BT, I'm an avid angler, too, and have inadvertently caught shark (never taken one home to eat, though). So, I'm all too alarmingly familiar with how seafood starts to spoil within minutes of its death and exposure to air. It applies to "frozen, freshly thawed" too -- I'm especially wary of thawed crab which ammonia-izes very quickly.
So I always ask the monger, "exactly how fresh," and I always ask for a personal close inspection of the flesh, including a sniff. If it has even the faintest ammonia smell, I politely pass. Don't ever eat it! I don't think it's toxic, but that ammonia smell will ooze from your pores for the next two or three days.
I've prepared steaks of thresher shark tonkatsu style, panko-breaded and shallow fried, drizzled with a bottle of traditional tomato/prune sauce. On a bed of cabbage strips slawed lighty with mayo and lemon juice. And have served it to guests who were completely fooled into thinking it was pork.