The reasons behind a massive fish die-off at Lake San Antonio remain a mystery.

On Friday, July 5, Monterey County Parks staff noticed hundreds of small baitfish washing up dead on the shores of Lake San Antonio, which is both a county-owned reservoir and a recreation area managed by the parks department that allows for day use, boating and camping.

Initially, officials thought it was due to the extreme heat – air temperatures exceeded 110 degrees in some places – but when bigger fish like bass started washing ashore in the subsequent days, park leaders took the precautionary measure to close the lake until the cause of the fish die-off could be determined.

The initial hypothesis that the water was depleted of oxygen due to the heat didn’t bear out – after samples were taken, oxygen levels came back within the normal range.

So now it’s a waiting game for more test results to come back, which unfortunately is at a time when demand to recreate at the lake is peaking.

“It’s a bummer,” Parks Director Bryan Flores says. He adds the lake’s facility brought in about $114,000 in revenue over Memorial Day weekend for camping alone, and there have been several weekends since when the county’s revenue exceeded $100,000.

“Reservations are full, we’re starting to make a comeback, we have some triathlons that want to come back next May, and the lake’s coming back,” Flores says.

County parks officials are honoring annual passes for Lake San Antonio at Lake Nacimiento during the closure.

Lab results won’t be back for some days – the timing is unknown – but Flores says the latest hypothesis is that the die-off is related to some kind of algal bloom. But for now, it’s all just speculation.