Rats tend to live two to three years. They can become pregnant by the age of three to four months, with a one month gestation period, and a litter of rat pups tends to be eight to twelve babies, but sometimes more. After they're born, the mother can become pregnant again.
Consider those brief lifespans and the size of the litters, with the number of decades that have passed with no other food present but other rats. Dozens of generations of rats eating each other, with the strongest rats surviving to eat those, breed more, et cetera. Likely you're looking at multiple colonies nested in different parts of the ship, but ignoring that and just going from a survival of the fittest perspective, at this point those are some mean fucking rats, descending from generation after generation of mean fucking rats.
I love rats and I would be terrified to set foot on that ship.