Around 2011 there was a huge uptick in headlines of mass groups of wild animals from a range of species being found dead all over various parts of the world. Often there was never any conclusive explanation for what caused each herd/swarm/flock to die off.. for fish it could often be blamed on lack of oxygen in water but many other cases seem to have gone unsolved.

[1] http://news.nationalgeographic.com/news/2015/01/150113-mass-...

[2] http://news.blogs.cnn.com/2011/01/07/mass-animal-deaths-scru...

[3] http://www.newyorker.com/magazine/2009/05/25/the-sixth-extin...

[4] (Ignore + skip past the sensationalization, this is a comprehensive list of websites citing instances of large groups of animal deaths.. notable if only for breadth and variety of different website 'sources' covering the past 5 years) http://www.end-times-prophecy.org/animal-deaths-birds-fish-e...