I was writing in the comments, but this became a bit wall-of-texty, so I choose to do it in an answer instead, where I can at least break it up into paragraphs.
First things first
I want to first discuss whether or not you actually needs protein shakes, given your light training regimen.
You mention in your comment that:
it is advertised pretty much everywhere that you need twice grams of protein of your body weight in kg
Yes, and the key word being "advertised".
Have you heard the phrase "breakfast is the most important meal of the day"? If we dig deeper, we'll learn that it was actually a man named Will Keith Kellogg who first coined this phrase. You may have heard the name Kellogg. The man started a company which sells breakfast cereal. So as you might assume, he was very happy when people started spreading the rumor of breakfast's immense importance.
Similarly, the whole 2-2.5g of protein per kg. of bodyweight holds very little scientific substance, and is essentially just a rumor spread to sell more protein powder. What's actually important is that you find out which amount per day works for you, because you don't necessarily have the same training regimen, or the same metabolic rate, or the same sleep schedule, or the same adaptations etc. as the persons who rely on 2g per kg.
I'm sure there are people who require 2g per kg bodyweight, but if your muscles aren't in need of that much protein, it will just convert the excess to fat, and store it.
This can also be said for the idea that you have to consume protein right after your workout. Because what kind of high-protein food is very easy to pack and eat right after a workout? Protein shakes, protein bars... This is why they sell this crap at the gym!
Fact of the matter is you don't need breakfast. In fact, when you wake up, your body naturally increases your blood sugar level to get you started. Further spiking your blood sugar level with high-sugar breakfast cereal (like Kellogg's stuff) increases the risk of Type-2 Diabetes.
This is not to say that all breakfast is bad. Just be aware of what you eat.
So, to your questions
Can tuna in brine replace protein shakes?
Tuna is food, and a protein shake is a food replacement. So what you're asking is if food can replace a food replacement.
The answer is yes.
Protein shakes should only be used when your protein intake needs to be higher than what you can consume through regular foods. It's a supplement after all. Whenever possible, your diet should be regulated with food rather than supplements, and add the latter only when necessary.
However, I urge you to check the mercury content of the tuna before eating cans of it every day. Particularly albacore tends to have way higher mercury content than, say, skipjack.
Although I drain the brine as well as I can, I am still afraid that I consume more salt than I should. Will this cause some adverse effects in the long run, compared to protein shakes?
I can't make a definitive statement about the salt contents of the tuna you buy. But an adult should eat no more than 6 grams of salt per day. If you keep this in mind, I'm sure you'll be fine.
