Great line Smalma, this is the kind of discussion that keeps attention.
Bottom line is that managers receive data from technicians on the river and make management decisions based on this information regardless of its precision or accuracy. I find it very unlikely that anglers simply aren't reporting their wild catch. Its much more likely that anglers think reporting their catch will limit their fisheries. And in some cases this may be true. Not sure about OP but some fisheries compare the relationship between catch data during the season and the preseason forecast to test the accuracy of the forecast (in season update) (Any OP mgmt Bios out there?). In this case it would be in anglers best interest to report all released fish.
Many of you have identified a deficiency in the management process but have not offered a
solution. How can we be sure the reporting of released fish is accurate? Bottom line is that creel data provides managers with a number. This number coupled with other indices of stock strength is used to manage future stocks and in some cases avoid unknowns. If for some reason this number is not an actual reflection it will surely limit managers ability to accurately detect changes in stock densities over time.