Having no idea of the species, but considering the time of year, I can't help but wonder if (many of) these incidences weren't examples of "summer kill"? Apparently the globe is warming and summer kill isn't exactly a novel phenomenon... A similar instance to "July 26, 2013: Mass fish die-off in a river in Moscow, Russia" was documented on 'River Monsters,' when Mr. Wade went in search of a sturgeon in eastern Russia. The water was measured as being unseasonably warm and the salmon suffered as a result.
"July 29, 2013: Thousands of fish die “due to heat and storms” in Handsworth Park, Birmingham, England
July 31, 2013: 3 TONS of fish die due to “lack of oxygen” in a river in Pilsen, Czech Republic"
... sounds like the heat probably did it again...
Of course, global warming isn't entirely to blame. In this case, pollution was the culprit: "July 29, 2013: Hundreds of dead fish wash ashore “due to pollution” on beach in Veracruz, Mexico." Reminds me of that cargo train that spilled into the Cheakamus a few years back...
If you believe that humans are causing global warming then maybe the mysterious link here is us; maybe we're to blame for all this.
Of course, when you put these events on paper it initially seems like a lot but think about how many other lakes there are in the world and how many fish DIDN'T die. I imagine that the events described are <0.000001% of all aquatic habitats. And, again, I can't help but wonder how rare such events really are. The article says that locals claim to have never witnessed anything like this before, but who were these locals? People who live on the coast/lakeshore year-round, or people who don't get out much? Mid-twenties or mid-eighties? Relatively new to the location or born-and-raised?
I'm not trying to diminish the importance of the article's message -- as I do believe that we are capable of destroying our environment -- but this article does strike me as a bit sensational and leaves me with more questions than answers.