American mergansers and belted kingfishers that eat large parr, particularly during the low water of dry summers, fulfilled the prediction (based upon statistics) that the periodic scarcity of Atlantic salmon is due to a factor operating on the salmon when near the smolt stage. One year's bird control more than doubled the number of descending smolts and correspondingly increased the sea catch of salmon related to the experimental river, as demonstrated by the occurrence of fish marked as smolts in the river. Bird control may thus remedy periodical scarcity. This serves to explain the favourable influence on numbers of fish that man's presence has been observed to have.