The fisheries of the Kyoga lakes and the likely impact on them by human activities in the catchment
Item Type monograph
Authors Twongo, T.
Publisher Uganda Freshwater Fisheries Research Organization
Download date 27/09/2021 01:14:30
Link to Item http://hdl.handle.net/1834/35599 ... -:11-. ~_.:.. ,. r '-'1
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'T ~--n E: F:- I E~ 1--8 E F.: I E": S 0 r::.- H<:Y"DC'::;f~ l_Ai" P.I/",! D -r H E L J[ iI< E::: IL_ ···R.... IMPACT ON THEM BV !--!l U 1""'1 J¢~~ R...... X (.~ C~ -D"- I ~~.)' I -r :I: E S :r N T" D--~ E"': C':: "4 "T" c~ 1--1 1"'1 E N -;; :! 1 .. , • .. . ! By T. Twongo Uganda Freshwater Fisheries Research Organization, P.O. Box 343, Jinja ,q'f! .. ~,,' y .~ ..::.~\ THE FISHERIES OF THE KYOGA LAKES AND THE LIKELY IMPACT ON THEM BY HUMAN ACTIVITIES IN THE CATCHMENT BY T" TWOI\IGO Uganda Freshwater Fisheries Research Organization, P"O. Box 343, JinJa ABSTH(.4CT A review of the historical development of the fishery resources of the Kyoga basin shows that the fishery based on the alien fishes which surplanted the indigenous species in the 19605 and early 19705 has been evolving. The record ca~ches of the 19705 and early 19805 were mainly influenced by increased nutrient supply and enlarged lake volume" The sudden collapse ifl the fishery particularly of the Nile perch in the mid 19805 was most probably imposed by bad fishing regimes. However, future trends in the fisheries of the Kyoga basin will most likely be controlled by nutrient availability and dynamics as well as by the impact of mans activities in the Kyoga catchment. if' • J. ~ -:~. ....' I "nRClDUC'r I 01\1 The current commercial fishery in the Kyoga basin is daminate-?d by two ali i,?r"l f i s h",}s : t h0? l'h 1 €.? t i L,-~ p i !JJ...lfLt.!.f..h5.§. ~I and the N:i. 1e pEl r c h L.2.1;g§. D..Lt9":'~..:i..£..I::~g.. T Il8 t. \'lJ0 d om i il c\ n t species are native to la~e Albert, the River Nile below the Murchison Falls, Lake Turkana, and to many waters in Central and West Africa. These two fishes were introduced into the Kyoga c:atchmE.~nt t)E'twE~en 19~i4 ,,~rld 1.960 (Hamblyn, 1962:; f:3toili'2m.i:ln <:l.nd F~og e ""S , 1970), tog ~'? t he r" with b"JO t 1. 1a p i <:1 s Q.r:.Q.9..c tLt:~Q.!D..t:~~. l.~l.g~.~.~..t...ir;sJ::.\.?- and T.~~.Lapi...S\. ;,:iJ.1Ji~1 os'ti·?n·:5ibly to boost fishet-y production. the same fish species were l~ter introduced into Lake Victoria with a similar mot~ve. Following ,the establishment of these alien fish species in the Kyoga basi~ the diversity and abundance of the indigenous commercially important fishes declined so much that many of tl1em have virtually disappeared. On the other hand production of the Nile tilapia and the Nile perch in their new environment increased markedly through the 1960s and 19705 making fish output from the Kyoga basin second only to that of lake Victoria, nationally. However, since the early 19805 production of the Nile perch in the Kyoga basin declined dramatically leading to a collapse in the commercial fishery of this species by 1.985 (Twongo 1986; Marriotta, et aI, 1988). According to some experienced fishermen on Lake Kyoga, even the catches of the Nile tilapia in this lake have also declined. This paper reviews and discusses available information on the fisheries of the Ky6ga basin with a view to highlighting factors that have influenced, and those likely to affect future fishery production in this important centrally located aquatic resource. The Aquatic Enviranment~ The aquatic habitats of the Kyoga basin are said to have been formed during the Pleistocene when an old river valley~ possibly the upper reaChes of the Kafu River, was flooded due to backponding and/or spill-over from Lake Victoria, facilitated by tectonic activity (Beadle, 1974). A generally shallow dendritic aquatic system was formed in a sourcer-like depression. The present Kyoga aquatic system (Fig.i) is comprised by 2 three major lakes namely Kyoga (1821 km , length 190 km), Kwania (508 kmz~ 105 km) and 8isina (Burgis and Symoens, 1987). Other components of the system include many small, shallow water bodies often referred to as 'lakes' such as Lemwa, Nakwa and Nyaguo. These are typically situated along the courses of tributaries .such as the Mpologoma, Lumbuye and Okwere; and are surrounded by extensive swamp. Total surface area of open Ir • , )-' 'i, water of the Kyoga lakes is currently about 2700 km 2 while the permanent and seasonal swamps along the numerous tributaries cover about 2000 km 2 of an enormous catchment area covering about 74,700 km 2 , including the lakes. The permanent swamp con!:~ist.~; mainly o'f e;.~tensive matr:5 of papyt-u~,; (h:;Vl;L£~L..h,\,S!, flaPYJ.'"L!~~2_). Large expanses of these occur along the mid and lower courses of t.h€.~ t 1'"' 1. butari E:!S amp t.',,- in<;,1 in t.o eelS tet'n L2\ kE) I The environmental transformation of the Kyoga aquatic habits which accompanied the dramatic rise in water level between 196i and 1964 (Fig.2) are described by Ogutu-Ohwayo (1984) and Twongo (1985). The net result of this rise in lake level was to extend t.he open water area of the major lakes. The extensive submerged and emergent macrophytes, papyrus, and other wetland communities which typically fringed these lakes, and often formed floating islands (sudd) w~re inundated and slowly dSFayed. Disappearance of large expanses of floating papyrtJs to the south of Lake Kwania has practically extended this lake southwards to the channel of the Kyoga Nile. Only several large floating islands now separate Lake Kwania from t.he Nile (cf Fig 1 and Fig.?). The rise in lake level also substantially increased the depth of Lake Kycga and Lake Kwania to the current mean of t~ree and four metres, respectively. ASPECTS OF HYDROLOGY The hydrology of the Kyoga basin is summarised in Table 1, taken from Burgis and Symoens (1987) who quote reports of HYDROMET Survey of 1974 and 1984. Lakes of the Kyaga basin are comparable to a flow-though system. the main, and by far, the largest inflow into Lake Kyoga is from Lake Victoria via the Victoria Nile. The Nile current continuously flushes the Western portion of the lake emerging as the Kyoga Nile, the only outflow to the lake. Many tributaries draining Mt. Eigon and Mt. Kadam which, respectively, lie to to the eastern and north e~stern boarders of the Kyoga catchment, tilter through large expanses of seasonal and permanent swamp to converge into Lake Kyoga. Inflow through these eastern tributaries, for instance the Mpologoma, Lumbuye and Okere, is much less than the inflow through the Victoria Nile. This inflow however, increases greatly during the rainy seasons, (March to May and September to Noyember) when the tribut.aries normally reach full spate. Important tributaries also drain the catchment areas to the south (e.g. the Sezibw2 feeding Lake Kyoga~ and to the north e.g. the Okweny and Abalang emptying into Lake Kwani~ (Fig 1). According to calculation by Burgis and Symoens (1987) rainfall over Kyoga Lakes is less than the rainfall over the land catchment, and the rainfall over the lakes is 105s than the e\/C\porati.on from t.hem, :in a "not"·mal yeal~I'. ""'he I'-J":lt.("~r- b,:::\lancE)~ 'f:Dr 4 ;; !'> ~J the Kyoga basin given in Table 1 indicates a small positive be.lanCE? "for- the basin in a "nDI"'mal ye<:w".- HOll-Jev£:?r~ ifT'i:JOI'-t,~'H'lt fluctuations in lake level e.g. of Lake Kyoga occur monthly (Fig. 2), and annually (fig.3). These fluctuations appear to exert considerable environmental impact on the aquatic ecology and fishery of the Kyoga basin (Twongo, 1988). LIMNOLDGICAL CONSIDERATIONS Detailed information on the limnology of the Kyoga aquatic environment is quite scanty. It appears that bathymetric maps have not been made for Kyoga and Kw~niathe largest lakes in the basin. Therefore, while it is, for instance, known that Lake Kyoga is generally deeper closer to the northern shore, and much deeper along the Nile channel, it is not possible to visualise the topographic characteristics and spatial extent of various depth zones. Yet such information would be useful while evaluating primary and fish productivity patterns. Bathynletric information would, of course, be very important when considering navigation routes. Previous studies on physico-chemical and biological limnology of the Kyoga basin are few and limited in scope, and include the work of Evans (1962, 1982) on Lake Kyoga. A more detailed study of this aspect was reported by Mungoma (1987). Though rather limited by spatial and temporal coverage, the study indicated: a. That the inflow from the eastern and north easter~ portl0n of the Kyoga catchment makes an important contribution to thE' !J c1isso 1 '.led min'?I'··a 1 pOD I" 01' L.:::lkf!2 ! 804 , Sand 8i03 .Si). b. That the westward spartial reduction in the conductivity of the Kyoga watersis"due to the diluting -effect of the "dissolved mineral pool~! by the Victoria Nile inflow as indicated by horizontal variations in conductivity. This phenomenon is illustrated further by conductivity values in Fig. 4. However~ more detailed studies are required to elucidate the influence of inflow from various catchment areas on the levels and dynanlics of various inputs of ecological importance in the lakes of the Kyoga basin. It is hoped that research on Lake Kyoga currently underway at the Uganda Freshwater Fisheries Research Organization (UFFRD) with IDRC Canada, and IFAD/World Bank fundirlg, will supply much of the required infOrmation THE FISH FAUNA AND THE FISHERY The earliest detailed accounts on the species composition habitat preference, and econo~ic importance of the indigenous fish fauna of the Kyoqa basin have been qiven by Worth1ngton ( '1. c:" .... q .' i c- .... ~ i.. -,..., r=' ~.' c·_·, f .... \. ·f·- '-'d 1- I " 7,.,., ) • ·_Jumf" (J 1_ 1 .•..::> 1n 01 ma t_1.01'1'. .1;": ,.- ,-:::>umll'ldl.- - ,l_,:>~". ,- I ~ll c;,.L _= .•c.• ,J • , #-' f:, The fishery was basically subsistance'oriented using simple fishing gear composed mainly of a v21-iety of basket traps, locally mae hooks and a type of seine made from papyrus stalks. Fishing pressure was generally low until gillnets were intl-oduced into Kyoga basin on a large scale after the scientific investigations by Rhodes in 1947 (Stoneman and Rogers, 1970). The effectiveness of gillnets is illustrated in Table 3 which gives a good idea of the commercial catch composition and the major fishing gears of Lake Kyoga in 1951,before the alien fish introductions. Clearly, the fishes of economic importance w~re still as reported by Worthington (1929). However, the most important fishing gears were now gillnets, contributing 37.2% of total catch, followed by hooks (33.8%) and basket trap (29.0%). T hF." data ind i c a t.es t hi.'1 t. . ·t1A10 ind i CJ i:-?n ou!::; tila p i. ,;:\ S Q... f1.??~.~~l..L~.!:} t.lL~. i':.l.r)d Q. :::';l!::=.i.i:~J?..;j,.1..~,.2. ~..,(.? r·t? pal~ tia 11 y SElL] 1·-£~l,.1 a ted in !f:ipace. Dill n(·z! ts ,'J hie h are basic~llly an open water gear landed 70.6% of the Q.. ~.2.~::.~lJ..§n.:"~.\'.\.2. and (Jn I y :2 i.J. • 4 % CJ 'f the P. • Y.:.§..r::.A.~..g.:Lt.i :2. • 0 nthE' 0 t h E0 i' hand basket traps which are set along shoreline swamp, landed ma:.i. nly fJ.. y..s:.r.f~.t-2j~J.i.2. <71 ,,~Yl. D f toted vJe i. CJ h t. for t ht".! S PL~IC.i e',::» an d only 29.4% Df Q.• §.§.f~g.1.Q!J ..tU.2.. Hoclk!:; capturE:1d m.:~inly c: a I;'n i v 0 r 0 u Si f~:~ . 2!?~(;,.tt.i:.fl.Id..;L<;'~.L:;.!. and p r· E' d it.ot-Y t:. " 9..9.fJI!:."i<;.. c:i n c:1 f;.. rng.~:~.2.Si!.f.!}.!;.?4.J.~.~.~?.. • The",e f i !:O; h E~ S wer E) t h f:!~ n m0 j'. E,) a bundan tin t. ile d f" E.' Pe j'.' waters of Lake Kyoga, particularly in the Nile Channel. However~ the above distribution patterns and indeed the entire faunal composition of the fishes of the Kyoga basi.n changed radically under the influence of alien fish introductions. Intl'·oduced spF~ciE's and the Kyoqa Fif.5hery • .Four f ish spec i €0!5 n c:\m€'~ I y t h(;.? 1\1 i 1e t. i 1a pia qX:.~..9..~-:;JJ.r:.9.j.IL:i:.2. l=lj,J.Q.t.;!,..f..h~~i , Urgo c;.!1I:.9.!.!J.;!".?-. J.. ~h~f';'9ji.t.:'1-J:-;; ..t..l:.:l..?.:. I),.L~!.p..i..-?~. .~~lJ. ..Li:):., and the pn".!ditory Nile pel'"'ch b-.<... ·t€2. t1i,J..2:;;?.i.;....t...f";9.2. ~'JE~rf~ inti~oduced intcJ thE.' Kyoga basin between 1954 and 1960. The purpose of the introduction was ostensibly to increase fishery production by converting the then less utilised fishes like the haplochromines into thE- mon:'? acceptablE~ 1\lil(;;") p€:~t-ch fles:ih. T..~.:t.LP.i.. l.'Jould utilize the abundant aquatic machrophytes in the basin. However, it would appear that the choice of Lake Kyoga for the introduction of the Nile perch was influence by the need to obt~in information about aspects of the biology and ecology of the preditor before the proposed int0oduction into Lake Victoria would be made (EAFFRO, 1954/1955; Gee 1969)" Of course, Nile perch appear~~ in lake Victoria in the early 1960s before any meaningful data on- its biology and ecology had been gathered from Lake;~ Kyoga. The fish introductions were initially made into Lake Kwania and Lake Kyoga from where they gradually dispersed into the minor lakes. Direct introductions into the minor lakes were also made. '7 Fig.5 summarises the general trends in the relative importance i~ the commercial fishery of the major indigenous and introduced fish ~-;;pecie:-3. It ~;houlcl be i1otE?d th.:::\t. although P.. 1~.U:; ..Q.?2.:!;'J.CJJLi. and I ..;.:il.Ltt wen~' the ·f:.ir···:st to Pt-O~;pE'ir. in the i1E'W Emvir·onmE'nt" these tilapias did not develop into a major commercial fishery but remained~ and still are, ecologically important only in the shallow weedy littoral zone of the large lakes. They are, however, more important in the smaller minor lakes of the Kyoga basin. It is clear from Fig 5 that the Kyoga basin fishery had firmly shifted from the indigenous species base to domination by introduced Nile perch and Nile tilapia by the late 1960s. This shift occurred much earlier in the major Kyoga lakes where the introduced fishes declined much faster. The Nile perch fishery developed much more rapidly than that of the Nile tilapia, and maintained higher catches until mid 1970s. However, the contrib0tion of the two introduced species to the total catch remained ~bout equal bet.ween 1975 and 1982 (Fig 5; and personal observation by this author), before the commercial fishery of Nile perch collapsed beginning about 1983 (Twongo 1986; Marriott eta1, .1988). The Wetland Fishery The magnitude of the fishery resources in the wetlands of the I they catch in the wetlands. This prot.ein contribution becomes more and ~ore important to the natives as the cost of alternative sources of protein like meat and poultry escalates. Two categories of fishes namely resident and migratory traditionally occurred in the wetlands of the Kyoga basin. The Si> ma i n r- E! siden t s p e c i e sinc: 1u d E? r~" §.s:..tl.Jj:.f?f?_;t~.~~~?2. ,:1 n d ~';;"L~1.r..L~~t~. s p p ~'Jhich are typic:~":\lly' ~'H'?tland f3~K::'!:"~ l,o.Jith thE.' ability tel utilize a r" t.,J, m0 s P h e I~ i c: 0 ){ y 9 en " q . @.~:!i.!.;lLl~~~D...!-:.u :'.!. ii:\ n c:1 Q" ~.f~ 1'" .:L§.t:L.t.;L~.~ DC C L\ 1""" I'" e d ~ in tHe clear water channels of the tributaries. The migratory species undertOOk seasonal visits from the main lakes up to the tributaries mainly tD spawn, and the major species included i~.~tL;.L1:.J;!.f~ ffi.Y..!.?..I.!.::!.2." bc_~J;?~~g y i.~tQ.Li!~.!:l~l? , £g..r..P..~~.2. 5 p petc .. F is', introduct.ion into the Kyoga Basin has affeated these wetland fisheries as well. The migrat.ory species were most affected as they disappeared f~Dm the main lakes. Recent reports on a study in Doho wetlands of Tororo District by the Ministry of Environmental Protection (Echat, personal communication) indicate ~:~ • ",," ~ B t hat Q .. .l~~.hi[~.Q s t:.:i:S;...};.!::.lE.. I~ e pIae €:.~ d Q. .. ~(t...\?..f.;.!::!.1 ..?.IJJ:'.U.2, .:='. n d ~;:l. .. Y.:::~~ I~ j:. a .P...i-J...:i....E!. ,,:3, ~,; the major fish species in the main water channels of the wetland t.ribut..::.. ries" The wetland fishermen of t~e Kyoga basin are basicaly sUbsistance oriented. They use simple gears such as basket traps, hooks and a limited supply of gillnets. Spears could still be used to catch fish during the dry season ~here fish have bee1'1 f C.lI~ C E? cI to conq t-egat (;? tl Y 1'" €.~ C E:' E'din c;l Jr.) .i:\ t. e t.., " g.. .. i~ill.:.~..iJ.}' q.p.i..s..!:!:,~ i ~3 often dug up from the earth during the dry season as soon as their aestivation burrow are located. Total F'ish Ci'ltc:h 'rhe I~elic:\b.il.i.ty cd: availab.lEl commerci2~1 Cii\tch dElta, em thE) fisheries of the Kyoga basin has often been debatable. Fig.6 show~:; commer'cial eEl'tch c:Iat.:::i, compiled fl-'om unpublished da.ta Df the Fisheries Department. Although the absolute magnitude of this data may be in doubt, the trend in the catch should be quite useful. The commercial catch maintained an upward trend into the late 19705 before falling slightly to hold fairly steady until 19805. The dramatic fall in fish output from the Kyoga basin which started in 1983 (not on Fig 6) was mainly d0e to the collapse in the Nile perch fishery as a result of indiscriminate use of seines with small meshed coc:lends off-shore (Twongc, 1986). T)"IE' U~:5e of t.hi·:; qf~,::'~I'" in the s.;hi£.~llo~'J c:ondi tions C".lf Kyoqa l·;:,k.E"!~" is apparently so effective, most probably due to the shalluwness of .~ .~ ~_ p·ft~·'i' t'hE'<::'eM ....1<31,:-..><:::t'.:::::.,. tl-,."t._.c., th'".. "- '-;JI:-c."'''''p''' cOlddr-e.J.. devP]0f.)pd_ .. "._L ir"'j"o.1_•. ::Hl .. ,t·-i'.':::> ? ~f cont.roll.inq· th02 Ni Ie pel"'d-I 1:1O.pula1:-:rbn:;n-"·'<:yogi1 lakos "_ By ~ 1988, the Nile perch. fishery had net recovered sufficiently and constituted only ~5 .. :~~% of the commE~t-cial fi·:;het-y.. H(O)nce in l.akE'~ Kyoga t.he catch data for 1988 was almost entirely contributed by the Nile tilapia (see Marriotte et al,1988)" DISCUSS I DI\! The Nile perch and Nile tilapia boom of t.he 19705 demonstrated beyond doubt the significance of· the Kyoga basin as a major source of less expensive fish protein, employment, and revenue for Uganda and her people. The large number of well est.ablished fishing villages all round Lake Kyoga and lake Kwania~ for instance (Fig. 7)~ and the numerous fishing canoes estimated in 1988 at 3459 (Marriott at aI, 198B) are ample evidence for t.his significance. Apparently lackinq yet, however, is the basic understanding of t.he environmental and ecological dynamics of the resource base of the Kyoga fishery, and the likely impact on this fishery by man's activities in the Kyoga catchment. An at.tempt is ~ac:le here at widening this understanding through a brief discussion of the informaticin reviewed in the paper" • , '" C'1 Early records on aquatic productivity, if there are any, are very few. Ho~ever, some information may be deduced and hypotheses made based en the review in this paper~ 1. Inflow from the extensive catchment area of over 74,000km2 appears to playa very important role in the supply and dynamics of various nutrients in the Kyoga aquatic system. However~ the extensive wetlands plugging the mid and lower courses of. the tributaries to the lakes of the Kycga basin likely act as effective sink for many of the nutrients, possibly to the extent of impoverishing the final lake inflow. On the other hand, the wetland vegetation is a vital silt trap, in view of the heavy silt load often observed in the upper courses of the tributaries especially those draining mountainous areas, and relatively silt-free inflow, say at eastern Lake Kyoga. 2. The rise in the level of the lakes of the Kyoga basin brought about a very important and significant increase in the productivity of the aquatic habitats of the Kyoga basin. The elevated lake levels inundated the extensive macrophyte communities .and other wetland vegetation which typified the original Kyoga lakes. All the submerged and emergent flor0 placed below critical water depth for survival drowned. The subsequent decay of vegetable matter, and leaching of newly submerged soils' released nutrients which set off a burst of primary and subsequent productivity comparable to what h0S beGn observed in Af~icanman-made lakes (Hading, 1966). New niches such as were to be found in decaying wood and rhizomes of papyrus were exploited by various invertebrate faunaincludj.ng cJragontli€.~s;~, m~:\yflies, G_s~Li.sLi:Jls\ Sipp \':?t.c. To c1-:.'\te:, i:?\/E.'r'"l eEls'...l':::'.1 examination of the bottom deposits of Kyoga lakes reveals large quan ti t.it?s of vE~getal::)].e debt-is at val'-ious !3tagE':)'::; o'f decomposition. The relevant question to ask is how much this source of nutrients contributes to nutrient dynamics of the Kyoga basin, and how long it will last. On Fishery Production There is no doubt that the steady rise in fish production in the Kyoga basin throuh the 19605 to the all high peak of the late 1970s (Fig.6) was the product of the coincidental rise in water level in the Kyoga basin culminating in the ecological effects described above, and the introduction of the Nile perch and the Nile tilapia. The expansion in Lake area and depth, and the tremendous increase in aquatic productivity occurred at the time the introduced fishes had just got established. They therefore, took maximum advantage of the favourable environmental situation. The Nile tilapia and the Nile perch were bese suited to take advantage of the new environment of abundant food and space in the Kyoga basin. The success of the Nile tilapia over f.., , ... 1~., '" ~~ " 10 Q...~...!?-,'?_(;1:~ ..L~~:E.t..hl§:. and p. . ~~.r=.+..i-:~.tli:_l.tE.~ ~.J c\ 5 d U €0 t Cl i t~; ~' habits (Lowe MacConnel, 1958; Balirwa, 1990) and fast growth t-at~? j Indeed these and other factors have endeared the Nile tilap ~ ia to fish culturalists throughout the t~opics and subtropics. In J." the Kyoga basin the Nile tilapia enjoys lake-wide distribution. While the fry and juveniles are concentrated in the Shallower and often weedy littoral zone, adult distribution ranges from shoreline swamp to the deepest waters. In Lake Kyoga for instance large specimens appear to concentrateinthe Nile c~annol where it is deepest~ co-existing with large Nile perch The Nile perch appears to have been placed in the most favoured setting to take advantage of the new environmental conditions 61 increased lake volume and abundant food resources in the Kyoga b~sin. Itis opportunistic in its food habits and feeds across several trophic levels (Ogutu-Ohwayo, 1986). The predito~ enjoys a tremendously high reproductive success embodying extremely high fecundity Ogutu_Ohwayo (1988), and hardly any natu~al enemies in the Kyoga basin, even at juvenile stage, in the Kyoga b~sin, and it has an apparently high growth rate (Acere, 1985). The performance of the Nile perch in the altered KY9ga lakes may be deduced from Fig. 5 when compared with Fig 6. Its rapid rate of ent~y into the comme~cial fishery completely oLltstripped that of the Nile tilapia until 1974 when the preditor began to decline (Fig.S). Since the~e was no pa~ticular chande in the fishing regime at that time to affect the Nile perch, it is plausible that certain environmental vari~bles, possibly food and, may be~ space began to limit the population Qf the preditor. The urgument for space as a limiting fattor is weakened by the subsequent overall increase in fish production over the following four years (Fig. 6). The excess production was then contributed by the Nile tilapia. The use of commercial data alone in the above analysis may be weakened by considerations of fishermen's preferences and gear availability. However the decernible trends would remain valid. On the impact of man's activities. The introduction of alien fishes into tIle Kyoga lakes was man-made, and it has brought about irreversible changes in th8 faunal composition of these aquatic habitats. Most local observers have welcomed the change as beneficial to the social-economic well-being ,of Ugandans. However, the aquatic resources of the Kyoga basin appear to be highly suceptible to the effects of man's activities. The following exarnples illustrate. ~ -~ ~.. • ../ ... \1 \ 11 1. The use of seine nets off-shore. The Nile perch fishery in Lake Kyoga suddenly declined in 1983 and had virtually collapsed by 1985 (Twongo, 1986) in respon$e to man's indiscriminate use of seine nets with small meshes in th8 bag (1.::.'," ._; ::::;.~5", on the averaq&?). Th~? o'f'f'-"5hol~e ~5E",ine apP';.:.~aTecl in the Kyoga fishery in 1982 as a modification of the traditional beach seine whose use was limited by availability of seinable beaches. It was hailed as an important break-through by fishermen because it facilitated a break with the beach, and made it possible to avoid theft of passive gear then rampant in Kyoga lakes. The offshore seine became an economic necessity although it clearly was a disastrous gear for the Nile perch fishery. Control of the use of the aff-shore seine appears to be fundamental to the rivival of the two species fishery of the 19705 and early 1980s in the Kyoga basin. Thanks to the enormous reproductive success of the Nile perch, recovery of the preditor' was already underway by 1988 (Marriott et al,1988) following suspension of the seine net when the gear became uneconomic to operate around 1986. In fact, the Nile perch had started to re-appear in the commercial fishery, but so had the off-shore seine. To date the use of seine nets on Lake Kyoga and Lake Kwania is wide-spread once again. To the real fisherman and dependants at the fishlandings in the Kyoga basin the seine is a social-economic necessity in view of the highly expensive alternative, the gillnet. In 1988 the Kyoga commercial fishery reflected a virtual monoculture of the Nile tilapia (see Fig.5). This is an ecologically inefficient culture system in a natural lake situation, and would not optimize fish production in the ."~ l~kes of the Kyoga basin. .:~ i 2. Land use practices. The extensive catchment ar~a (about 74,OOOKm2 ) draining into the Kyoga lakes poses a variety of potential hazards to the fishery arising from man's use of the catchment. First there is the potential and, in parts, real hazard of siltation arising from soil erosion. Heavy siltation would pose grave environmental problems in shallow aquatic habitats like those of the Kyoga lakes. Two major agsllts of soil erosion namely overgrazing and bad agriculturai practices particularly in mountainous terrain, and along the lake shor-e are present in the Kyoga catchment. When it rains the exposed, loosened soil is swept into the run-off afld may end in the lakes via the tributaries or direct along the lake shore. However, the aquatic habitats of the Kyoga basin have built in silt sinks in form of temporary and perma~ent wetland communities along most of the tributaries feeding the lakes. The above natural silt sinks are, however, becoming threatened in some areas by man's wish to exploit the wetlands by growing crops like rice and vegetables which thrive in these erl\iit-·onment. .. '. This advent.urne by m,:'.j.n POSl-:?':5 othE'r' c::h.0\J. J.ani;V:.'·s to aquatic resource management. There is no doubt that the ... ") ".... " 12 indiscreminate clearing of wetlands would lay open the down-stream aquatic habitats of the Kyoga basin to heavy siltation and other potential pollution hazards. Wetland drainage and reclamation, of course, destroys the attendant t ishel~·ie~'5. On the oth":-I"· Ilane! ~I t-ice and VE~(.:.:I~:;?ta.ble (Ji~·m.,;ing ~'IIouJ.d be viewed as quite important for food and income by those who would wish to exploit wetland resou~ce5 agriculturally. These are problems from different sectors of land use and thus call far multi-sectoral solutions. They constitute man's impact on the envirnment around him. The third potential hazard would surface in the event that serious consideration is given to the use of wetland environments to the extent of starting large scale irrigation schemes iI<' comparable to Kibimba Rice Project, with large reservoirs. The likely dependence on the catchment for the replenishment of essential nutrients has been demonstrated is the review section of this paper (Mungoma, 1987; Marriott et.al, 1988). But it is known that very often these large irrigation res2rvoirs form massive nutrient sinks thereby interfering with the supply of nutrients to the aquatic habitats below. In such an event where vital nutrients are affected the impact on the aquatic productivity down stream would be considerable. Besides, large agricultural experiments, especially in wetlands, are often accompanied by heavy use of herbicides, pesticides, and fertilizers of various kinds. Aquatic biota including fish are highly succeptible to this form of chemical pollution, direct and/or through the food chain. +. ·F~' GQJJJ~ L..!d.~J_Q.t:L$_ Future trends in fishery production in the Kyoga lakes will .~ be governed byg /' f ~ 1 " Continued stability in the supply of basic nutrients 1·:', essential for productivity processes" Current knowledge on their supply and dynamics in Kyoga lakes is highly limited. 2. Viable management strategies particularly with respect to fishing gears such as the off-shore seines and the newly introduced cast nets, so as to ensure optimal participation of both the Nile tilapia and the Nile perch in the production proces~es" The essential ingredients for management decision making on the fisheries of Kyoga lakes include: 1. Long term data on supply and dynamics of the essential nutrients, and on fish stocks and catchos. ..... ~--'~-, ¥,¥ .' '" ..,.,0" • " j -::' - "-' 2. A good understanding on the social-economic conditions of the fishing communities in as far as they would affect formulation and implimentation of fishury management measures. 3. A co-ordinated multi-sectoral approach to resource development and management in the Kyoga catchment because development inovations in one resource sector often affect other sectors, especially the aquc:ltic n,~soun:es 51 tuated in tt-H0 "envir'Onrnel'1'tci.1 cesspool". HEFEF~E::I\ICES Acere, T.O., Observations on the biology, age, growth, and 1984 niat.ur".ity ,:.{nd 5e;{uality Cl'f thf:~ 1\!11£o:, pelr'C!-'I:, b:,atf..~. Q_j.J.Q,1:..';,~ .. ~;..hl?_ (Linne), and thE' gl'''m\lth of its fishel'''y in the northern waters of Lake Victoria. Bal in",a, J.S., The effect of ecological changes in lake . 1 CJc,~O Victoria on the present trophic characteristics CJ 'f P.r:J~,Q\;JJ.r.:..9...£!L:t~, I) i J q.t.:i\;..ld.~ 1 n r' ~~ 1 c:\ t ion to i hE' species rCJle as a stabilizing factor of biomanupulc:d:.icm" ill. 1:::'ishelr'i,E":;; o'f th(:? A'fl-ican great lakes. A symposium onrasource use and conservation of African Great Lak~s. University t of Burundi, Bujumbura. Bead 1 ~~, L.C., The inland waters of tropical Africa. An 1974 introduction t.o tropical limnology. london, ~~ Longmans, '365PPn Bur-gis:. 1"1. and J.J. Symoens, Lake Kyoga and associated Lakes. 1987 In.. ("ifl'''ic,an ~'IE,\tlands and !shal10w ~·Jatel'''' bo(jies. EPIFFRCi , East P,'ft"'ic<:,lfl FreS',h',-.)atE~t- F'.i.sh. Has. Dr-g. 0D..D...!!.B..\£.f.2.• 19::~,4/ .19~i~i. Jinjcl. Fc h,,::, t, ,J. 1990. Personal communication. Evans~ J.I-I. The distribution of phytoplankton in some 1.962 Central East African waters. Hydrobiol. 19~299.315. 1982~ Some new records and forms of algae in Central East Africa. Hydrobiol. 20.59-85. '"i!.,L • ,....1 \ .. \ \ \ \, 14 GeE', J. 1"1. , A composition of certain aspects of the biology of 1969 b:.!::'!..t~..~} D..:!...LQ::.t..l..f.:J.,.\.2. (L.innie) i.n ~~OmE,\ [:ast ('d:,r·.ican Ld.:es B.§.Y...:!...... ,.;2.g.9J...~_....Qg..t1:....._Etf..r.~ • 80 ( ::~; ..... Lj.) ~ 2'+ LI· _.. 6.1. 1"·lad ing, D. Lake Kariba. The hydrology and Development of .1966 fisheries !n Lowe MacConnel ed. Man-made lakes . Academic Press. 218 pp. Hamblyn, E.L., Nile perch investigations. Rep. E. Afr. Freshwat. .. l c;h2 Fish. Res. Org., (1961)~ 23-28. Lowe t'I 19 ~58 t~1 .~J...g ..:tj,J~-:"s\. Linn e in East. Af r·.i c an wc~ ters (Pisces~ Cichlidae). Rev. Zool. Bot. Afr. 57 (1-2) 129,-1'70. t'lar-ri 0 t t. , S.P., P.R. Manacop and T. Twongo. Report on the survey 1988 of Lake Kyoga. Agricultural Development Project, North East Uganda. Entebbe. 1"lungoma, S., Horizontal differentiation in the limnology of a e l i87 tropica], r i Vt"2I'''' lake (Kyoga!, Ug,?.nda). !:::l.Y:r:l..r:.9.g.. ;.~.f?J ....':... .1 ~.;f~. , 8 c; ..... (,' 6 • I l' Drac h ....·!'·lf2Z c~ , F.L., Observation on the trend in the fisheries of 1971 L71. Unpl.lbli:-shed IVls. r r~~" ~ PE~f"C l,...<.llf~.2. g.t.~.s::.l:':.2. , CJ<;Ju'tu'''''()hwayo, Predat.ion by the Ni], e h n·t1.. 1984 introduced into Lake Kyoga (Uganda) and its effect on the population of fish in the Lake. M.Sc" Thesis. University of Dar-es-Salaam, Tanzania. ------,1986 Reproductive potential of the Nile perch L S\..t..f:?..?.. !J.,'Ll.9...:.t.tf... ~.:!..?. L. .' and the ~?~;; t <:\ b 1.i s h Iii e n t 0 f t. h (2 species in Lakes Kyoga and Victoria (East Africa) t Hvd 1·..·0b i (J 1. 162:: 1 (j::::;···200 • .~;. ~ !3t.oneman ~ J. and J.F. Rogers, Increase in fish production 1.9'70 achieved by stocking exotic species (Lake Kyoga, Uganda). Occas. Pap. Fish Depart. Uganda (3) 16-19 ,~. ! • .' ,--"," e> 15 Twongo, T., Recent trends in the fisheries of Lake Kyoga-Uganda 198::'i At CIFA WOI'"'kshop on predi tor--'prey relationships, population dynamics and fisheries productivities of large African lakes, Lusaka, Zambia, 29 September 2 October 1985. Subsequently published as CIFA Occas. Pap. No. 15 (1988). ' ------,1986, Current trends in the Fisheries of Lake Kyoga Uganda.. ~t the Uganda Freshwater' Fisht?I'"'ies Research Organization seminar on current state and planned development strategies of the fisheries of Lakes Victoria and Kyoga. ------,1988, The impact of drought on the fishery resources of Uganda. At Workshop em dr-ought mi tigation, HYDROMET Survey, Entebbe, Uganda. Worthington, E.E. A report on the fishing survey of Lake Albert 1929 and Kyoga. London. Crown Agents 136 pp. ~ ). ,-,. 1 Table 1. The water balance of the Kyoga Basin in a "normal year". Source: Burgis and Symoens, 1987. r~ -iQq m3 mm ",--,. - ~/ - Precipitation over the lake 1220 5.55 Inflow from land catchment 6~8 2.91 Inflow from Lake Victoria 5622 25.58 Out.flow :':18 CJ7 26u83 Evaporation 1516 6.9 "" I T ,.;....,. ~ ....~ .. , )-," 2 Table 3: Commercial catch composition and the major fishing gear of Lake Kyoga in 1951. Source: Deprt. Game Fish. Ann. Report Dec. 1951. Fish species Total c.§-!;c!J. ~~e ha\vested by: wt.(lb) I.. age nets hooks traps Oreochromis eg,culentus 143,586 37. :::; 70.6 29.4 g.• variabilis f74,022 24.4 28.6 71. :; Protopterus aethiopicus 61 , 29~5 15.9 0.3 98.6 1..1 Baqrus docmac 49,816 12.9 ~5. 4 95.9 0.7 Clar:-...;i:.as mos~ambi_cus 22,279 5.8 6.6 91.0 2lt4 Barbus al t.ianal is 8,016 2.1 71.8 21.2 7.0 !::1prl!lYrus spp 4,304 1.1 99.2 0.3 0.5 Schilbe £!lY.stus 990 0.2 96.0 4.0 LabeQ. vic tori§'!.Q..!:l2. 436 0.1 76.4 .;23" 6 .1""'\"'"\ 1'", t'-~.R1..Q£.b.r::..o mi..2. s P P 194 0.0 ...:...::. . ..::. 77.8 Iot~L__. ::-::'84:\936 lei() " 8% :::;7.21.. . 3:3. 81.. 29. 01. ct11~~ ' -' . .. /, "".;') ll' ~~:: \ Table 2: Early distribution ~f Lake Kyoga habitats and fish fauna (After Worthington, 1929) SO-ore 1 ine_§.~af.!.!Q Wide bel t of ~,per.!::L~ pap.-Y.r~.?. and species of ~o.§.?...!,a and Phr:E.9.mite?., vir·tucll1y ~;;urrounding the Emtii~e lake. Fishes: F'r-'otC:1ptE'ru!:', aet11iopicus and small species of Clarias. ~------_._------_.------.-._------_._------WELter ~ Zone ,Depth less than three metres; extensive growth of floating or submerged macrophytes including Nymphaea, Fisti" , Ceratop.t}y.~w...:.mr.J E,g,.!:..="l£.'L\-]get:on almost all a.round tl'1e 1 c., k e . F i s Ile~:;;': QX=-~oc hr:.om i~ va [email protected]·s;, Protem tgrL.!.?. aet.b.,tQ.P-i..f.l.l.§,,· Ha.l21ocll.r.Q...fllis (most. speciE's), Ra !:?J;.r...Lll.~QQ.9..l~, ~1 r q ~?n tea, small mormyrids. Note: there was an abundance of inverebrate fauna including prawns and insect larvae. Open Wat~ Depth about three metres, rather narrow and frequently invaded by sudds (floating islands). Fi~;;t·H=S.: g'r"eochrornis E?sc::ulentus, Bagl'-us docmac, Clal"'iEIS gC:lri~.I2in.!d.§., Barl2.!::ls ~dtianalis~, Bar-'gus spp., ~Lm.ilb£~ illYs t.l:~~~_, L'::l bs:e. ~.i.1; torianus:1 tLQn'!!'yr~.Is" s p p., 2Y,nodon1;i ~ v i <;.tQ..r:::l,,~D..H?., s. s p. , Ra~;; tr...Ln ~o bo 1 C:l a t-q en tea, Ha pI Ol;,,,b.!:"_Q.m issP P • ( seve r-' a I ) ~ ha beD..: Y i..£..1..o r 1<::\11 u~. Economical)y important species (lakewide): Qr~~2.ct-!.!"omis var"i..c\bilis., Q. esculentus, Baqr-us doc:maC:~1 C 1 at'.:" i.::L.~" Sls!F iepinus ~ E:r.:ot:.Oj2.:tE,~t::.:l-..\2. Clett-liopicus,~ fu:.u:.Pus, spp., t~f;.. t!,i'!j;l_~ fD.Y.§j;...l::lli.·