The 1946-47 catch of 68,656 is approximately 23 'rats per acre over the 3,000 acres of producing marches. The additional sea-rims and ridges were not trapped. To trap this number of 'rats per acre the marshes must carry at least 30 to 35 animals per acre […] . If ignited by lightning or other agencies during dry spells these will suffer damage to 'rat populations.