Wikipedia says the inland natives used this fish grease as a condiment like butter. I've also heard that their diet was low in fat, which seems a bit odd, since I would think they could get the necessary fat from their normal game animals; certainly the northern natives closer to the center of the continent didn't have access to this grease.
I've also heard that the coastal folks and the inland folks smeared this on their skin, either plain or as the base of grease paints. Was this effective as a mosquito repellant?
It was not uncommon for interior groups in the Plateau and Northern Great Basin to go through periods where fat was extremely scarce. The fat of land mammals varies dramatically by season and on a longer time scales. Interior anadromous fish have significantly less fat than coastal fish. So eulachon trade was important in moderating impacts on the diets of interior peoples. If I recall correctly, eulachon oil was traded from the coast to the Intermountain west from California to the Arctic. My reading suggests that it was routinely poured over dried or cooked fish or other meats. I have not seen any references to using the grease as an insect repellant. For more information, I would look at one of Hilary Stewart's discussions in her several books on NW natives like this one.
Edit: provided link.