Happy New Year, everyone.
Generally, fish and game cops are constrained by the same constitutional requirements as regular cops.
That said, there are a couple of differences. First, WesF noted above that there is a RCW section that allows for a fishcop to check any and all things that you have with you in the field that are being used to attempt to take fish. This includes fish boxes, lockers, back of truck, under the seats in your boat. Even the back of your vest.
That law would not allow a fish cop to check a film canister for pot, as you're not fishing with it, and it couldn't hold a fish.
Just because a law says it's OK, that doesn't really necessarily mean that it's constitutional. There is another basis for this type of search.
On the bottom of your fishing license where you sign it, there is language to the effect that you agree to allow a fish cop to search you and your gear (which is generally known to include your truck and boat) when approached by a fish cop in the field, and to show him/her all your necessary licenses.
When I worked at the Attorney General's Office representing WDFW, this topic would occasionally come up. Legally speaking, all of the attorneys there were of the opinion that these searches were OK.
Lastly, I still feel that asking first would diplomatically make sense. Just because they have the right to do it doesn't mean they have to take full advantage of it. Personally, I would like to be asked if I would mind, and if I said no, then I'd expect to have them insist. I'd just like the opportunity to say yes.
Fish on...
Todd.
_________________________

Team Flying Super Ditch Pickle