I read this thread a few days ago, and made a point of checking salt content of different types of tuna when I went shopping next, just out of curiosity. I found that there was no real difference in salt levels between tuna in sunflower/olive oil, and the tuna in brine - all at around 0.8-1 gram of salt per 100 grams (0.3-0.4 g sodium), depending on make. However, there was tuna in spring water which only had 0.3 g salt / 100 grams.
Just thought I'd let you know in case anyone is interested...
Sometimes I get curious about stuff and have to check it out...