A lake in Nevada has suffered a wipe-out, with the loss of around 100,000 fish due to low oxygen levels - but the cause of the problem remains unknown.
In December around 3,000 fish died in one area of Sparks Marina due to lack of oxygen, but the Nevada Department of Wildlife thought at the time that the incident was localised.
But now tests for dissolved oxygen at different areas around the lake have revealed that levels are too low for fish — and scientists can find no signs of any fish still alive.
It's now thought that while many of the fish involved in the December die-off washed ashore, others sank to the bottom, giving officials a false idea of the scale of the problem.
"It looks like we lost the whole fishery," Chris Healy of the Nevada Department of Wildlife said. "As to why, we don't know."
The 100' deep, 77-acre man-made lake is 15 years old and is stocked with trout, catfish and bass. The water is reported to be very clean.
One suggested theory for the diminished oxygen levels is that surface water in the lake may have chilled very quickly during December's cold snap, during which temperatures in the area plummeted. As a result there could have been what's described as a 'violent turnover' where oxygen-poor water from the bottom of the lake rose to the surface.
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