Weird things found in fish filets before cooking -- any guesses what they were?

(Before anyone asks, we didn’t eat the fish. I wound up exchanging it at the store for a different product).

It was rockfish, if that matters. I was deboning the filets prior to cooking and kept finding these odd things embedded in the flesh of the filets—small, transparent, round, very firm. They reminded me a bit of silica beads, and that’s probably the best visual comparison I can offer. One was much larger than the others, but most were very close in size. Most of them were embedded in the meat, a few were encased in pus. They were in most of the filets (only one was entirely free of them).

We couldn’t figure out what they were. Anyone have any guesses/insights?