The fish began to die, and it didn’t stop.
But no one knew why — until recently.
Locals fishing in the Oder River in Wroclaw, Poland, noticed the first dead fish on July 28, CBS News reported on Aug. 17.
The Oder River — a 520-mile-long river that runs along the border between Germany and Poland — was typically considered a relatively clean river, The Guardian reported.
Fish continued to die, surfacing over a week later and over 100 miles downstream at the German border on Aug. 9, Deutsche Welle reported.
More fish continued to die. Over 500 Polish firefighters removed more than 220,500 pounds of fish carcasses from the river by Aug. 17, CBS News reported. The massive cleanup operation used dams, boats, and drones, The Guardian reported.
German authorities warned people not to touch the water — or allow their pets and livestock to come into contact with it either, Deutsche Welle reported.
At least 600,000 pounds of fish in total ended up dying across 300 miles of the Oder River, AFP and CBS News reported.
But a question lingered: What was causing the fish to die off?
The disaster baffled experts, reported BBC, Deutsche Welle, and The Guardian. Officials began to hypothesize the cause: high mercury levels? Toxic waste dumped in the river? Chemicals? Pesticides? Low water levels? High water temperatures? Chlorine-filled industrial waste water?
Every guess was wrong.
The fish died from Prymnesium parvum, known as “golden algae,” according to a report from the German Environmental Agency on Sept. 30, Deutsche Welle reported. Polish authorities came to the same conclusion at the same time, according to AFP.
The usually relatively clean Oder River became too salty over the summer, turning the river into “brackish water,” AFP reported.
And golden algae thrives – or blooms – in brackish water, experts said, per AFP, creating a “man-made environmental disaster.” Golden algae blooms produce toxins that kill fish and any creatures with gills, according to Texas Parks and Wildlife.
Experts, however, still don’t know what caused the river to become saltier so suddenly, Deutsche Welle reported. They recommended investigating further and improving response systems to avoid a similar disaster from occurring again.