Your tuna would not be safe canned at 8# pressure. If it was canned within 24 hours you could reprocess it at 12# for 100 minutes. If it was longer than that I would not recommend consuming it. You risk botulism poisoning.
Please go to the OSU Food Preservation website:
http:/extension.oregonstate.edu/fch/food-preservation and down PNW 194 Canning Seafood. It will give you instructions for properly disposing of under processed foods.,
Thanks for using Ask an Expert