What is this bone from? In San Diego, California

Saw this while freediving, about 18 inches long. I pick up trash and throw it away when I get out of the water. Originally thought it was plastic from a kayak handle so I picked it up to throw it away but it had little tendrils of what I assume was flesh/cartilage. The (spine, I’m guessing) bone stayed in one piece, very solidly fused together when I picked it up. We have a lot of sea lions, harbor seals, and large sharks in this area if that helps.

When I found out it was definitely not trash, I let it go so I don’t have anything to reference but this underwater picture I took on a GoPro.