE

Central Fisheries Research Institute

Chifanga, Zambia

INTRODUCTIONFishery management and developmen t be

it in developed or developing countries aimat the same objective, namely production ofa maximum quantity of fish commensuratewith the available resources, ad infinitum.Although priorities may differ from countryto country, this objective is motivated by:(a) the need to provide the country's citizens

with more animal protein-food;(b) the need to export surpluses to earn

foreign exchange;(c) the need to raise the standard of living

of the fish community, through increasedearnings; •

(d) the need to curb unemployment throughdevelopment of ancillary industrieswhich are recognized powerful employ­which are recognized as a powerfulemployment generation.

The need to provide more protein topeople in developing countries is much moreacute than in advanced countries, since notonly is the quantity or per capita animalconsumption low, but the variety is alsoseverely limited by religious and! or dietarytaboos. Efforts to bridge the gap are oftenhandicapped, among other things, by higherrates of population increase.

Having recognized the need for fisherydevelopment and management, we find our-

selves confronted with the big question ofhow and where to start. Do we simply in­crease the fishing effort by allowing morefishermen in our fisheries and providing themwith bigger craft and gear? If so, by whatpercentage must each of these be increased?Or must we, like agricultural farmers, addfertilizers into OUf natural fisheries to im4

prove their fertility? We even wonder (if weare not wise enough) whether the achieve­ment of our objective might not be ac­celerated by the importation and introductionof live north-sea herring into our freshwaterlakes and rivers. Since any of these movescould have the most disastrous consequenceswhich may be unpredictable, the one startingpoint is ecological studies.

While I am fully aware that CIFA wasprimarily set up to deal with freshwaterfishery problems pertaining to Africa, theuniversality of ecological problems, demandsthat I treat the subject with a broad world­wide outlook. I, therefore, intend to discussthe subject in general but paying specialattention to local problems.

ECOLOGICAL INTERACTIONSIn order to fully appreciate the role of

ecological studies in fishery development, onemust have a basic knowledge of the factorsthat influence the relative abundance, be-

144 A. B. E. MABAYE

haviour, and spatial and seasonal distributionof fish in any given natural water body. SinceI have no reason to believe that only eco­logists are attending this meeting, I willbriefly consider ecological interactions inecosystems.

The Abiotic ComponentThe volume, surface-area and quality of

water in all lakes and rivers depend on themorphometric characteristics of the lake orriver basin, size and soil quality of the catch­ment area and the amount of rainfall. Lakesin the Rift Valley of Africa resulted fromgeologic disturbances while those in CentralNorth America were created by continentalglaciers (R EID 1961). They can also beformed by volcanic and stream action. Arti­ficial reservoirs are characterized amongother things by dendritic basins.

There are four main interdependent, physi­cal factors that control biological productionin natural waters. These are associated with(a) radiant energy input, (b) nutrient inputand loss, (c) oxygen supply, and (d) theinteractions of morphometry and motion.Temperature" controls the rates of anabolicand catabolic processes and as the maindensity determining factor, it exerts a funda­mental influence on motion and stratifica­tion. Of great importance to the totalmetabolism of an ecosystem is the rate ofproduction at the producer (plant) level, forthe entire trophic structure of an ecosystemis ultimately dependent on this level (REID1961). But photosynthetic rates are regulatedby temperature and availability of light, andavailability of light not only depends onincoming radiant energy and wa ter trans­parency but frequently also on the ratio of"stirred depth" to "illuminated depth"(MORTIMER 1969).

JONASSON (l969J observes that the stra­tification of oxygen in Lake Estrom followsthe same trend as the temperature, with thehypolimnion being devoid of oxygen for

several months. Oxygen depletion in thehypolimnetic waters is also probably dueto the high oxygen demand of the sediments(BEETON 1969). As a result of the com­bined effect of morphometric characteristicsand temperature (Table I), the depth limitof oxygenation in Kariba. which is a warmmonomictic lake (COCHE 1968) and LakeTanganyika, is approximately 20 m and 200m respectively. The influence which thesefactors exert on the vertical distribution offish in both lakes becomes quite apparent.

Annual fish kills on the Kafue involvingmainly Gnathonemus spp. are a direct resultof low dissolved oxygen levels at the onsetof rains (ARMSTRONG. personal com­munication). In the .same fishery in 1965,an estimated 10,000 Synodontis macrostigmaperished from predation by grey-headedgulls, Varus cirrocephalus and other pisci­vorous birds in a shallow sandspit into whichthey had been forced to migrate followingthe de-oxygenation of a flood plain (TAIT1965). The locomotory response to low oxy­gen which may be termed the oxykineticresponse may be expected to move theorganism from regions of lower to those ofhigher oxygen concentration even when otherconditions may be unfavourable (FRY 1969).Oxygen measurements made in 1961 at Mid­Mwela (Lake Tanganyika) indicated that anupwelling of deep de-oxygenated water closeto the shore, was the main factor causingfish mortality (COULTER 1961). Oxygenis thus one of the most important dissolvedgases in ecosystems.

Nitrogen and phosphorous have beendefinitely established as consistent growth­limiting factors in natural water (BIGGARand COREY 1969); therefore, the import­ance of dissolved salts in natural waterscannot be over-emphasized. Total dissolvedsolids is not a measure of the fertility orproductivity of a given water body but canalso have a significant influence on the extentof mixing or circulation of water within the

Table 1. Zambian Fisheries and Their Characteristics

Surface Maximwn Maximum I Muximum I Mean I Conductio ptt Dissolved SurfaceArea2 Length Width Depth Depth vity Solids TemperatureKm Km Km m m u mhos mg/l

BANGWULU 9,850 72 39 5

I

35 8.3

KAFUE 2,833 240 9.7 207.8

I7-8.4 I 93 17-33°C

-.KARIBA 5,250 320 40 120 29.5 100

I

7.5-8.0 70 17-J2°C

LUKANGA

I I

- -

IMWERU 4,560

I

96.5 48 25 8 55-180 6.6-9.3 75 I9-JO'CI

MWERU·WA-NTIPA

TANGANYIKA 32,993 673 48 1,436 520-610 7.2-7.8

UPPER ZAMBEZII \

~

OJ

~~~giii

~

~

146 A. B. E. MABAYE

system. Lake Langsee (Austria)", possessesa monimoHmnion that has not mixed withthe upper waters for over 2,000 years (REID1961). The waters of the monimlimnion con­tain a heavy concentration of salts derivedfrom sediments by biochemical processes andare usually anaerobic. In meromictic Jakes.the oxygen distribution reflects the essentialtwo· region structure of the water mass. Theeffect of holomixis and meromixis whetherresulting from "salinity" or temperature,controls the distribution of fish and otherorganisms through oxygen and other con­comitant factors.

Carbon dioxide not only buffers the en­vironment against rapid shifts in acidity­alkalinity states but is also the gas used byplants in photosynthesis (REID 1961). With­out it, carbon assimilation would not bepossible.

The Biotic ComponentPlants

Bacteria are small organisms which aredifficult to define as most of the characterswhich enable us to recognize an organismas a bacteriU'll1 are negative ones (HAWKERet ai. 1960). In Birge's concept, bacteriastand at the base of the fertility of lakesas they do also in soil (McCOY andSARLES 1969). Bacteria are the primeagents for the return of the dead organicmatter to the soluble state. Without themthe carbon of the world would soon be tiedup in a complex organic state in dead tissuesand phyosyntbesis would stop for want ofcarbon dioxide. Also, bacteria are the primeagents for the return of soluble organic nutri­ents to the biological system. By using suchnutrients in their own growth, heterotrophkbacteria become particulate matter and foodfor protozoa which in turn are food for

'higher metazoa leading ultimately to fish(McCOY and SARLES 1969).

Some aquatic fungi are well known fortheir harmful rather than beneficial relation-

ship with fish. One such fungus is thephycomycete Saprolegnia which causes in­fection of small fishes.

"The whole fisheries industry depends onphytoplankton and algae must rank high asoutstanding contributors to world food sup­plies (SIMON et al. 1966)". Algae are cer­tainly one of the most important classes ofplants in the economy of ecosystems. Thehabitats which they colonize are so diversethat few classes of plants could be moreeurythermal and euryhaline. One of themost important groups of algae is phyto­plankton the composition of which may varyconsiderably according to seasons. This Iswell illustrated by Lake Estrom where inspring (March-May). Bacillariophyceae arethe dominant group within a depth of 0-4 m,while in summer (June-July) Chlorophyceaebecome the dominant group from 0-10 min the sub-littoral zone. In autumn (August)Cyanophyceae are the commonest group ata depth of 0-5 m, Bacillariophyceas takingover again in September at 0-6 ffi. loe rateat which these groups finally sink to thebottom is different. The value of phyto­plankton as food for bottom fauna is thusvery different in' different seasons and de­pends on lake rhythms (MULLIGAN 1969)".Phytoplankton also oxygenate the water, con­vert inorganic materials to organic matterand serve as food for zooplankton. Dis­cussing the state of Lake Mweru Fishery,SOULSBY (1959) writes: "It is significantthat of the three top species. two (T. macro­chir and Tylochromis mylodon) are plank­tonic feeders to a large extent and hereinlies the difference between the productivityof Lakes Mweru and Bangweulu. In thelatter system, the predatory Hydrocynus isone of the top two. If productivity be gaugedin terms of catch/lOO yards of net, theMweru figures average out at 25.1 lbs andthe Bangweulu area approximates to 10.8 lbs.Water sampling by Research Officers hasindicated that there are not sufficient nutrients

in the waters of Lake Bangweulu to supporta good plankton population, while conduc­tivity readings in Lake Mweru show thatthe water is very rich by comparison andthus able to support heavy populations offish which are close to the foot of the foodchain. In the north of Lake Mweru, a sea­sonal plankton bloom occurring in Septemberand October of each year is so closelyassociated with this fish (T. mylodon) thatthe fish and bloom are given the same name,'Ntembwa'."

In general, the food of Tilapia mossambicamortimeri in Kariba consists mostly of algae,mainly gathered from a substrate rather thanfree-floating algae (MATTHES 1966). Inareas where muddy bottom is easily stirredup, large quantities of mud rich in algaeand organic matter are often ingested.

Field observations show that fishermenrarely catch Tilapia mossamhica mortimeriin gill-nets set in the open lake, but have thebest catches of this fish when they set theirnets among submerged objects in shallowerareas. It is reasonable to conclude that avail­ability of periphyton" and mud in this habitatis one of the main fish attractions. Thus,although the fish is generally regarded as anominivore, it would appear that in Kariba,its main food consists of epiphytic epipelicand epilithic algae. On the debit side reducedfish production in some parts of the worldis often the result of excessive filamentousalgal growths (MULLIGAN 1969, quotingLAWRENCE 1958).

In large deep water bodies in which thelittoral zone occupies a small area, phyto­plankton account for most of the primaryproduction, but in small lakes with exten­sive littoral zones, the aquatic macrophytescontribute most of the primary productivityMULLIGAN 1969, quoting STRASKRABA1965). The role of benthic macrophytes inthe aquatic environment is probably moreextensive than that of groups of plants con­sidered so far. In brief, they (1) produce

ECOLOGICAL SlUDIES 147

oxygen through photosynthesis, (2) shadeand cool the sediments of the littoral zone,(3) regenerate substances from the sedimentsto the water, (5) provide surfaces for attach­ment by bacterial periphyton and aquaticinsects, (6)' serve as food, nest-building mate­rial and sites for egg-attachment for aquaticinsects, and fish, (7) protect small fish frompredation, (8) convert inorganic material toorganic matter, (9) serve as food for gamebirds and animals, and (10) anchor the soilin place by means of their attenuated rootsystem (MULLIGAN 1969).

The harmful effects of some aquatic plantsare quite considerable. Free floating plantslike, for example, Salvinia· and Eichorniaspp. which have their roots dangling in waterbeneath the plant, extract nutrients from thewater phase in competition with phyto­plankton and by shading water from sunlightthey inhibit production (REID 1961). Wherethere is a heavy growth of rooted aquatics,the water may become over-saturated withoxygen and incidental to this over-saturation,a very high pH value may occur which canbe fatal to fish (FRY 1969). On the Kafue,plants like Oryza barthii: Vossia cuspidata,Phragmites mauritianus, etc., contribute tothe richness of the water body upon deathand decay, but these same plants if inundatedbefore complete decomposition (or burning)may raise the BOD of the system resultingin fish kills (CHAPMAN et al. 1971). OnKariba, the high fish production of 1963was accompanied by an explosion of Salviniaauriculata. From 1963 to 1966, fish produc­tion declined by about 75% (Table 2) andstarted showing an upward trend in 1967.Although it is known that the phenomenonof a period of high productivity being fol­lowed by a trophic depression phase, is acharacteristic feature of all man-made lakes,it is not unreasonable to postulate that thepresence of Salvinia auriculata might haveaccelerated the decline through this directand indirect form of de-eutrophication. This

148 A. B. E. MABAYE

Table 2. Lake Kariba Fishery: Production offish during the period 1960-1970 ex.pressed asfresh weight equivalent in metric tons

N~re: Lake Kariba was created in 1958, whenthe water of the Zambezi River startedto be impounded behind the Kariba Dam.The average operating level of the lakewas reached in April, 1963, during thefifth year after the closure of the dam.

favourite haunt of sunfishes and perch, con­sists of cladocerans while the zooplanktonof great fishing areas of the world (whichare usually in plankton rich regions) is largelycomposed of copepods (BROOKS 1969).Some c1adocerans found in Kafua fisheryare Pseudosida szaLayi, Diaphamosoma ex­cisum, D. sarsi, Moina dubia, CoridaphniacornUfa, Bosmina longirestris. etc., whilecalanoid copepods include Tropopiaplomuskraepclini with Mesocyclops leukarti andThermocyclops negleetus representing cycla­poid copepods (CAREY 1965}. On LakeTanganyika, some members of the Malaco­straca playa significant role as food for com­mercial fish species. YDung of Lates mlUiaefeed mainly on Caridinae (COULTER ]964)while altbough it will feed on any foodavailable in the open waters, Stolothrissatanganicae seems to prefer Limnocardimj spp.which occur in great numbers in the lake espe­cially at certain season (MATTHES 1966).Alestes macru[Jhtha{mus includes in its foodCan"dina nilotica and the large ostracedCyclestheria hislopia (JACKSON 1961).

The littoral zone of lakes and the vege·tated parts of rivers usually abound in animpressive assortment of young and adultinsects including orders like, Ephemoroptera,Odona!a, Trichoptera, Hemiptera, Coleop­tera and Diptera, most of which are repre~

sented on the Kafue (CAREY 1965). Youngfishes find food and shelter in the vegetationin the fringe zone of the floodplain marginwbere run-off and new plant growth help tomaintain high 0.0 (LAGLER cl ai, 1971).Growth of fishes is rapid during this seasonwhen tbe bulk of the animal component foodconsists of insect "larvae" and crustacea.

Among the zoobenthos, Tendipedidaelarvae, for example, Chironomus spp. areconsumed in great quantities by many fishesas are the culicid Chuuborus and the Dip­teran Simulium (REID 1962). In Zambia,the food of Mormyrus longirostris, a fishof commercial importance found in lakes

Fish llroduction(metric lOris)

4541,8142,7213,4311,881IAi I1.157

7871,137

8242,096

Year

19601961196219631%4196519661967196819691970

AnimalsAquatic Invertebrates

Zooplanklon plays a very important rolein aquatic ecosystems. Nearly all fish subsiston zooplankton shortly after they hatch andcommence feeding, and some adult fish con~

tinue to find much food in the open waterzooplankton (BROOKS 1969). Zooplanktoncomprise numerous groups of invertebrates.From one-third to more than half of zoo­plankton in the vegetated littoral zone, a

point wiIl be discussed in more detail later.Another obnoxious aquatic plant is Fichor­

nia crassipes which has been observed ontbe Kafue. Tbrougb the Department of WaterAffairs. the Zambian Government has start­ed combating the possible spread of thismenace, biologically by the use of parasiticmites and cbemically by employing bormones(HEDBERG 1971).

It is well kncllvn that aquatic plants hamperfishing activities, especially in places likeMweru-Wa-Ntipa, where they form floatingislands.

;

Kariba. Mweru and Bangweulu consistsmainly, of larvae especially of chironomids,as well as Caddis, Oligochaetes, etc. (JACK­SON 1961). Another group of zoobenthosthat plays an important role in the economyof aquatic ecosystems is Mollusca. In thefood-web, snails are consumed in large num­bers by certain fishes such as catfishes, sun­fishes and suckers (REID 1961). The maincomponent of the food of Chrys;ehthys spp.in Zambian lakes is molluscs especiallysnails, (JACKSON 1961). Thus even in thisbrief outline it will be noted that in commonwith most fishes of the world our Zambianfish subsist on planktonic and benthic in­vertebrates; some when young and othersthroughout life.

FishesUsually thought of as highly predaceous

animals, vertebrates, of which fish are amember, represent the culmination of theaquatic food chain (REID 1961). Althoughthis is generally true, the overall ecologicalpicture is complicated by a wide range ofsize and feeding adaptations among verte­brates. Fig. I will illustrate this point quiteclearly even though only fish (and only afew of them) are given. The complexity ofthe food-web will be appreciated even morewhen it is realized that not only do thefeeding habits of th~ fish change with agebut also that most of the fish will take morethan one type of food, either out of choiceor because of variation in abundance of pre­ferred food type. Most freshwater fish arefacultative planktivores, feeding on zoopJank­ton when large forms especially Daphnia,are plentiful but switching to some otherfood, like small fish or benthic invertebrateswhen the supply of large zooplankters fails(BROOKS 1969).

Other rertehratesSnakes, crocodiles, birds and mammals are

all involved in the dynamic interactions of

ECOLOGICAL S1UDIES 149

organisms m fresh water ecosystems. InZambia, the first group is presented byGlypholycus bieolot and Boulengerina an­nulata both of which are piscivorous snakesfound in Lakes Tanganyika and Mweru­Wa-Ntipa. In the same Jakes, we also findCrocodilus catafraetus which in Mweru-Wa­Ntipa feeds almost exclusively on CZarias,in spite of the preponderance of Tilapiamaerochir there (FRYER and ILES 1972).The avian community boasts of numerousrepresentatives and here one needs onlymention the almost ubiquitous Phalacrocoraxafricanus and P. lucidus which feed almostexclusively on fishes. The number of P.africanus in the BangweuJu swamps is esti­mated to be between 5,000 and 10,000 birds.If 10,000 is the correct figure, their annualconsumption of fish is about 260,000 kg or260 t (FRYER and ILES 1972). Among themammaJs, the otter. Lutra maculicolJis isprobably the most outstanding piscivore.

The interaction between fish' and mammalsis by no means limited to' predator-preyrelationship. On Kafue where lechwe, Kobusleche kafuensis are estimated at 95,000 head(HEDBERG 1971), the grazing of the flood­plain vegetation and its subsequent returnto the system in the form of dung plays animportant role in the recycling of nutrients(CHAPMAN et al. 1971). The role of theseand other mammals like Hippopotami inmaintaining the biological productivity ofour fisheries though not yet quantitativelyassessed is probably quite significant.

PURPOSE OF STUDIESHaving briefly glanced at the interactions.

interdependence, coactions and other rela­tionships of the different components ofaquatic ecosystems, we are now, I believe,in a much better position to understand andappreciate the place of ecological studies infisheries development.

Freshwater bodies (fisheries) of Africa canbe divided into two main categories. namely

FIGURE 1.

_SCHEM~~_Cl~jSIFICAT~ OF SOME ZAMBIAN FISH A~CORi;JING TO FEEDIN~ HABITS

M.~

lates _~

~~~f. mossomblcu5

-<Ao

;>

l"

'"!

FAUNAFISH

~

~,// . ~

/ ~Plant Feeders Animal Feeders

/~ /~/ .~ ~

.7;' 7\ ;A~'~ ~~,"T'<=~~' .,~~, ."O""~/:[" ··"·'~·:T::OC~"O>

I I' I r I1.. macrochir L r~ndalli ~.1.. alti"'elis ,,/ .Q schengo ---StO[O;hrisso.!e£ Q.. 'macro[opldotus? Q.. mabusl1. m yl04g.n ~ T ••_u· ~

..-------'---- .----........

- -.........~ "- ,- .... ~~,.\

........~

PI$CIVOROUS

!:t v ittatus

!::i. odoe

•A.

UNEXPLOITED Exploited .Ecological studies of exploited fisheries are

generally motivated by:(a) the need to find ways and means of

increasing yield above existing levels.and

(b) the need to determine the cause of adecline in yield.

(a) Increasing YieldEcological studies conducted in an ex­

ploited fishery may reveal the presence ofan unexploited part of the usable stock.This is well illustrated by the discovery ofa sizable population of Alestes macroph­thalmlls on Lake Mweru. This fish was notbeing harvested by fishermen because being

If the unexploited water body containsedible fish, it may also contain usable stockwhich is defined as the weight of all fisheslarger than a minimum usefnl size (FRYERand ILES 1972), The usable stock is con­tinuous]y added to as a result of youngindividuals reaching this size and being re­cruited to the stock and by the growth ofthose individuals which have already beenrecruited (i.e., a +b in Sketch Diagram, A.)At the same time, death from natural causesor natural mortality (c. in Sketch), is con­tinuously reducing the usable stock, so thatthe three "forces" result in the establishmentof a system that is in a state of dynamicequilibrium. Starting a fishery in a givenwater body introduces a second "negativeforce", the yield (d. in Sketch B.), whichwill be an additional component of mortality.Since this action immediately distnrbs theexisting eqnilibrium and conld lead to aprogressive depletion and eventual extinctionof the stock, one of the fishery biologist'sproblems is to determine the effect of thenew factor so that its magnitude and intensitycan be defined, To do this satisfactorily onewill need to study the ecology of the systemfairly thoronghly.

ECOLOGICAL STUDIES lSI

Death

c. Natural-----~

Deathd. yjeld

c. Natural----~

Usable Stock

EXPLOITED

Usable Stock

Growth b

Recruits a

Growth b.

-----~

B.

Natural Lake or RiverUnexploited

The mere presence of a water body withinour national boundary does not imply thatit's teeming with fish. It could be completelydevoid of fish or full of inedible trash fishor snakes which would be of no use toanyone. We have seen how the composition.abundance and distribution of fish in a lakeor river depend on the physico-chemical andbiological characteristics of the environment.

It would be a waste of public funds tostart acquiring new fishing gear and puttingup fish markets only to find later that therewere no fish in the water body or that wehad acquired the wrong type of gear andbuilt inadequate market facilities or that theywere wrongly sited.

The problems to be considered in thestudy of an unexploited natural water bodycan be ilInstrated by the sketch diagrambelow modified after FRYER and ILES1972.

---~

Recruits a.

natural and man-made, which for the pur­pose of our discussion can be sub-dividedinto unexploited and exploited.

KAFUE FISHERY

152 A. B. E. MABAYE

of a small size it can only be caught in gill­net mesh sizes 5.1 cm and smaller, whichare illegal for commercial fishing. Withoutecological studies, this discovery would havehad to come about as a result of an "acci­dent" and that perhaps after many years.

Ecological studies also help in spottingsituations of overfishing and underfishing. Anexample of the latter in Zambia is that ofthe pre-impoundment studies on the KafueFishery which revealed interesting and mostuseful information which may be illustratedas indicated below:

(b) Investigating Cause of Decline in YieldYield may decline following a decline in

either total usable stock or the exploitedpart of it. By exploitable stock I mean thatpart of the usable stock comprising speciesandJor sizes of fish favoured by the fishingmethods in use. A decline in either of thesestocks may be due to:(1) An increase in natural mortality which in

turn may have been caused by increasedpredation. water pollution or some un­favourable change in the physico-chemi­cal condition of the environment;

(2) Decreased rate of recruitment causedby natural calamities or fishing mal­practices;

(3) Increased fishing effort so as to make(c+d) much greater than (a + b) inFig. 118;

(4) Decrease in growth rate resulting fromthe development. of unfavourable en­vironmental con(1itions, e.g., lack offood, or a sharp rjse or severe drop intemperature.

Labeo altivelis (Luapula Salmon) whichin 1947 formed well over 40% of the totalcatch from Lake Mweru' only totalled 2%in 1959 (SOULSBY 1959). Today one canalmost count the total number of specimenscaught in one year on one's fingertips.Although different people attribu te this de­cline to different causes JACKSON (1961)states that "gravid fish were killed in thou­sands during the spawning run and caviaremade from the ripe eggs in the ovaries wasa popular delicacy in the Belgian Congo",If this is true, the "Luapula Salmon" storywould be an instance where fishing mal­practices have led to a complete depletionof a fishery. Had the ecology of the fishbeen undertaken in time during those pre­independence days, there can be no doubtthat suitable management measures wouldhave been taken to protect the fishery.

Lake yield declined by about 15% between1963 and 1970 on MweruJLuapula fishery.

----~30,600 ts. Yield 3,000 t

c. NaturalDeath 27,600 t

Usable Stock

June-September, 1970

Recruits a.

Growth b.---~

Between the high water level of June andthe low water level of September, 1970,27,600 tons oi fish of exploitable size werelost. Of the 30,600 t, fishing mortality (Yield)only accounted for 3,000 t (8%) whilenatural mortality apparently accounted for27,600 t (92%), (LAGLER et ai. 1971). Thissituation would have been almost impossibleto detect without ecological studies.

Where a state of overfishing has beendetected, steps can be taken to reduce fishingeffort which can be done in one or by acombination of the several ways known tofishery managers, as for example reducingthe number of fishermen, imposing a banon hannful fishing methods, restricting fish­ing to certain times of the year to certainareas of the fishery, etc.

This is not reflected in Table 3 because thedifference was compensated for by increasedproduction from the river section of thefishery which began in 1965 following theimprovement of marketing facilities there andthe issue of fisheries Joans to fishermen byGovernment. Catch statistics examinatjonshowed that Tilapia mucruchir comprised33% of total catches from the lake whilecatch per unit effort for all species averaged10 kg. In 1970, these figures dropped to 7%and 4 kg, respectively. Ecological studiesconducted recently suggest that the declinein population of T. macrochir was causedamong other things by declining water levelsin the intervening period (WILLIAMS. per­sonal communication). There are also in­stances from other countries where declininglake levels have resulted in reduced recruit­ment. FRYER and ILES (1972) report thatthe productivity of shallow Egyptian deltalakes depends on the nature and quantity offlood water supplied to them from the Nile,as was shown by the unusually low dis­charges of 1927 which were followed in thespring of 1928 by poor phytoplankton pro­duction. The low lake levels also affectedthe shallow jnshore breeding sites of Tilapia.Much destruction d the young fishesoccurred and 1929 was a poor year for fishyield in nearly all of the lakes. FRYER andILES conclude by saying that natural fluctua­tions in the level of reLTUiLJnenl to fisheriesare beyond the control of man at least forthe foreseeable future. but I believe thatwhere ecologkal studies reveal a positivecorrelation between recruitment and a naturalphenomenon Hke water~level. fLshery mana­gers could alleviate the adverse effects ofthe phenomenon oy taking such measureslike the regulation of fishing effort duringthe first few years following the calamity.More often than not, however, the eflel:lsof a natural disaster which might otherwisehave heen mild are worsened by increasedfishing effort during the period of recovery,

EOOLOGlCAL STllDIES 153

and since the rate at which the populationis rebuill is greatly dependent upon thenatality-mortality relationships (RHO 1961),extreme and uncoIJtnJlled exploitation mayreduce the numbers below a minimum levelnecessary for population regrowth.

Man-made Lakes"Fisheries are an instantanccv:: 0..:cGndilry

pay-off from a man-made lake, a pay-off thatcan continue when backed by research andmanagement" (LAGLER 1969).

So dependc:nt is the continuance of the

"pay-alI" on research (and management) thatthis statement could almost be applied in itsen tjrety to na tural wa LeI badjes.

Unexploited Before CreationPre-impoundment and follow-up ecological

studies arc essential even in unexploited waterbodies because they yield inlormation on:(I) trends in quality and size of the new

envhonment;(2) changes in species composition and dis­

tribution of fish;(3) availability of unused niches and the

need for introducing new fish specie~

and food organisms;(4) the need for obnoxious weed control.

In Zambia, we have [his type of wa[~1

body in Lake Kariba. Pre-impoundmentstudies carried out between 1956 and 1957showed that Labco alth'clis and L. congorowere the most abundant and potentially im­portant commercial fish present, while TiJapiamossambica mortimeri and T. rendalli werein moderate quantity. Of the predators, tigerfish Hydrocynus vittatus, was the mostabundant. By 1963, five years after inunda­tion, the relative abundance of these specjes.was beginning to change, as shown in Table2. The highest yield was harvested that year,with Labeo altiveNs and L. congoro con­tributing almost 50% of the total numbers.The red-breasted bream, Tilapia rendallistarted appearing in commerdal catches in

154 A. E. E. MABAYE

1964. On account of the selectivity of thegill-net method of fishing, the trends onlyrefer to that part of the usable stock har­vested by 10,2, 12.7, 15.2 and 17.8 emstrelched mesh gill-nets. In 1963 depart­mental experimental nettings showed that:(ai In 5.1 em gill-nets, H. vittalus and A.

imberi were the most abundant species;(bi 7.6 em gill-nets, H. vittatus and Synn­

dontis zanlbezensis;(c) in 10.2 em nets it was the two Labco

species and H. vittatus;(d) in 12.7 em nets, the two Labeo species

mainly, although T. mossambica mor­timeri figure~ more frequently than insmaller nets where it was almost non­existent;

(e) in 15.2 em nets, T. mossambica morti­meri was the most abundant species(ALLEN 1964).

These results serve to remind us of thelimitations of gill-netting as a samplingmethod. However, that the trends shown bythe commercial catch figures are not totallyincorrect is shown by results of ecologicalinvestigations in which a combination of gill­netting, echo-sounding and fish-poisoningmethods were employed. These show:(a) a drastic drop in the population of L.

altivelis and L. congoro;(b) fluctuations in abundance of H. vittatus;(c) a steady increase in catches of T. mos­

sambictf rnort!meri up to 1967 andgradual decline thereafter; and

(d) a steady increase in T. rendalli.This is in general agreement with the

results of ecological studies, which classifiedL. aItivelis among the "Low Density" groupwith less than 150 specimens per hectareand a total yield of 3 kg/ha while T. mos­sambica mortimeri is not only one of the"High Density" species (mean of 2,122 speci­men> per hectare) but also tops the list intotal yield with 468 kg/ha (BALON, inpress). T. rendalli is in the "Medium Den­sity" group with 957 specimens per hectare.

There has, therefore, been a real changein species abundance since the time of pre­impoundment studies on Kariba. That somechange would occur following inundationwas well·known. What is of practical. in­terest from the fishery point of view is thecause of the decline ill population of thetwo species of Labeo between 1963 and 1969alter having apparently adapted themselvesto the new environment for five years.

The decline in popUlation of the twospecies of Labeo from 1963 to 1969 appearsto have resulted from poor recruitment,which may have been caused by one or acombination of three ecological factors, viz.,(a) lack of suitable breeding grounds, (b)predation, and (c) lack of food:(a) T.ack 0; Suitable Breeding Grounds

It is probable that the two Labeo failedto find suitable breeding grounds follow­ing inundation of "traditional" ones in1956. If this waS the case, fish harvestedbetween 196D 'and now consisted ofgenerations spawned before inundation.Growth would have been rapid between1958 and 1963 due to favourable con­ditions created by reduced predationpressure and food abundance. This hypo­thesis is supported by departmental gill·nettings which showed a completeabsence of juveniles of this genus aswen as those of T. mossambica mortf­meri. Once n~tural and fishing mortalityreached a certain point, the numbers oftbis generation would rapidly dwindle-­there being no replenishment from anysource. The life span of these fish, whichis estimated at seven to nine years(BALON, in press), support the trends.

(b) PredationDuring the pre-impoundment studies(JACKSON 1961) found that Labeo wasthe biggest single genus preyed uponby tiger fish, H. vitlatus, especially dur­ing the dry season. Since Labeo werethe most abundant group before and

1

several years after inundation. it is pos­sible that tiger fish continued to preyheavily on this genus. Although preda­tion by itself would have been unlikelyto reduce the population to such alarm­ing proportions, its combination withother unfavourable conditions wouldbring about this effect.

(c) Lack af FoodWhen discussing the role of aquaticplants earlier on, I made brief referenceto "the colossal spread in Lake Karibaof the water fern, Salvinia auriculasa.which at one time covered 175 squaremiles with a thick carpet precluding in­gress of light and downward diffusionof oxygen. HEDBERG (1971) andHARDING (1961) estimated that thefern covered an area of more than 1.0 mdeep in some places. This is more thanone-and-a-half times the total area in­habited by fish (BALON, in press).The weed reached its peak between 1963and 1964 when the young lake was aboutsix years old. This was a time when thetrophic depression phase would havebeen expected to commence.By inhibiting primary production andextracting nutrients for its own growthover such a wjde area. Salvinia musthave accelerated this phase quite con­siderably. One of the multi-effects ofsuch a process would be to reduce thequantity of plankton, periphyton, bot­tom fauna and ultimately fish. Underthese circumstances, the first fishes tofeel the effect and hardest hit wouldbe the highly specialized and thereforeless adaptable group, which among thefive would be Labea, since it feedsalmost exclusively on periphyton. Lackof food would in addition reduce theirability to escape from predators andthis would initially benefit predatorswhich would be expected to increase.This appears to have been the case

ECOLOGICAL STIJDIES 155

with H. villa/us which reached its maxi­mum proportion in 1965. By 1967 thecombined proportion of the two Labeaspecies had dropped by about 88.5%from its 1963 level, while that of H.villaills had declined from 7.8% in 1965to 4.2%.T. mossambica mortimeri which is anomnivore would be expected to wanderfarther afield in search of food andthus would be caught in gill-nets set indifferent habitats. This hypothesis issupported by field observation. Its pro­portion in catches would continue toincrease partly as a result of this andpartly due to decreasing numbers ofLabea sp.

From 1966 the Salvinia mats began toclear. Although no data on the rate of pri­mary production arc available, it is reason­able to assume that the overall ra te improved,although perhaps at a much 'lower level thanat the peak of the high trophic phase. Asthe environment began to stabllize. marginalvegetation began to develop and with itmacrophagous species like T. rendaW andDistichadus schenga. These two species, infact. became so prominent that they account­ed for 10.3% and 5.9% of the 1970 totallandings respectively. The appearance ofSYllodontis zambezensis ,in the commercialfishery to the levei of constituting 8.5% ofthe 1970 harvest also indicates improvedconditions at the bottom fauna level. Theimmigration of Alesles laleraUs into LakeKariba. a fish unknown in this region before1963, must have had some effect on thewhole system, especially as it was so success­ful in the new environment that. by 1970, ithad the highest density, with 29,454 speci­mens/ha (BALON, in press). It was alsobetween 1966 and 1968 that the Lake Tan­ganyika fresh water sardine, Limnalhrissamieden, was introduced into the lake. By1969, tiger fish fed almost exclusively on

156 A. B. E. MABAYE

Limnothrissa mieden (WOODWARD, per­sonal communication).

The effect of this would he to relievepredation pressure on Labco spp. The slightincrease in their overall percentage of totallandings in 1970 might well have been partlydue to this.

However, rapid immigration of large num­bers of individuals often have deleteriouseffects on the original population primarilythmugh increased competition for breedingterritory, cover and food (REID 1961). Itis probably competition for planktonic foodin open waters and the development ofaufwl1chs among submerged trees that mighthave driven T. /llossambicu mortimeri intorelatively shallow uncleared are:as-hence itsrecent decline in commercial catches.

In spite of all these fluctuations, however,results of ecological studies indicate thattotal annual yield from the lake can be in·creased from the presen t 2,000 to 10.000metric tonnes without harming the fishe:ry(BALON, in press).

Exploited Before CreationWhen a reservoir is to be created on a

fishery under exploitation, fishery managersare naturally anxious about the consequencesof such a move. This anxiety is not withoutjustiflcation since the changes wrought bythe new ecological regime could have dis­astrous elfects on the fishery. This is whyif little is known about the ecology of thefishery concerned, pre-impoundment studiesbecome nece.ssary in order to determine eXlst4

ing characteristics of the environment, fishpopulation composition, abundance and dis~

tribution and other parameters so as to beable to predict the likely effects of the watercontrol scheme. Since such waler bodies areoften -multi-purpose. this information is ofvital importance to Governments which mustllecide on national priorities. The final deci·sion to go ahead with the planned projectmay well depend on the results of thc in·

vestigations, for no one c~n expect a wiseGovernment to embark on a project designedto provide electricity to a small village inone corner of the country when this wouldmean complete destruL:tion of the fishingindustry which may be the mainstay of thecountry's economy.

In Zambia, our Government was wiseenough in carrying oUl. with FAG assistance,ecological studies before the creation of theKafue Dam. As a result of these investiga­tions, we have a better knowledge of thepotential of the fishery. We know, for in­stance, that the present level of annualharvest !Table 4) can almost be tripledwithont causing undue harm to the fishery.Government also knows how the interestsof Power, Wildlife and Water Supplyauthorities can best be served without jeo.pardizing the fishing industry.

STUDIES ACCOMPLISHED AND/ORIN PROGRESS IN ZAMBIA

Although the W9rk done differs in depthand extent from fishery to fishery, it maybe summarized as follows;(a) Mweru! Luapula Fishery

The Tilapia macrochir biology studiescarried out between 1961 and 1962 en·abled Government to save the fisheryby declaring the fish's major breedinggrounds a prohibited fishing area. Thissaved the fishery from what appearedto be a certain collapse at the time.Current investigations have revealed thepresence of an Alesfes macrophfhalmusfishery which will soon be exploited.Annual yielll from the fishery is given inTahle 3.

(b) Lake Tan&anyikaLimnological and Limnothrissa miodenstullies enabled the department to suc­cessfully transplant this valuable fishinto Lake Kariba where catches fromexperimental fishing are already beingsold to the local coal-mine community.

Current studies on the potential of thebenthic fishery have given us informa­tion on where and at what fishermenmay expect reasonable catches. We havealso gathered more data on the speciesand size composition of commercialharvest in the pelagic fishery. Annualyield from the fishery is given in Table5.

(c) Lake KaribaLimnological studies enabled us to in­troduce the Lake Tanganyika sardineinto Lake Kariba as has been referredto above.Studies completed in 1971 provided uswith information on the fish speciesavailable, their relative abundance andperhaps most important of all estimateson maximum sustainable yield whichwill enable us to plan the means andlevel of exploitation.

(d) KafueFrom studies completed in 1970, weknow what Government shOUld do toprotect the fishery from the effects ofthe water regulation scheme. Fish stockshave been assessed and widely heldfears of overfishing have been allayed.Plans of exploiting tonnes and tonnes offish that annually go to waste are afoot.Annual yield figures are given in Table 4.

(e) Lake BangwevluStudies carried out in this area someyears ago have added immensely to ourpresent knowledge of the Jake fishTaxocene. More detailed ecological stu­dies are in progress. Total annual yieldfigures are given in Table 6.

(f) Upper ZambeziFollowing our three-year studies, thelimitations of the fishery were estab­lished. Ways and means of getting thebest out of this not-sa-rich fishery havebeen drawn up. We now know that theMaale/o method of fishing is not asharmful as was previously thougbt.

ECOLOGICAL SruDlES 157

(g) Mweru-Wa-NtipaIt is hoped to start ecological studieson this fishery between 1973 and 1974.Till then, exploitation will be carriedout with caution (Table 7).

(h) Lukanga SW(l/npNo ecological studies have been carriedout on this fishery. It is hoped to do soduring the current National Develop­ment Plan.

0) All Commercial FisheriesAnnual fish production for the wholeRepublic fluctuated between 19,000 and30,000 metric tons during the ten-yearperiod from 1960 to 1970 (Table 8).Since independence in 1964, total an­nual fish production has not fallen below25,000 metric tons. It reached a recordlevel of 32,000 metric tons in 1970 whichindicates that the intensive ecologicalstudies carried out during this periodare beginning to bear fruit.

It will be seen that it is only after eco­logical studies that we c'an determine thesize and type of fishing effort, where andwhen to fish, which fish to introduce intowhich fishery, etc. Our knowledge of theenvironment will tell us that. acclimatizingnorth sea herring into our tropical freshwater fisheries would be doomed to failureand also that introducing new fish into afishery that was at its maximum carryingcapacity would be taking an unnecessaryrisk.

I would like to conclude by saying thatresearch workers throughout the world areoften accused. sometimes not without justi­fication, of pursuing activities that bear littleor no significance to the hard and arduousday-to-day life of the man in the street.Indeed, there are many people who lookupon research workers' activities as nothingmore than a vehicle for getting themselvesto a public platform where they can startchanting philosophical platitudes in order tosatisfy their pedantic ego. It is partly for

YCIJr Fis'l production Year Fish production(metric tons) (mptric Ions)

1960 2.453 1960 1621961 3.934 1961 1.7001962 5.640 1%2 5911963 7.037 1963 4201%4 7.983 1964 7601965 6,598 1965 1,6761%6 6.089 1%6 1,6711%7 2,872 1967 2,8621968 4,6% 1968 3,7111%9 5,724 1%9 4,1201970 5,552 1970 4,165

158 A. B. E. MABAYE

Fish produc/ioll(metric tons)

5.9685.4785.8045.8006.8527.9877.8107.4051,IlSl7,4237,782

Fish production(metric lOllS)

2.6691.8141,8836.2587.4663,8164.0397.0936.3274,7156,839

Year

1%0196119621963196419651%61%719681%91970

Year

1960196t196219631%4196519661967196819691970

Table 6. Bangweulu Fishery: Production of fi:ihfrom the lake and swamps during the period1960·1970 expressed 'as fresh weight e(JuivalentJll metric tons

Table 7, Lake Mweru~Wa-NtipaFishery: Produc­tion of fish during the period 1960-1970 expressedas fresh \',:eight equivalent in metric Ions

Table 5. Lake Tanganyika Fishery: Production offish during the period 1960-1970 expressed as freshweight equivalent in metric tons

Fish production(metric toTlS)

6.6265,8965.m75.6195.8146,911

7.2265,7435.7075,8785.594

Year

19601%1196219631964196~

19661%7196819691970

this reason that while it would have beenthe easiest thing in the world for me to statein general terms and in a few lines, the roleof ecological studies in the management offish stocks, I have outlined problems thatare normally encountered in different fresh­water ecosystems, why they should he tackledand have also given local examples. In thisway, I have endeavolJred to show that weare carrying out projects the end results ofwhich are intended to benefit people not inAmerica, Europe or Asia but the ordinaryman and woman jn the villages of our greatcontinent~AFRICA.

Table 4: Kaine River Fishery: PIOduction offish during the period 1960-1970 expressed asfre5h weight equivalent in metric tons

Table 3. Lake Mweru and Luapula Fishery:Production of fish Juring the period 1%0-1970expressed as fresh weight equivalent in metrictons

'--------------- -- - ----

REFERENCES

Table 8. Annual production of fish during theperiod 1960-1970 from the major fi'sheries eX­pressed as fresh weight equivalent in metric tons

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ECOLOGICAL SlUDIES 159

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