Minecraft wiki entry for Trident, associated text: combining two damaged tridents on a crafting table or the 2×2 inventory grid, which removes any enchantments.
combining two damaged tridents in a grindstone, which removes any enchantments except for Curse of Vanishing and Curse of Binding
combining a damaged trident with another trident on an anvil, which preserves enchantments.
applying the Mending enchantment.