A kelp frond is a kind of comestible. It is a vegetable, preserving both vegan and vegetarian conduct, and can be fed to herbivorous pets, though it counts only as food and not as a treat like apples. It is only generated under water: 1/10 chance of being generated in each pool square and 1/30 chance of being generated in each moat square.
Perhaps the easiest way to harvest kelp frond is to cast detect treasure while blind, levitate over the water and apply a bullwhip down. Juiblex's and Medusa's lairs are good places to collect kelp, but watch out for the eels. By the time a player has access to lots of kelp, food is usually not an issue anymore. Another method is to polymorph into a gargoyle or other non breathing monster and simply pick it up.