Technically they can't do anything.. Of course, if they have a real hardon for you, they can make up some "reasonable grounds" (that's the CDN version of "probable cause" that you learned about in your favourite TV show) and still search your vehicle, but in general they back off. If you have nothing to hide, tell them "I do NOT consent to any searches of my person or property." Same goes if you do have something to hide.. If they want it that badly, they should go through the trouble of getting a warrant instead. Also, don't fall for the good ol' scare tactic of "I can't help you if you don't consent." His job isn't to "help you." His job is to charge you and get you convicted.
Actually the new rage is broadening the range of "arrestable offenses." Technically, not wearing a seatbelt is an "arrestable offense" in some jurisdictions. That means that he can charge you with something like that and search your person/vehicle if he decides to be a real dick, but you'd be no worse off than if you just gave him your consent.
This is coming from a law-abiding citizen who values his privacy and who has already refused consent to cops in the past for that reason. Another very important reason is that you don't want them to expect from everyone to lie down and allow them to perform warrantless searches. They would start treating everyone who refuses to give consent as a criminal. If more regular people give them a strong NO, they learn that they need to go through the proper channel and give up on the sport of fishing.