cats have been carried on ships for many reasons - most importantly, to catch mice and rats. on a ship, these rodents can damage ropes and wooden parts of the ship. once electricity was discovered, rodents would also damage ships' electrical wiring. also, rodents threatened the ship's cargo - the things it carried. rodents would eat the food carried to feed the crew. if the ship was carrying things like grain as part of its cargo, rodents could also cause the ship to lose money by eating the cargo. also, rats and mice spread disease, which is dangerous for ships that are at sea for long periods of time. for example, rat fleas can carry plague, and rats on ships may have been one of the main reasons why the black death spread.