Fish diversity in the Cahora Bassa reservoir (Mozambique) some twenty five yearsafterthe dam closure: what has been the effect of the change from the lotic to lentic environments on the Zambezi River fish fauna?

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REVISTA MOÇAMBICANA DE INVESTIGAÇÃO

PESQUEIRA

RIP No.32, pp 2-14, 2012

Fish diversity in the Cahora Bassa reservoir (Mozambique) some twenty five

years after the dam closure: what has been the effect of the change from the

lotic to lentic environments on the Zambezi River fish fauna?

by

Jorge M. Mafuca

1

and Roger Bills

2

1

Instituto Nacional de Investigação Pesqueira (IIP) Private Bag: 4603, Maputo - Mozambique Jorgemario@sapo.mz

2

South African Institute for Aquatic Biodiversity (SAIAB)Private Bag 1015, Gramstown 6140, South Africa r.bills@saiab.ac.za

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Mafuca and Bills RIP No.32 pp 2-14, 2012 3

Index

Abstract... 4 Resumo ... 4 1.Introduction ... 5

2. Material and Methods ... 8

2.1. Study area ... 8

3. Results and Discussions ... 8

3.1 The fish fauna ... 8

3.2. Changes in Catch composition ... 9

4. Conclusions and recommendations ... 12

5. Acknowledgements ... 12

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Mafuca and Bills RIP No.32 pp 2-14, 2012

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Abstract

The Cahora Bassa Lake is one of the many man-made impoundments which can be considered as a river in a very slow motion, due to its short water residence time (about nine months). As a consequence of these characteristic, seasonal variations in water level occurs with magnitude that attains some 10 m per year. The status of the fish diversity in Cahora Bassa area was assessed before and after the impoundment, however, most of the surveys in Cahora Bassa took place in the mid eighties and a reassessment is long overdue. In order to ascertain the present status of the fish diversity and the possible changes that took place over time as a results of the changes in the ecosystem, a survey was undertaken using gill-nets, seine-nets, electro-fisher, rotenone poison and by means of interviews with fishermen in September 2008. The survey covered the Lake Cahora Bassa and some affluent catchments. Preliminary results showed that most of the indigenous riverine fish species were able to colonize and establish in the newly formed lacustrine environment. Certain alien species, such as Limnothrissa miodon and Oreochromis niloticus, have established and are presently important species in the semi-industrial and artisanal fisheries respectively. At least one indigenous species from tributaries (Sargochromis codringtonii) is becoming abundant in the lake whilst species such as Oreochromis mortimeri, Tilapia rendalli and Opsaridium zambesense are dwindling. On the other hand, the eels (Anguilla marmorata, Anguilla mossambica and Anguilla bengalensis labiata) are virtually absent from the area

Key-words: fish diversity; species composition dynamic; Cahora Bassa dam;

Resumo

O lago Cahora Bassa é um dos muitos lagos artificiais que pode ser considerado um rio em movimento lento, devido ao curto tempo de residência de água (cerca de nove meses). Como resultado desta característica intrínseca do lago, ocorrem variações periódicas no nível de água cuja magnitude atinge cerca de 10 m. O estado da diversidade piscícola neste ecossistema foi estudada antes e depois da construção da barragem, mas estudos recentes datam dos anos 1980, quando o lago ainda se encontrava em processo de maturação. Assim, de forma a avaliar o estado actual da fauna ictiológica da albufeira de Cahora Bassa e as possíveis alterações ocorridas como resultado das mudanças no ecossistema, foi levado a cabo um inventário usando diferentes artes de pesca como emalhe, arrasto, aparelho de pesca eléctrica, rotenone, e através de entrevistas aos pescadores, em Setembro de 2008. O inventário foi levado a cabo ao longo da albufeira e na sua bacia hidrográfica. Os resultados deste estudo mostram que grande parte das espécies que existiam no rio antes da formação da barragem conseguiu colonizar o novo ecossistema; Algumas espécies invasivas como a Kapenta (Limnothrissa miodon) e a tilapia do nilo (Oreochromis niloticus) se estabeleceram na albufeira e actualmente são importantes nas pescarias semi-industrial e artesanal respectivamente; Pelo menos uma espécie de peixe, conhecida como residente nos rios tributários se estabeleceu na albufeira e se está a tornar abundante (Sargochromis codringtonii) enquanto algumas espécies como a Oreochromis mortimeri, Tilapia rendalli e Opsaridium zambesense estão a reduzir. Por fim as enguias, (Anguilla marmorata, Anguilla mossambica e Anguilla bengalensis labiata), que no passado habitavam e frequentavam a área, estão virtualmente ausentes nesta área.

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1. Introduction

Damming rivers for whatever purpose results in changes in the original riverine ecosystem from lotic environment with rapid changes in nutrients and water levels, to a more or less stable lentic environment. These changes bring about alteration in the distribution and abundance on the aquatic organisms as they adapt to the new environment. Such changes, that are reported to have occurred in many of the man-made lakes such as Volta (Petr, 1969); Lake Nasser (Raheja, 1973), Kainji (Banks et al., 1974); Kariba (Begg, 1974), involve population structure, species diversity, abundance, and even biological attributes such as breeding behavior, seasonality and biology. In Lake Kariba, for instance, pre-impoundment surveys showed the presence of only 28 fish species within the area to be impounded, however at least 45 species are known to inhabit the lake today (Marshall, 2000).

Cahora Bassa is the second man-made lake formed through impoundment of the Zambezi river at the Cahora Bassa rapids. Though the first closure happened on 5 December 1975, changes in the dam area started to occur long before that with the closure of Kariba, when flooded conditions in the area prevailed year around resembling more closely the reservoir-river like environment, due to continuous discharge from Kariba (Jackson and Rogers, 1976). According to these authors, such a scenario had favored the occurrence of Oreochromis andersoni, an originally Upper Zambezi and Kafue species. It also had resulted in an extended breeding seasonality for some species such as Hydrocynus vittatus and changes on feeding habits of some others, in response to changes in prey abundance. Together with Lake Kariba, Cahora Bassa is part of the Middle Zambezi system that stars from Victoria Falls and ends in Cahora Bassa falls (Jubb, 1967; Marshall and Bills, 2004). This is the most modified part of the Zambezi system. The physical characteristics of this area is well described by Jubb (1967) and Marshall and Bills, (2004) who also indicated that the fish fauna of this region is relatively depauperate, comprising of about 45 indigenous species comparing to 71 species in the Upper Zambezi; 82 species in the Okavango Delta and 62 in the Kafue System (Marshall and Bills, 2004). This scenario is probably derived from the fact that this section was originally a single sand channel inhabited by tigerfish with almost no cover for small fishes. Therefore, damming resulted in habitat extension and complexity which obviously provided lots of scopes for stragglers from tributaries and upstream to become established.

Like in most man-made lakes, there are basically four impacts at work in Cahora Bassa that mold the composition and structure of fish fauna. First is the change from lotic to lentic environments; Second is the change in nutrient flux/cycles in the lake from the initial filling (high nutrient levels) to a progressively oligotrophic system; Third is the cutting off of the migration routes for certain species; and fourth the impacts of alien introduced species. The fish community of the lake is responding to each of these factors and is changing with time. The response to the first three impacts has probably reached its ‘climax’ in that 30 years on from dam closure/lake formation those species that do not favour the lacustrine environment have disappeared or dwindled e.g. Opsaridium zambezense (Bernacsek and Lopes, 1984). Likewise the eels that were trapped in the lake have most probably died of old age and few or no new recruits get through turbines, resulting in their virtual disappearance. On the other hand, introduction of aliens in other parts of the system have had an effect in Cahora Bassa, has these species have became established into the lake. The spread of Limnothrissa miodon downstream from Lake Kariba was interesting and has been a positive economic development. The spread of Oreochromis niloticus, which also occurred in lake Kariba (Marshall and Rogers, 2000) has been extremely rapid and in just a few years it has almost entirely replaced O. mortimeri to the point where the latter is now listed in the IUCN red List (Marshall and Tweddle, 2007).

Some authors conducted fish surveys both before and immediately after the Cahora Bassa dam closure and the results are reported in Jackson and Rogers (1976)”. Further studies were conducted seven years after the dam closure (Bernacsek and Lopes, 1984). However, these studies were conducted by fisheries scientists with variety knowledge of taxonomy and in addition, most of these failed to target smaller species, due to the methods used. This study was thus, conducted with the objective to fill this long gap by assessing the possible changes in species composition and ecological and biological effects from the lotic to lentic scenarios over the past 25 years after the dam closure.

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Table 1. Location of the sampling sites during 2006 by gill-nets and the 2008 survey at Cahora Bassa dam.

Location Latitude Longitude Cantchendje 15o 33.000' 32o 37.000' Camandu 15o 43.639' 31o 15.644' Zumbo/sede 15o 37.497' 30o 28.383' Zumbo/sede 15o 37.622 30o 30.448' Cayembwa 15o 39.437' 30o 34.234' Chissavo I 15o 39.145' 30o 46.665' Chissavo I 15o 39.078' 30o 46.529' Catowa 15o 16.968' 31o 14.770' Catowa 15o 46.704' 31o 14.503' Nacanda 15o 41.942' 31o 18.916' Nacanda 15o 42.073' 31o 18.990' Magaissa 15o 40.183' 31o 21.862' Mucanha river 1 15o 39.386' 31o 37.092' Mucanha river 2 15o 39.497' 31o 37.019' Muandzi 15o 37.662' 31o 46.387' Muandzi 15o 37.481' 31o 46.533' Duangwa river 1 15o 34.926' 31o 53.048' Duangwa river 2 15o 34.812' 31o 52.971' Nhenda 15o 36.237' 32o 01.781' Nhenda 15o 36.216' 32o 01.936' Cooperativa 15o 45.399' 32o 01.184' Cooperativa 15o 45.110' 32o 01.234' Daque river 15o 46.580' 32o 07.863' Daque river 15o 46.192' 32o 07.863' Manherere 15o 37.904' 32o 06.810' Manherere 15o 37.982' 32o 06.862' Sitche river 1 15o 31.640' 32o 08.311' Sitche river 2 15o 31.651' 32o 08.841 Sitche river 3 15o 33.371' 32o 08.537' Catete 15o 32.316' 32o 09.492' Catete 15o 32.170' 32o 09.477' Nhambando 15o 41.516' 32o 17.074' Nhambando 15o 41.472' 32o 17.262' Capimbe velho 15o 32.177' 32o 17.271' Capimbe velho 15o 32.011' 32o 17.305' Mbambazane 15o 32.442' 32o 19.870' Mbambazane 15o 32.446' 32o 19.546' Nhantidzwa 15o 37.240' 32o 26.238' N'cassi 15o 33.191' 32o 26.267' N'cassi 15o 33.043' 32o 26.092' N'cassi 15o 33.317' 32o 26.286' Nova Chicoa 15o 33.191' 32o 26.267' Nova Chicoa 15o 35.609' 32o 24.560' Nova chicoa 15o 35.689' 32o 24.767'

Location Latitude S Longitude E Nhaukulira 1 15o 36.817' 32o 40.518' Nhaukulira 2 15o 36.320' 32o 40.350' Nhaukulira 3 15o 36.380' 32o 40.410' Nhaukulira 4 15o 38.020' 32o 41.590' Nhaukulira 5 15o 33.210' 32o 40.520' Nhaukulira 6 15o 32.357' 32o 41.101' Nhaukulira 7 15o 32.132' 32o 40.217' Nhaukulira 8 15o 32.430' 32o 38.480' Butho Stream 1 15o 35.480' 32o 42.540' Butho Stream 2 15o 35.470' 32o 42.430' Butho Stream 3 15o 35.280' 32o 42.240' Muze River 1 15o 01.527' 31o 11.501' Muze River 2 15o 00.216' 31o 14.241' Ncanha River 14o 57.536' 31o 23.309' Wooded stream 14o 57.506' 31o 38.013' Fingoe stream 1 14o 59.130' 31o 41.250' Fingoe stream 2 15o 09.260' 31o 53.150' Fingoe stream 3 15o 09.100' 31o 53.070' Fingoe stream 4 15o 08.170' 32o 00.400' Capoche River 15o 08.100 32o 42.190' Safari camp 1 15o 39.191' 30o 35.588' Safari camp 2 15o 39.106' 30o 35.144' Zunguluque 15o 37.050' 30o 26.372' Muanhame river 1 15o 39.487' 30o 40.249' Muanhame river 2 15o 39.195' 30o 41.087' Águas quentes 1 15o 39.241' 30o 57.060' Águas quentes 2 15o 39.195' 30o 57.119' Messengezi 15o 44.374' 31o 04.527' Choe Bay 15o 45.112' 31o 36.434' Ncanha River mouth 1 15o 37.495' 31o 37.052' Ncanha River mouth 2 15o 36.407' 31o 37.246' Ncanha River mouth 3 15o 36.320' 31o 37.180' Year 2008 Year 2006

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2. Material and Methods

2.1. Study area

The study was conducted within the Cahora Bassa Lake and some catchment rivers. The lake is located in Tete, central province of Mozambique, between the latitudes 15o 29’ and 26o 00’ S and Longitudes 30o25’ and 32o 44’E. It is the fifth largest man-made lake in Africa, second constructed on the Zambezi River (Silva et al., Undated), 246 km long with a maximum width of 39.8 km. The shoreline length is estimated at 1.775 km (Bond and Roberts, 1978). For management purposes, the lake is divided into seven sub-basins namely Zumbo, Messenguezi, Magoe, Chicoa, Carinde, Mucanha and Garganta. The location of Cahora Bassa Lake and the sampled sites can be viewed on Figure 1 and Table 1.

The hydrology of the reservoir is essentially controlled by three major inflowing rivers, the Zambezi, Panhame and Luangwa. Since dam closure, the lake level has fluctuated from a maximum of 327.64 m (April 1978) to a minimum of 312.97 (January 1982) with designed annual water level fluctuation of about 10 m (Bernacsek and Lopes 1984; Gliwicz 1984). The climate is influenced by convergence of three distinct air currents; the Southeast; the Northeast monsoon and the Northwest Congo humid current (Silva et al., unpublished report). According to Vostradovsky (1984), three seasons occur in the Lake: (1) the hot rainy season (November to April); (2) the cool dry season (May to August); and (3) the hot dry season (September and November). The mean annual water temperature fluctuates between 23o C (in winter) and 30o C (in summer), the mean annual rainfall is 650 mm (Bernacsek and Lopes, 1984).

2.2. Data collection and processing

Biodiversity sampling was conducted using different gears and approaches and at different periods. Initially, a lake wide gill-net survey, using multi-mesh series of standard monofilament gill-nets (1, 1½,2, 2½, 3, 3½, 4, 4½ and 5 inches equivalent to 2.54 - 12.7cm) was undertaken for three weeks from 22nd October to 6th November, 2006. A more detailed survey using gill-nets, fyke-nets, D-shaped hand nets, a 1.5 m throw net, long lines, seine-nets, electro-fisher, fish poison (rotenone) and interviews with local fishermen was conducted in September 2008. The later survey covered the lake and surrounding wetlands. Typically, more than one method of sampling was used at each site, according to the habitat. The areas sampled during the two surveys are illustrated in Table 1. Samples of fish collected were identified in the field, to the lowest level possible using Jubb (1967) and Skelton (2001) and fixed in formalin 10%. Later the samples were transferred in the lab into 70% ethyl alcohol for long-term preservation. Photographs of the fish species collected and sampling sites were also taken. Tissues were also taken and preserved in 95% ethanol for future genetic analysis and the voucher specimens for these were deposited in the fish collection at the South African Institute for Aquatic Biodiversity (SAIAB) for future reference purposes. Some historical data on catch and effort for the artisanal fishery were used to compare abundance of some species over time and these data were obtained from the Fisheries Research Institute of Mozambique (IIP) database.

3. Results and Discussions

3.1. The fish fauna

Table 2 shows the species recorded by different workers either within the region or within the area where the Cahora Bassa dam lies today. All the names of species have been updated using their current names. Hence, the species that was previously recorded as Leptoglanis rotundiceps has changed to Zaireichthys rotundiceps; Varicorhinus nasutus to Labeobarbus marequensis; Hippopotamyrus discorhynchus to Cyphomyrus discorhynchus; Barilius zambezense to Opsaridium zambezense.

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9 A total of 43 species were identified in the Cahora Bassa Lake and its catchments during the survey. This represents a higher diversity compared to previous work conducted in the reservoir (Jackson and Rogers, 1976; Bernacsek and Lopes, 1984), after the impoundment or those undertaken in the dam area prior to impoundment (Morais, 1974). These findings are also comparable to the records from Jubb (1967) for the Middle Zambezi, in terms of number of species though they differ from the findings of Bell-Cross (1972) in the same area.

In general there are differences between the findings of the different workers in the area. Some of the differences can be explained by differences on sampling intensity and diversity of sampling gears employed though for the work done in 1983 it is not clear what sampling procedures were employed making it difficult to ascertain the reason for such considerable differences.

Other differences are not the result of sampling and appear to be due to environmental changes induced by the damming of Cahora Bassa and also due to alien fish invasions. For instance, the complete absence of all the three species of eels previously reported from the area (Anguilla marmorata, Anguilla mossambica and Anguilla bengalensis labiata) from Cahora Bassa in all the studies conducted after the impoundment (Jackson and Rogers, 1976; Bernacsek and Lopes, 1984; Vostradowisky, 1984) reflects the negative impact that this infrastructure has brought to the Zambezi river eels populations. The other species that was known to migrate into the river as far as the Kariba gorge prior to the Cahora Bassa dam closure is the Zambezi shark, Carcharinus leucas (Bell-Cross and Minshull, 1988). This species is completely absent from the studies conducted after the dam closure. Some species on previous checklists are obvious misidentifications e.g. Oreochromis mossambicus which was recorded by Jubb (1967) in the Middle Zambezi. According to Bills (1999) and Skelton (2001) this is typically a Lower Zambezi system species and its distributions go as far Eastern Cape.

Other species are new in the area and their occurrence owes to introductions upstream. Such are the cases of Kapenta (Limnothrissa miodon) and Nile tilapia (Oreochromis niloticus). The Kapenta was first confirmed by Lindem (1983) who did the first hydro acoustic survey in the lake and reported Kapenta density ranging from 12,000 to 18,000 fish/ha. The species was introduced into Lake Kariba between 1967/68 with the objective of filling the pelagic habitat which was formed by damming (Bell-Cross and Bell-Cross, 1971; Begg, 1976; Mandima, 1999 and Mandima, 2000). Today it supports a thriving fishery of about 250 fishing rigs and the annual catches have surpassed all the previous estimates. Bernacsek and Lopes (1984) predicted a potential yield of about 8,000 t and Marshall (1988) predicted about 12,000 t, while current catches have reached 20,000 t per year.

A survey conducted in Cahora Bassa lake revealed that Nile tilapia started to appear in around 2002/3 (Buque, 2007). Since the species was reported to have been established in Kariba and in the Kafue systems, for sometime now, where it is replacing the indigenous Oreochromis mortimeri and O. andersonii (Bills and Marshall, 2004), it was an expected immigrant of the Cahora Bassa area. It appears that the 2001 floods may have triggered a rapid expansion of this fish species into the lake. Obviously, this species was not recorded by all previous researchers (see Table 2) except for Buque (2007).

The species Kneria auriculata was not found, despite all the effort to trace it. This species was first described from Muze River, which is an affluent of Mucanha. Three days of sampling were devoted to locate the species. The river from which it was collected was located and, by the time of our sampling it consisted of some few stagnant pools on the river-bank. No flow existed neither along the river nor on the Mucanha River. Changes on the catchments may have resulted in draught out of the river source turning the river into seasonal, thereby making it difficult to fish fauna survival.

3.2. Changes in Catch composition

Figure 2 shows the trend of the species recorded in the artisanal catches from 2007 to 2009. Eighteen species where recorded in artisanal catches, of which the most dominant species were the Nile tilapia (Oreochromis niloticus), the tiger fish (Hydrocynus vittatus) and the chessa (Distichodus schenga). The kariba tilapia (O. mortimeri), was one of the dominants for the years 2007 and 2008, but it seems to have declined dramatically in 2009 whilst the green bream (Sargochromis codringtonii) is beginning to appear in the catches.

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Table 2. Species reported in different studies conducted and the areas where the studies took place (MZ-Middle Zambezi; CBA-Cahora Bassa

Area; CBD-Cahora Bassa Reservoir; CBD & C- Cahora Bassa reservoir and Catchments).

Species Jubb (1967) Bell-Cross (1972) Morais (1974)

Jackson & Rogers 1976)

Bernacsek & Lopes (1984)

Present study

Region Sampled MZ MZ CBA CBD CBD CBD & C

Amphilius uranoscopus X X

Anguilla bengelensis labiata X X X

Anguilla marmorata X X Anguilla mossambica X X X Aplocheilichthys johnstoni X X X X X Aplocheilichthys katangae X X Barbus annectens X Barbus barotseensis X Barbus cf. banardi X Barbus eutaenia X Barbus fasciolatus X X X X Barbus haasianus X Barbus kerstenii X Barbus lineomaculatus X X X X X Barbus manicensis X X Barbus paludinosus X X X

Barbus radiatus radiatus X X

Barbus trimaculatus X X X X Barbus unitaeniatus X X X X X Barbus viviparus X X X Brycinus imberi X X X X X X Brycinus lateralis X X X X X Carcharhinus leucas x Chiloglanis neumanni X X X X X Clarias gariepinus X X X X X X Clarias theodorae X X X Ctenopoma multispine X Cyphomyrus dischorhynchus X X X X X X Distichodus mossambicus X X X X X X Distichodus schenga X X X X X X Heterobranchus longifilis X X X X X X Hydrocynus vittatus X X X X X X Kneria auriculata X X Labeo altivelis X X X X X Labeo congoro X X X X X Labeo cylindricus X X X X X Labeo molybdinus X X X X X Labeobarbus marequensis X X X Limnothrissa miodon X X X Malapterurus shirensis X X X X X X Marcusenius macrolepidotus X X X X X X Micralestes acutidens X X X X X Mormyrops anguilloides X X X X X X Mormyrus longirostris X X X X X X Nothobranchius orthonotus X X Opsaridium zambezense X X X X Oreochromis mortimeri X X X X X Oreochromis mossambicus X Oreochromis niloticus X Petrocephalus catostoma X X Pharyngochromis acuticeps X X X X

Protopterus annectens brieni X X X X X

Pseudocrenilabrus philander X X X X X Sargochromis carlottae X Sargochromis codringtonii X X X X Schilbe intermedius X X X X X X Synodontis nebulosus X X X X X Synodontis zambezensis X X X X X X Tilapia rendalli X X X X X Tilapia sparrmanii X X X Zaireichthys rotundiceps X X X X Total 47 55 35 30 18 43

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11 Before impoundment the tilapiines O. mortimeri and Tilapia rendalli were the commercially important species (Morais, 1974) and so remained many years after the dam closure (Bernacsek and Lopes, 1984). However, as shown in Figure 2, there has been a drastic reduction in the contribution of these species to the artisanal catches and they are being replaced by the Nile tilapia (O. niloticus). Therefore, it is clear that, such has happened in Lake Kariba, the original tilapiine population is being displaced and replaced by O. niloticus and conservation concerns raised about particularly the kariba tilapia (O. mortimeri) in Lake Kariba (Bills and Marshall, 2004) are equally applicable in Cahora Bassa. Considering that this species is endemic to the Middle Zambezi system, and that it is almost extinct from Kariba and Kafue (Bills and Marshall, 2004), the only other place where it was left is in Cahora Bassa and, probably in the Luangwa system. But, as it is shown in Figure 2, this species is also disappearing from Cahora Bassa, it is becoming urgent that conservation actions are taken to prevent its extinction. In fact, only one specimen was recorded during the 2008 survey and it is rarely seen in the artisanal catches. Though some steps have been taken, such as placing it in the IUCN red list (Marshall and Tweddle 2007), much more regional initiatives are necessary to identify possible refugia where some viable populations of this species may survive.

Although Jackson and Rogers (1976) recorded some specimens of Sargochromis codringtonii, this species was never abundant and rarely occurred in the artisanal catches. However, from the Figure 2, it seems that there is an increase of this species in the catches. This may be an indication of colonization of newly formed environment by the species and probably they are also benefiting from reduction of other tilapiines.

0 10 20 30 40 50 60 B ry cin u s im b er i C la ria s g a rie p in u s Dist ic h o d u s m o ss a m b ic u s Dist ic h o d u s s ch en g a C yp h o m yr u s d is co rh yn ch u s H et er o b ra n ch u s lo n g if ilis H yd ro cy n u s v it ta tu s La b eo a lt iv eli s La b eo co n g o ro M o rm yr o p s a n g u illo id es M a la p te ru ru s sh ir en sis M o rm yr u s lo n g ir o str is O re o ch ro m is m o rtim er i O re o ch ro m is n ilo tic u s Sc h ilb e in te rm ed iu s Sy n o d o n tis za m b ez en sis Tila p ia re n d a lli Sa rg o ch ro m is co d rin g to n ii C on tr ibu ti on t o the a nn ua l C at ch (% ) Species 2007 2008 2009

Figure 2. Catch composition of the artisanal fishery in Cahora Bassa during 2007-2009. Data source: Pescart database, Mozambique Fisheries

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12 The tiger fish has always been the abundant species in the area. Its population may have benefited with establishment and abundance of the L. miodon, the most important food item (Cabanelas, 2005) and became cosmopolitan. Obliviously, the apparent reduction of the species in the catches is a consequence of reflection of the dynamics of the prey population which has seen some fluctuations during the past five years (Mafuca and Pegado, 2005).

4. Conclusions and recommendations

The present fish diversity in Cahora Bassa is a result of combined impacts resulting from dam wall construction e.g. cutting off of river migration routes and lake maturation which changed nutrients dynamics and habitat diversification and alien fish species invasion. Some species that were known to occur in the area where the dam exists are now absent as a result of their inability to negotiate the dam wall e.g. the anguillid eels and the Zambezi shark. One species (O. mortimeri) is endangered as result of competition exclusion and hybridization by the alien O. niloticus. The rapidity of the decline of the indigenous Oreochromis, in the last decade, is remarkable and warrants further investigation both in Lake Cahora Bassa and the lower Zambezi region. Other alien species, e.g. the clupeid L. miodon, does not seem to have had negative impacts on the indigenous species but rather has enhanced the populations of tiger fish and the dynamics of the later seem to harmonize with the changes on the populations of the first.

It is clear that with the construction of Cahora Bassa there were both negative and positive impacts to fish diversity and more importantly to fisheries. Impacts on eels and probably sharks could have been minimized if management measures such as fish ladders or fish ways were considered during dam construction. On the other hand, the construction of the dam has allowed for creation of new, diverse and complex habitats to be formed, such that allowed for invasion of tributary and upstream river species.

The same things apply to alien fish species invasion. The occurrence of L. miodon in the lake, has allowed for development of a thriving commercial fishery and artisanal catches have boomed with the establishment of O. niloticus, despite the well known negative impacts it has on biodiversity. However, the long term sustainability of the situation is unclear and requires continued monitoring.

Very little is known about the degree of dispersal of these invasive fish species within the system and down to lower Zambezi. Therefore, there is an urgent need for the assessment of their extent and of their possible impacts on habitats and fish diversity. Ideally a long-term monitoring program should be established to track future impacts and changes in fisheries communities. This could be linked to fisheries livelihoods.

5. Acknowledgements

This work was conducted under a project co-funded by ICEIDA and the Government of Mozambique. Hence, the authors wish to thank the two institutions for the financial support. We also wish to thank SAIAB staff in general for varied assistance throughout this project but in particular to Bernard Mackenzie and Kholiwe Dubula for help in sorting samples and accessioning material into the SAIAB collection; All the IIP-Tete Office personnel for all the logistic support and Mr. Mario who captained the research boat.

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6. References

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