I don't think that's ink, but rather fish oil. This actually makes sense considering real-life salmon do in fact have lots of fish oil (or more accurately, Omega 3). Plus the Salmonid band that plays the Salmon Run songs is literately called Omega 3. Their water is murky because of the large concentration of Salmonids there, explaining how they are able to spread it by just moving, unlike the Smallfry companion who probably hasn't returned to the water in a long time after meeting Agent 3.
It's possible that without water, Salmonids can't produce their oil.