They were a source of protein in the gruel! Seriously, here is a quote from this source: "Many accounts from Royal Navy sailors 1760-1815 note that to enjoy fresh meat aboard ship for a change - perhaps during a year at sea - catching rats and feeding them pieces of ships’ biscuit to fatten them up for eating was a well-known practice aboard larger ships." SOURCE: http://www.sea-thieves.com/education_pages/seathieves_edu_diet-food.htm