Nile River And Lake Victoria Eco System

Describe Some Food Chains And Finally Design A Food Web For Your Ecosystem Showing The Appropriate Trophic Levels

Introduction

Food chain is a linear sequence of organisms that starts from producer organisms and ends with decomposer species (such as grass or trees which use radiation from the Sun to make their food) and ending at apex predator species (like grizzly bears or killer whales), detritivores (like earthworms or woodlice), or decomposer species (such as fungi or bacteria).

Food chain is a series of organisms interrelated in their feeding habits, the smallest being fed upon by a larger one, which in turn feeds a still larger one, etc.; it is the chain from a food source to the ultimate consumer. It is a succession of organisms that eat other organisms and may, in turn, be eaten themselves.

A food chain is a linear network of links in a food web starting from producer organisms; it shows how the organisms are related with each other by the food they eat.

Food web is a connection of multiple food chains. Food chain follows a single path whereas food web follows multiple paths. From the food chain, we get to know how organisms are connected with each other. Food chain and food web form an integral part of this ecosystem.

The trophic level of an organism is the position it occupies in a food web. The trophic level of an organism is the number of steps it is from the start of the chain. A food web starts at trophic level 1 with primary producers such as plants, can move to herbivores at level 2, carnivores at level 3 or higher, and typically finish with apex predators at level 4 or 5. The path along the chain can form either a one-way flow or a food ‘web’. Ecological communities with higher biodiversity form more complex trophic paths.

Lake Victoria Eco System

At 68,460 square kilometers, Lake Victoria is approximately the size of Ireland and three times the size of New Hampshire. Situated at 1136 meters above sea level, it is the world’s largest tropical lake and the second-largest freshwater lake. As Speke discovered, Lake Victoria is the source of the white Nile. Out of proportion with the lake’s large size is its depth, which never reaches more than 84 meters. Its mean depth is 40 meters, which is extremely shallow for a lake of this size. Putting its depth into perspective a little more, 40 meters is not even the length of an Olympic swimming pool. Of course, none of these facts was available to Speke when he first viewed the lake. Rather, what struck him were the attractive possibilities for colonizing the region, the potential for economic development, and the sheer attractiveness of the pale blue waters of the lake.

That blueness would eventually disappear. Although we usually associate pristine blue waters with healthy lakes and streams, the truth is that blue water is not very fertile. Algae require various nutrients to grow, and without them cannot reproduce very quickly. All of the life in the lake depends on the productivity of its algae. As with many lakes, the ecosystem of Lake Victoria was based on complex interactions among species in many trophic levels. At the bottom of the food web were producers (organisms that convert energy into a form that can be used by other organisms) such as algae and diatoms.

There were several fish and animal species in and around the lake which directly or indirectly depended on the algae population. Even the bones of Pleistocene Nile perch which people regard as a strictly newly introduced species have been found on Rusinga Island. The major fish species found here before the introduction of Nile perch include omena (sardine, Rastreoneobola Argentia), Kamongo (Lungfish, protopterus sp.), Mumi (catfish, clarius mossambicus), Ningu (catfish, Cyprinus Carpio), Odhadho (catfish, Labeo), okoko (catfish,barbus docmac), Fulu (Haplochromis sp.), Ngenge (Tilapia, Tilapine sp.),eel and others. The number of Mbuta (Nile perch, Lates niloticus) had dwindled and disappeared for reasons which have not yet been described by scientists. Other animals such as otter, a few crocodiles, hippopotamus, and fish-eating birds also constituted the ecology of this lake. The food chain is complex, with smaller fish species such as sardines and haplochromines feeding on phytoplankton (blue-green algae, diatoms etc), zooplanktons (copepods) and dead decomposing animal and plant material (detritus).

The pattern of feeding by tilapia species is rather complex since in addition to zooplankton and phytoplanktons some fish fingerlings, insects larvae, molluscs and leaches have been found in their gut. Most of the catfish species are piscivorous but may also feed on prawns, dragony and other insect species. The otters, crocodiles and other birds feed on a variety of fish species. The relationship between the hippocampus and the fish is strictly symbiotic. Hippos only feed on live foliage and their excrements may constitute the detrital material upon which several fish species depend. Man is at the top of the food chain feeding on both plant and animal material including the crocodile and the hippopotamus. Haplochromis constituted the highest population of the lake followed by tilapia.

Lake Victoria Food Web

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Describe The Population Dynamics Of Your Ecosystems And How It Impacts The Environment Of Your Ecosystem

Dramatic changes occurring in the Lake Victoria ecosystem

Dramatic changes are occurring in the Lake Victoria ecosystem. Two- thirds of the endemic haplochromine cichlid species, of international interest for studies of evolution, have disappeared, an event associated with the sudden population explosion of piscivorous Nile perch (Lates: order Perciformes, family Centropomidae) introduced to the lake some thirty years ago. The total fish yield has, however, increased 5-fold from 1970 to 1990, but this yield is now dominated by just three fish species: the introduced Nile perch (lates niloticus), Nile tilapia (Oreochromis niloticus), and a small endemic pelagic cyprinid (Rastrineobola argentea); these three have replaced a multispecies fishery. Contemporaneously the lake is becoming increasingly eutrophic with associated deoxygenation of the bottom waters, thereby reducing fish habitats. Conditions appear to be unstable. There is now an urgent need for intensive limnological studies to understand the relationships between these events and to learn from this unintended large-scale field experiment how to manage the lake to avoid further loss of biodiversity and destruction of fisheries.

The Nile perch in Lake Victoria are usually referred to as Lates niloticus but the exact taxonomic status of these fish and some related species has been called into question (Harrison 1991). In this account I shall simply use the generic name Lates. The fish is very large and fecund, reaching sexual maturity after about 2 years and producing several million eggs at each spawning. It may live for 20 years; in Lake Victoria body weights of 35-50 kilograms are common and a fish of 1 79 kg has been recorded (Achieng 1990).

Limnological and fisheries research on Lake Victoria have had close connections with the Freshwater Biological Association, so it seems opportune to report here on the present state of the lake. The first research station on Lake Victoria, the East African Fisheries Research Organization laboratory based at Jinja, Uganda, where the Nile flows out of the lake (Fig. 1), was planned by Dr. E. B. Worthington while Director of the FBA. As a member of the 1927-28 Survey of Victoria Nyanza and its fisheries (Graham 1929), Dr Worthington knew Lake Victoria very well. A founder member of the FBAs staff, R.S.A. Beauchamp, became EAFROs first Director (from 1947 to 1961). The composition of staff at EAFRO was based on the FBA plan, with a chemist, algologist, invertebrate zoologist and fish biologists. The close connection with the FBA continued for many years; various members of the FBA staff joined or visited EAFRO (including myself and Jack Tailing), and Geoffery Fryer from EAFRO joined the FBA staff (Fryer & Tailing 1986).

Changes to the fish fauna

Two special events have had profound long-term effects on the lake: (1), introductions of several exotic fish species, i.e. tilapias and Lates, between 1953 and 1962 and (2), exceptionally high lake levels due to high rainfall, from 1961 to 1964; this helped the introduced tilapias to get established (Welcomme 1966) and also accelerated eutrophication and other limnological changes (Hecky 1992, 1993). The rapidly increasing human populations led to rising demands for fish. Overfishing occurred because the scientists’ advice to prevent this was ignored. To counter the declining tiiapia catches the exotic Tiiapia zillii was introduced (from Lake Albert, via Uganda dams) as a ‘complementary’ species. With T. zillii came other exotic tilapias, Oreochromis leucostictus and O. niloticus, which had also been stocked in these dams. These exotic tilapias have now replaced Victoria’s two endemic tiiapia species, and O. niloticus, which coexists with Lates in other lakes where both are indigenous, is now the dominant tiiapia in Lake Victoria’s catches.

More controversial was the introduction of Lates, despite scientific advice against such an action (Fryer 1960). The fish came from Lake Mobutu (Albert), with a few stocked later from Lake Turkana (Rudolf). A few Lates were discovered in Lake Victoria near Jinja in 1960 (possibly these had escaped from stocked dams though it now seems that some were introduced clandestinely about 1954), so deliberate introductions were made near Entebbe in 1962. For about 20 years, Lates populations increased gradually and then suddenly irrupted in the Winam (Nyanza) Gulf, Kenya, in the mid-1970s. Lates arrived in numbers at the south end of the lake in 1982 and abundant 10-cm juveniles appeared here in 1985, indicating that Lates were spawning in the Mwanza area.

HEST monitored the changes in haplochromines as the Lates populations increased. Of the 123+ species caught at their sampling stations, ca. 80 had disappeared from the catches after 1986. Extra• polation of the Mwanza Gulf data to the entire lake would imply that approximately 200 of the 300+ endemic haplochromine taxa (recognized but not all formally described) have disappeared or are threatened with extinction (Witte et al. 1992a,b). The large hap- lochromine species vanished first (many may have been selected out by trawling), then rare species and those with most overlap in habitat with Lates. Least affected were haplochromines living in rocky areas or in very shallow water with plant cover. Haplochromines had been estimated, from early trawl surveys, to make up ca. 80% of the demersal fish biomass (of more than 200 kg per hectare) in sublittoral areas (Kudhongania & Cordone 19.74a,b). The disappearance of this huge biomass was thought by the HEST team to be a major cause of cascading changes in the ecosystem (Goldschmidt et al. 1993). These hap• lochromines were mostly detritivorous and phytoplanktivorous fish (an estimated 40 % of the biomass in the sublittoral, 6-20 m deep), particularly important as primary and secondary consumers, whereas the currently dominant Lates (now more than 90 % of the demersal fish biomass and estimated at less than 100 kg per hectare in the sublittoral) and the pelagic cyprinid Rastrineobola argentea, are mainly tertiary and secondary consumers (Witte et al. 1992a; Goldschmidt et al. 1993).

At the south end of the lake, changes in the haplochromine fauna coincided with the arrival of Lates in the area, leading HEST to conclude that they were responsible for the main faunal changes (Witte et al. 1992a,b). Changes in the species and their food webs in the Mwanza area sublittoral (6-20 m deep) were summarised by HEST, comparing conditions in the 1970s, before the Lates irruption, and 1989 after it (Fig. 2). In the 1970s haplochromines dominated both in biomass and numbers of species in all trophic groups except the piscivores. By 1989, Lates had replaced piscivorous catfishes and haplochromines. Detritivorous/phytoplanktivorous haplochromines appear to have been replaced by the atyid prawn Caridina nilotica, and after the decline of the zooplanktivorous haplochromines, numbers of the cyprinid R. argentea increased greatly. Oreochromis niloticus, a more generalized feeder than the indigenous tilapias, replaced these. After the haplochromine decline, Lates fed mainly on Caridina, Odonata (dragonfly) nymphs, Rastrineobola and juveniles of its own species.

Limnological changes to the lake

Concern for the deteriorating state of the lake, its loss of biodiversity and signs of collapse of the indigenous fisheries, led to a workshop on ‘Biodiversity, Fisheries and the Future of Lake Victoria’, held at UFFRO Jinja, in August 1992. This was organized by the New England Aquarium, Boston, USA in collaboration with KMFRI, and was funded by the US National Science Foundation. About 40 participants focused on limnology, fish biology and conservation, and socioeconomics, policy and management, aiming to set up testable hypotheses, recom• mendations and research agendas (Kaufman 1992; New England Aquarium 1993). The many contributions to this meeting indicated that environmental changes in the lake basin were basically due to the increase in human populations, with intensification of land use and increased run-off of nutrients into the lake. There is also some urban and industrial pollution, mainly in the gulfs near the growing townships of all three riparian countries. Clearing riparian vegetation has removed plants which acted as natural filters, removing nutrients from waters draining to the lake (reforestation of lake shores would help). Climatic cycles leading to high lake levels (most marked in 1961-64) drowning riparian bush and swamps, contributed to accelerated eutrophication, Between the 1960s and 1990s, a 3-fold increase has been detected in the nutrient concentrations in rain falling over the lake; this may be the result of increased burning of bush and grasslands in countries round the lakes.

The Changing Ecosystem Of Lake Victoria

Effects on fisheries

By 1990 the Lake Victoria commercial fishery was based on three species: exotic Lates (ca. 60%), Nile tilapia O. niloticus (14%), and increasing amounts of the small endemic cyprinid, R. argentea (18%). Subsistence fishing still takes some other species. The change from the haplochromine-dominated fishery in the 1960s to the /.afes-dominated one twenty years later is shown clearly for Nyanza Gulf catches (Fig. 3). Over the lake as a whole, catches increased 5-fold between 1970 and 1990, from 106,500 tonnes to 561,700 tonnes (Greboval & Fryd 1993), but it is not known how much this was due to increased fishing effort or to other factors. These could include an increase in biological productivity of the lake, or the cropping of r-selected species (Rastrineobola and Lates) which have a higher turnover production rate than the small-brood /(-strategy haplochromines (Witte et al. 1992b). HEST’s evidence suggests that Lates were mainly responsible for the fish faunal changes at the south end of the lake. But in Ugandan waters lack of effective management, overexploitation with destructive fishing gear, and interspecific competition among the tilapias, were considered to be more important in the decline of Ugandan fisheries than predation by Lates (Kudhongania et al. 1992).

Conclusion and Future prospects

Lates cannot now be eradicated from the lake, but there is a distinct possibility – indeed likelihood if management is not improved – that Lates populations will crash. This would have very serious socioeconomic consequences for the riparian countries. Lates were at first disliked but are now regarded by local people as the “saviour’ since the enhanced catches lead to more jobs. However, so much of the catch is now processed for export that riparian people with little money have less protein than formerly. Also, the processing facilities under construction could take more than the estimated total population of Lates, which could easily be overfished, and there are already some signs of this; furthermore they are now cannibalising their own young.

The few haplochromines which have survived need protection, by preventing fishing in inshore waters and between rocks, where some small species still survive, although there is a temptation to catch them for bait for line fishing (Kaufman & Ochumba 1993). The original haplochromine complex of numerous species with highly specific habitat ranges (as discovered by the HEST team) was involved in very tight cycling of nutrients. There was an amazing number of piscivorous species (over 30 described species and numerous other taxa recognized but not formally described, Witte & van Oijen 1990), feeding mostly on other haplochromines, especially on their young stages. This complex has now been lost. However, it seems highly improbable that the high yield of the present fisheries could have been obtained by a carefully controlled fishery of haplochromines, because their specific habitats and low fecundity make them very sensitive to fishing pressure (Barel et al. 1991).

Lake management urgently needs a unified Lake Victoria Fisheries Commission, to unify statistics and legislation in the three riparian countries. If the Lates population crashes, there may be pressure for further fish introductions. We would need to know a great deal more about the limnology of the lake before attempting any such remedy. Using bays of the lake for culture of tilapias and other species already in the lake would be more advisable.

References

1. Achieng, A. P. (1990). The impact of the introduction of Nile perch, Lates niloticus (L.) on the fisheries of Lake Victoria. Journal of Fish Biology, 37 (Supplement A), 17-23.
2. Coulter, G. W., Allanson, B. R., Bruton, M. N., Greenwood, P. H., Hart, R. C, Jackson, P. N. & Ribbink, A. J. (2006). Unique qualities and special problems of the African Great lakes. Environmental Biology of Fishes, 17 161 -183.
3. Graham, M. (2009). The Victoria Nyanza and its Fisheries. Crown Agents, London. 255 pp.
4. Kudhongania, A. W., Twongo, T. & Ogutu-Ohwayo, R. (1992). Impact of the Nile perch on the fisheries of Lakes Victoria and Kyoga. Hydrobiologia, 232, 1-10.
5. Tailing, J. F. (2006). The annual cycle of stratification and phytoplankton growth in Lake Victoria (East Africa). Internationale Revue der Gesamten Hydrobiologie, 51,545- 621.
6. Worthington, E. B. (2000). Observations on the temperature, hydrogen ion concentration and other physical conditions of the Victoria and Albert Nyanzas. Internationale Revue der Gesamten Hydrobiologie.