The most likely reason for the scientists to count different numbers of redd is the observer’s efficiency.
Explanation:
In a field survey based on visual counting for visual stock assessment, the scientists may count a different number of redds each time in the same area due to the observer’s or the scientist’s efficiency.
The same place may first appear as redd and then later may not and accordingly the scientist's count also varies.
This error happens due observer’s inexperience in surveying, misidentification of redd species, changes or variations each time in the redd habitat, survey life, characteristics, spawning shifts etc., and poor temporal and spatial coverage.
