Hey Jim, guys -
If I recall correctly from law school, search and seizure laws are pretty darn weak these days. You need probable cause to search under the Constitution but there are a whole lot of exceptions. Nothing prevents an officer from asking to see what's in a bag politely. Even private citizens can do that (I get searched every time I go to a Braves game).
If someone declines to allow you to look in the bag, no big deal, just let them go on their way, BUT that becomes one of the factors in keeping an eye on a person for probable cause purposes. If an officer heard water sloshing on a pack or cooler, he would absolutely have probable cause to search, because that's not something you're typically going to hear (not a lot of water anyway).
There are many other reasons why an officer might be allowed to search a cooler, too, like enforcing the law on no glass bottles in the backcountry. Believe me, the rangers know this stuff cold. They will find a way to look.
As far as ages of trout go, I think 3-6 years is a bit short, though that might be very reasonable for a typical mountain fish. I know record trout live more than a decade, sometimes up to 12-16 years in good conditions (like lakes). Mountain fish are highly unlikely to live that long.
At the end of the day the rainbow trout is a very flexible animal. It adapts very well to greatly different conditions. We see body size as the main indication of that adaptability, but if you pull one trout out of the environment it's adapted for and put it in another one (stocking a hatchery stocker in a mountain stream, for instance), you've got to adjust your expectations.
Most of the hatchery websites I'm seeing from various state agencies estimate a hatchery 12" bow at about 2 years old. The Alaska Game and Fish website estimates trout begin to spawn at 3 years old and continue through age 7, varying greatly by density of population and size of water.
Anyway, the point is, they need to be caught out ASAP, because they probably have not spawned yet, but they certainly will if left in the river.