In northern Botswana, the term fish trap is used to describe a section of the Okavango River (or one of the many channels that flow from it and make up the delta) that becomes separated from the permanent channel supplying it with water. When this happens, fish can’t escape and become concentrated in shrinking channels and lagoons. As a result, a glut of easy pickings attracts large feeding parties of different bird species.
When the level of the Okavango River is at its highest, hun-dreds of square kilometres of floodplain are shallowly inundated. Sunshine stimulates the growth of aquatic vegetation, enabling fish to find shelter among the plants and in the deeper pools or depressions. In a typical season, fish numbers flourish from April to September, with the highest water level occurring in June. The phenomenon of fish traps is most likely to occur from September to January when increased day length and rising temperatures combine to reduce the water level. From December local rains may partially replenish the system.
At some point the water becomes so shallow that the fish are very noticeable and vulnerable to piscivorous birds on the hunt. About 400 bird species occur in the Okavango Delta and those that may attend a fish trap include egrets, storks and herons, bitterns, spoonbills, African Fish Eagles, kites, kingfishers, terns, harriers, Hamerkops, skimmers, stilts and ibises. The size of the fish, the water depth and the structure and quantity of the vegetation seem to determine which birds can benefit and will show up. Other predators, notably Nile crocodiles, may also be present in fish traps and this may influence how far the birds are prepared to venture into the water to catch their prey.
Those attracted to the bounty of food on offer are not limited to birds and crocodiles, and a varied range of other opportunistic wildlife may benefit from it. Nile monitors are sometimes found in or around fish traps, while mammalian predators that will take advantage of the fish feast include spotted-necked otter, leopard and African civet. I saw a young Martial Eagle launch an aerial attack on a mixed flock of storks, herons and ibises that were greedily gulping down the abundance of food. However, the Martial Eagle missed its target – an African Openbill – in the large flock and the abortive strike scattered all the birds for some time.
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