Skipjack tuna are doing fine, there's no significant concern at current populations. Check the supplier is at least involved with the MSC or the ISSF.

Radiation from Fukushima isn't a concern to us. Nobody would recommend you eat fish swimming just outside of course but there's no reason to avoid fish elsewhere for fear of radiation. This chart might help. :smiley:

might be too salty for cats

I wouldn't recommend it, maybe a mouthful every now and then as a rare treat. If you have to give fish, any vet will tell you it's better to give them plain fish such as cheap frozen white fish from supermarkets.