MANKATO, MNAt a dinner party Monday, host Jeanette Rojahn, 44, announced with great pride that the main course she was serving was shark. 'Can you guess what you're eating? It's shark!' Rojahn said to her seven dinner guests, who collectively muttered forced exclamations of surprise. 'I know, can you believe it? It's actual shark! I saw it at the Market Basket, and I thought, 'What the heck! Let's try shark!'' Rojahn's guests last feigned excitement in August, when the hostess served cactus. 