Chosen answer:During the several years it took to construct the ship probably, or in any of the supplies/food brought on board, or in the furniture brought on board. A single pregnant female rat can be responsible for thousands of rats in a very short space of time (the offspring are not too choosy about who they breed with).
A pregnant female rat could have made a home in a underneath a third class couch and had the other rats then all the females would have baby rats quickly.
Answer:In addition to the other answers, rats can easily get on ships by climbing the mooring lines that tie vessels to the dock and also go up unattended gangways. They can also use temporary overhead cables attached to ships while in port.
Chosen answer: During the several years it took to construct the ship probably, or in any of the supplies/food brought on board, or in the furniture brought on board. A single pregnant female rat can be responsible for thousands of rats in a very short space of time (the offspring are not too choosy about who they breed with).
Soylent Purple
A pregnant female rat could have made a home in a underneath a third class couch and had the other rats then all the females would have baby rats quickly.