That thing looks too big and complex for an intestinal parasite. Was it loose in the stomach or was it embedded in the stomach lining? Was it in the gut? If it was loose in the stomach I'd think it is some thing the YT ate. It looks a bit digested. The larger end could be the remains of the skull and gill rakers. My best guess as to what it is would be a small ribbon fish. They're not common in So Cal but they are around. Mike
