Well, don't put it on the menu if you can't catch one.
A snooty diner at a restaurant might complain that an item shouldn't be on the menu if the supply has ran out. Here, the employee of the restaurant is a bear, so instead of the complaint being about running out of inventory bought from a supplier, the complaint is about running of out fish because the bear couldn't catch one in the river. It's funny partly because, if a bear were to work at a restaurant, catching fresh fish might be a reasonable job because they do that in the wild regularly.