Crocodiles in lakes Edward and George: Where did they come from or did their fossils resurrect?

Since the year (2019) began, I have had an opportunity to have two field

excursions on Lake Edward, obtaining samples of fish and characterizing

the physical habitat and water conditions. As usual, this involves moving to

predetermined sites, and with the help of a collection of tools such as

gillnets, echosounders, GPS and others, we obtain the different data types.

During the excursions on this lake however, there are many things to take

of, and key among these are wild animals.

Lake Edward is located in Queen Elizabeth National Park, a distinguished

park that is actually a biosphere reserve. Several animal species that call it

home include elephants, loins, buffaloes, and many others including

threatened species. You may think that these animals have nothing to do

with somebody working in water because they are found on land but in

water you find hippos and the dangerous man eaters, the crocodiles. So,

you do not only care not to disturb the peace of the animals on land by for

example reckless hooting when driving but you do not have to mess with

them as contact with most of these animals can be fatal. Elephants kill and

do hippos, buffaloes and off course crocodiles and lions. The list is endless.

Off course my main interest is in what is in the water although recently you

must have been seem my article on elephants. Please revisit it here, it is

enriching. The excursions were wonderful. We caught many fishes, people

were engaged, we distributed some to locals for food (off course after

getting data and adequate collections) and personally, I encountered fish

species such as Labeo forskalii which I had never encountered (see pictures

below).  Field work is always interesting. A part from the “fishy” things, the

highlights of the excursions were hippos and crocodiles. These frightening

creatures overlapped with our fishing grounds. Let me focus on crocodiles

though.

Crocodiles seem to be many in this lake and if you fear a lot like me, you

may end up stopping at the lake shores especially if it your first time.

Indeed, I am thankful to fearless field assistants. Crocodiles are so abundant

that one was captured in our gill-nets and sometimes we retrieved fish

specimens partially eaten by crocodiles. May be some fish in our nets were

swallowed completely, who knows. Questions started coming up in my

mind wondering for example the importance of fish in the diet of

crocodiles, the susceptibility of fishers to crocodile attacks, whether you can

be attacked when you are on a boat, and the origins (whether native or

introduced) of the crocodiles. The following are some lessons learned.

Crocodiles in Lake Edward seem to be many. One of them was caught in our gill-nets

The per capita dependence of crocodiles on fish is apparently small (Cott,

1961). I think their predation on fish may be a concern if populations are

high in a specific area or if threatened or endemic species are included in

their diet. The crocodiles in the area are a menace to local people. They kill

to an extent that local politicians have picked interest.  The national agency

that takes care of Ugandan national parks (Uganda Wildlife Authority) has

responded to the outcry by relocating some crocodiles to crater lakes

deeper into the park and establishing protective cages on domestic

extraction points. People are also urged to take care and avoid them. These

measures seem not to work as people are still dying. It appears that you can

be attacked even you are in a boat because fishers have not been spared.

The biggest lesson to me was that crocodiles are not supposed to be in the

Lake Edward System which includes Kazinga channel and lakes Edward

and George.

The first account of the absence of crocodiles in the above system which I

came across is by Worthington (1932). He presents some notes on the

distribution of crocodiles in Uganda, noting its absence here and presence

in other water bodies including lakes Victoria, Albert Kyoga, Kwania, River

Nile, River Semliki (below the falls). Reports published later in 1960s

including (1969) have no reference to the presence of crocodiles in the lake

Edward-George system.  The reasons given for the absence of crocodiles in

this system yet they are found in the neighboring Lake Albert System is

because of a probable barrier of Semliki falls and extirpation of earlier

extant populations by extreme environmental change. The later hypothesis

is supported by presence of fossils of crocodiles in sediments under the

Lake Edward system. This thinking also explains the absence, in this system

of some fish taxa such as Lates and Hydrocynus that are present in the

Albert system. The former hypothesis presents Semliki falls as a barrier that

halted the re-colonization of crocodiles through Semliki river just as it does

for fish flocks either side (Witte et al. 2009).  

Jim (2009) and Spawls (2018) reported presence of crocodiles in Kazinga

Chanel and lakes George and Edward but wondered what could have been

their origin. Like they wondered, I am also wondering how they came

there.  Was it a deliberate or undeliberate introduction, did the Semliki falls

falter in their function or the fossils resurrected? The first three reasons are

possible and regrettable.  Worthington (1932) noted that re-colonization

through Semliki would be possible if the fringes of the river and the falls

were deforested. This is likely to have happened in recent times. This can be

excluded by undertaking a small study tracking movement of crocodiles

upstream Semliki beyond the falls. If this was not the means, a question

remains on who introduced it and for what. The presence of the species

there is regrettable because in most cases new introductions become

invasive and who knows, the species may already be or about to be

invasive. 

References

Cott, H.B. 1961. Scientific results of an inquiry into the ecology and economic status of the Nile Crocodile (Grocodilus niloticus) in Uganda and Northern Rhodesia. The Transactions of the Zoological Society of London, 29(4), 211-356.

Jim. G. 2009. Nilotic Lakes of the Western Rift: In H.J. Dumont (ed.), The Nile: Origin, Environments, Limnology and Human Use, Springer Science + Business Media B.V.

Spawls, S. et al. 2018. Field Guide to East African Reptiles. Bloomsbury Publishing, pgs. 544.

Witte et al. 2009. Fish Fauna of the Nile: In H.J. Dumont (ed.), The Nile: Origin, Environments, Limnology and Human Use, Springer Science + Business Media B.V.

Worthington, E.B. 1932. A Report on the Fisheries of Uganda Investigated by the Cambridge Expedition to the East African Lakes, 1930-31. Zoological Laboratory, Cambridge.

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