MEXICO CITY (AP) — Mexican authorities say a mass die-off of fish at a lake in western Mexico was not due to natural causes, but the causes are still being investigated and one research said that low water levels could have been responsible.

Over 53 metric tons of rotting fish have been removed from Lake Cajititlan in western Jalisco state, the state environment department said late Sunday. Workers using shovels, a bulldozer, boats and wheel barrels removed the dead fish, which are being sent for testing. Workers wore masks to ward off the smell of rotting fish.