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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