Some of you may have read "Last chance to see" by Douglas Adams. The book, a documentary about endangered species, describes how the Yangtze River Dolphin population declined drastically in just one or two generations. Those dolphins were not slowly killed by man for food. They simply died, because all those engine ships going up and down the river rendered their sonar useless.
It is now 3 years since a dolphin was last sighted despite a Large Expedition in 2006. They will not be officially declared extinct untill 2054 (50 years after a living speciment is last sighted) but there is little hope that they have somehow survived. As far as i know, it is the first of the species mentioned in "last chance to see", that will not be seen anymore.