© 9News, Colorado
The fish, which were mostly sucker fish with some brown trout, were discovered by Fort Collins resident Bob Jackson. Jackson immediately informed local authorities, who sent out representatives from the U.S. Department of Fish and Wildlife.
"I walked the shoreline on the north side of Willox and took some samples that I turned into the health lab," Shane Craig, district wildlife manager and game warden for Fort Collins, said.
Craig says the fish most likely swam into the irrigation canals as the temperatures turned mild and became trapped in the canal after being released from the Horsetooth Reservoir.
When the water levels in the canal dropped, so presumably did the oxygen levels, Craig says.
"We're pretty confident [the cause] is just low oxygen levels," Craig said.
Craig says the department speculated at first the fish may have died from a natural process called winterkill.
Winterkill occurs when snow and ice cover a body of water, and the sunlight reaching aquatic plants is limited. The plants then cut back on the amount of oxygen they produce.
If vegetation dies from lack of sunlight, the plants start to decompose, which uses oxygen dissolved in the water. When oxygen depletion becomes severe enough, fish die.
Craig says the department does not think the deaths were caused by winterkill at this point, but he added that mass fish death during this time of year is not uncommon. And, he says, the warmer temperatures have melted away much of the ice from local lakes and rivers, bringing the dead fish to the surface.
"It's just one of those things that happen from time to time," Craig said.
Like species extinction events? Not to worry unless that door slams shut on you.