With city names, the dative form is used to mean "in [location]." I think one of the grammar chapters refers to it as the locative, although a lot of textbooks avoid that. If you need the real grammar history wonk, here it is: https://en.wikipedia.org/wiki/Locative_case#Latin
It's all to do with the fish flying about and the fighting that ensues. On top of that, fish all over the floor cause slipping accidents to unhappy old men, and serious cases of gastro-enteritis. Sepulchrum also means tomb, but it's not accepted.