Why would fresh tuna be salted? I bought a big piece of tuna about a week ago. I carved it up and put most in the freezer. However, as I am the proud owner of a vacuum sealer (as of a week ago), the worst part of the tuna got vacuum sealed and put into the fridge.

Six days later, I took it out of the fridge to cook, and found that the vacuum was gone (it had been the first item I had sealed). I made a fish-pie out of the tuna. As this was the worst part, I didn't expect anything great of it, but it was salted, apart from having a strong flavor. I attribute the strong flavor to the cut (the dark part, mostly) and from having been in the fridge for a week).

One piece of prime cut that I had sealed in with the other meat, I reserved and grilled the next day. It was also salted.

Would this be because I salted it too heavily, or can it be because it had been sitting during one week? Is tuna meat salty on it's own? Any other suggestion?
As far as I know, raw tuna is not usually salted and/or pre-treated.

What may be happening is a case of perceived salinity.  For example, there are numerous studies that suggest foods with certain odors can be perceived as much saltier than they actually are.  If you have access to that second article (it is copyrighted and behind a paywall, so I can't post it here), it has a great figure on page 3 depicting the perceived sourness, bitterness, sweetness, and saltiness of a number of common foods, including tuna.  The aroma of plain tuna alone ranked higher than soy sauce on the saltiness scale, for example.  Tuna is relatively high in fat, and other studies suggest that high fat foods can also increase perceived saltiness.  Perhaps you were using a fattier part of the fish?

Finally, there is evidence that dehydration increases the saltiness of tuna (which actually seems somewhat intuitive), especially if extra salt is added before cooking.  Was your tuna previously frozen?  Freezing—especially if done improperly and/or if there are multiple freeze/thaw cycles—can act like dehydration.