Updated at 4 p.m.
Interim City Manager Pat Kilroy told Patch that the last time the lake had a fish kill, it was in 2008 or 2009, during an extremely hot, windless summer.
The subsequent oxygen dissolution led to the deaths of different species of fish.
In the case of the shad, said Kilroy, the cause of the deaths are probably either oxygen deprivation or a release of gases when the rainstorm earlier this week caused the barometric pressure to fall.
The gases could have been carbon dioxide or hydrogen sulfide, he said.
"Shad are sensitive," said Kilroy, and tend to be the first species hit when changes occur in the lake.
“It has something to do with their biology,” Kilroy told Patch.
The state Department of Fish and Game confirmed Friday that there is a massive fish kill in Lake Elsinore, numbering the dead Threadfin Shad in the "thousands."
"They’re floating in the river ... the biologists are on their way," said Andrew Hughan, information officer for the department.
The usual reason for a die-off is a lack of oxygen, which causes fish to drown, he said.
Preliminary reports indicate a dissolved oxygen issue, sometimes caused by an algae bloom and sometimes by a spike in water temperature, Hughan said
"This is not an unprecedented event," in lakes, Hughan said, but he could not say whether it has happened before in Lake Elsinore.