Tuesday, July 26, 2011

River Crossing Gone Wrong

The wildebeest migration arrived in the Masai Mara in early July, and by about July 10th, they had started crossing the river. This spectacle draws tourists from all over the world, to watch as huge herds of wildebeest leap into the river and swim through crocodile-infested waters to reach the other side. Typically a number of animals are lost during each large crossing due to crocodile attack, exhaustion or trampling from other wildebeest desperate to reach solid ground. However, sometimes very high losses can result when the wildebeest cross during very high flows or at a particularly bad place.

A wildebeest crosses the Mara as another one behind it is taken by a crocodile
On July 14-15, a very large river crossing went terribly awry, and thousands of animals drowned in the river. We were traveling to collect our water meters at the time for some more focal sampling, and when we returned, the river was literally full of wildebeest bodies. The folks at the Mara Conservancy tell us that the wildebeest had been crossing at a particularly bad place, and they believe they tried to cross at night, which resulted in the high mortality. River flows are a bit higher than normal for this time of year, which may have contributed to the event, but that doesn't seem to be the major reason in this case.

Crossing location where many wildebeest died
It is hard to imagine how so many animals could die during a single event, but we had the opportunity to watch an earlier crossing that occurred at the same location. Animals were streaming across the river and piling up on the far bank, which has a steep, rocky face that is difficult for them to climb. Even as some animals were stranded there, others kept swimming across and then desperately trying to get out of the water, trampling others on the way.
Wildebeest crossing the river

Scrambling up the river bank
Stranded below the rocky ledge
This huge input of animal-borne nutrients into the river is one of the focuses of my dissertation work, so although this was a tragic event, it also was a unique opportunity for me to capture some critical data on the effects of such a large pulse of nutrients into the river. We immediately deployed our water meters above and below the bulk of the carcasses and began collecting water samples. We also walked ~4 km of river bank counting wildebeest carcasses, and determined there were about 5,000 dead wildebeest in the river! 

Wildebeest bodies piled up in the river
 This huge pile-up of bodies also attracted large numbers of vultures and Maribou storks, as well as incredible densities of fat and happy crocodiles, who are seizing this time of fattening to start mating.
Vultures feeding on wildebeest carcasses in the Mara River
Crocodiles mating in the Mara
Although this a huge number of animals to perish in such a short time, it is a relatively small number compared to the estimated size of this herd, which is over 1 million, and these large mortality events are relatively rare. The last die-off in the river of this magnitude happened in 2007, when the Mara Conservancy estimated nearly 10,000 animals had died in the river over the course of several days. We hope to learn as much as we can this summer about how an event like this affects the river and all of the other wildlife that depend on the migration in various ways. 
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