D
I've never portaged fish from lake to lake.  There is enough stuff to carry without worrying about dying fish.
I thought everyone always brings enough food for some meals without fish, so it's not worth the hassle to me.
But if it's been a horrible trip fishing wise, and I've only had fish to eat once and it is getting toward the end of the trip, it might be tempting to fillet them up prior to portaging. ;D