Dunno about shark. Ive caught plenty of sharks on 30lb main, 40lb leader while fishing for jacks. If you catch em in the lip, 40lb will hold. Biggest one was over 2m, took me an hour on this light tackle.

Ive also had repeat bustoffs on 80lb, and never found out what it was. No bites on wire seems to point to mackeral, they are scared off easy. Sharks are not, they will bite on anything.

Are you sure its a clean bite near the hook, and not a rub off on rocky structure half way up the leader?

Only solution is to keep fishing the spot and hope to get one near the lips that holds, but you will lose a few before that happens, and thats heartbreaking.

Thats fishing, weve all been there.

cheers
Andrew