People for the Ethical Treatment of Animals volunteer Dina Kourda told Irvine's street maintenance chief the sign would remind drivers that fish value their lives and feel pain.
About 1,600 pounds of saltwater bass died on Oct. 11 when the truck hauling them to market got into a three-way crash.
Kourda's letter acknowledges roadside memorials traditionally honour humans, but she hopes an exception will be made.
Irvine spokesman Craig Reem says there won't be a fish memorial.
But PETA spokeswoman Asheley Byrne said they will go back and ask again.
It's not the first time PETA has asked to post a memorial for animals killed on their way to slaughter.
They've tried to honour pigs killed in Virginia and cows killed in crashes in Illinois, Kansas, and Manitoba, Canada, Byrne said, but none have been approved.
It's their first fish effort. They will continue trying for memorials when trucks carrying animals to slaughter are involved in crashes and there is a heavy death toll.
"They are on their way to slaughter, which is, of course, pretty hellish. To suffer an accident on the way and be left in the middle of the street is unthinkable," Byrne said.
Information from: Los Angeles Times, http://www.latimes.com