In a February 1921 edition of the Portsmouth Evening News I found this story about what must have been some sort of plague within the naval dockyards of the time.
As can be seen, in the previous month of January, 506 rats were destroyed in Devonport Dockyard with another 33 taken within the barracks and victualling yard. What is worse is that another score were killed in HMS Indus.
What the killing total must have been in Portsmouth Dockyard is not mentioned but, size for size, it must have been about the same, if not more.
The total for one month in all yards and establishments, which must have included Portsmouth, was 586 – in just a four-week period.
If the catchers were that good for the rest of the year, and this was only January, then more than 7,000 would have been destroyed in one year.