Run-off water from the fire-fighting operation reduced oxygen levels in the canal, causing the fish to die.

BBC: Fire-hit Erewash canal set to be restocked with fish

Mr. Dohlin realized that 300 to 500 freshwater koi fish would die when the saltwater surge flooded their pond.

WSJ: Getting Back In the Swim After Sandy

If the lakes dry up, he explains, the fish will die.

NPR: Young Alaskan Sees Changing Way of Life

Without their fins, sharks cannot swim and they sink to the ocean floor, where they're picked at by other fish and left to die.

CNN: Shark fin soup alters an ecosystem

Those fish didn't die in vain.

FORBES: Magazine Article

It releases a surge of warm water from the equator which kills off the plankton causing the fish which feed on them to die of starvation and driving the survivors south into Chilean waters.

ECONOMIST: The unkind child

This was just the first in a near-comical procession of lurking horrors from anaconda that can digest an entire deer to the slivery candiru (parasitic toothpick fish) that can lodge, die and rot in your urethra, from puraques (eels) that can send 650 volts of electricity into their prey to bullet ants whose stings feel like being shot (all dangers we thankfully avoided).

BBC: Taming the heart of the Peruvian Amazon

Those toxins can affect the central nervous systems of fish and other vertebrates, causing the animals to die.

CNN: Algae bloom kills record number of manatees

Clair are invaded by millions of inch-long fish flies that live only about two days and then die coating roads, sidewalks, yachts and cars in blankets of lifeless, crunchy bugs.

WSJ: Something Smells Fishy in These Tony Michigan Towns