Mentaiko is a dish in Japanese cuisine which consists of a sausage-like object filled with the spicy fermented roe of the Alaskan pollack. Although Mentaiko can now be found all over Japan, it is famous for being the meibutsu (locally famous cuisine) of the city of Fukuoka in Kyushu.
Mentaiko originated from a Korean dish. During the Japanese rule over the Korean peninsula prior to World War II, the most common way to travel between Korea and Japan was to take the ferry which ran between Pusan and Shimonoseki. Japanese travellers took to buying a local Pusan dish of pickled pollack eggs to take with them as a snack on the ferry and the food soon became popular in Shimonoseki and neighboring Fukuoka.
After the war, the dish spread throughout Japan, and still retains its fame as a snack to eat while travelling, and thus is sold at virtually every train station and airport. It is also used as a topping on a variety of dishes, including pizza, pasta, and inside onigiri, or enjoyed all by itself along with sake.