Do I "know?" Sorry, no.

The package probably says "refrigerate after opening." I'd want to use it up that day, if possible. Two days might be OK, but fresh fish goes bad really quckly. Smoked? You may have more time. Often manufacturers are conservative in their estimates of any product's freshness. If I could manage it, I might open the package as close to the beginning of the trip as possible and repackge single servings into vacuum bags. That would be dicey still, but I'd consider it. I MIGHT even freeze the repackaged salmon and carry it as deep in my pack as possible for maximum insulation.