Mexican authorities say a mass die-off of fish at a lake in western Mexico was not due to natural causes.

Over 33 metric tons of rotting dead fish have been removed from Lake Cajititlan in Jalisco state. The cause of the die-off has not been determined but the state environment department said late Sunday it was not due to natural causes. Workers using shovels, a bulldozer, boats and wheel barrels removed the dead fish, which are being sent for testing.

It was the fourth time this year there has been a die-off at the shallow lake, which sits between Guadalajara and Lake Chapala. Some had speculated abrupt changes in temperature or oxygen levels may have been responsible for earlier incidents.

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