I can assure you that the population of grouse moors go up and down like a yoyo. The Red Grouse is a wild bird. It is given a helping hand by heather burning to promote young shoot growth in the breeding season and keep parasites down like fluke but a cold wet spring can virtually wipe out a breeding season. One moor that I know well that is along and at the top of Arkengarthdale has lost three shooting seasons since 2004 and others have seen their number shooting days seriously curtailed.
The above is the main reason why Grouse Shooting is the top end of game shooting. Pheasant and Partridge being much more affordable due to being reared and released.
On the 1st May this year I was on Hathor from Wroxham to Ranworth. The river was very quiet and we saw virtually no Mallard, Moorhen or Coot. We did see however four apex predators in the form of Otters.
The Marsh Harrier has also become very largely non migratory and the roost at Hickling now has huge numbers of birds, young waterfowl are very much on his menu.
Ten years ago, if you sailed from Stalham to Ant Mouth, you would be chased by Mallard chicks looking for food all the way down river, when did you last see that? The Mallard is not exactly a retiring bird, it will nest if full view of humans so the argument of they are in the water where there are no boats does not hold water where Mallard are concerned.