The larger abalone had already been wiped out in the Monterey area by sea otters and withering syndrome even before the purple urchins ate most of the kelp. Otters are the primary predators for abalone and when the otters were hunted nearly to extinction then the abalone population exploded. But then otters were protected so they ate many of the abalone, and withering syndrome killed others. I still occasionally see small abalone while diving.

https://wildlife.ca.gov/Conservation/Laboratories/Shellfish-...