Keeping Fish Cold on the Go...
Your question is a good one, but not an easy one. There are many variables affecting fish storage while in the field. These include but are not limited to:
amount of fish, fish coverings, selection and rotation of ice, outside temperatures, and opening and closing of the cooler. It's not really that complicated if you think it out, but you sure need to plan it out for down the road care.
I carry an individual cooler my fish. I use block ice instead of crushed when I can get it. I carry a vacuum processor if I'm around electricity or heavy zip lock freezer bags without power. I have an ice pick and duct tape with me at all times.
I never subject fish to water. They are immediately cut and then processed via the processor or at least stuffed into the zip locks that I then suck the air out. Fish go on the bottom, and ice goes on the top. The cooler is then sealed with duct tape and stored out of the sun.
As my catch days continue, I will drain water and replenish with ice if I can get it, and then reseal. With that method my fresh caught salmon will hold for a good 5 days. If I lose the ice opportunity, I keep the cold water in and it holds for 3 days.
I always seal and I never peek. If I'm in a pinch and the catch is excessive, I go beyond my cooler bounds and I dry brine with salt and brown sugar into my zip locks, and then go to smoke via foil and ingenuity in the field. One way or another, what I catch comes home. Once I'm satisfied, then it's on to catch-and-release.
It's a challenge and all part of the game...
I found this article to be useful.
AlaskanAuthor has pretty much given all the tips. I too have kept fish for up to 5 days iced in a cooler.