after all the crops are gone will my villagers need to fish?....and if so how do i get them to fish

The crops will regrow after a while, and so will some berries, so you won't be completely out of food. There are certain prereqs for fishing that involve access to the ocean and level 3 of Harvesting. Until you get fishing, you should keep your population below 15 or you may have a food crisis on your hands.