So far as we know, the very first thing that Uri eats when he and Mafatu get to the island is a robber crab. We cannot be absolutely sure, however, that this is the first thing he eats.
At the beginning of Chapter 3, Mafatu is just regaining consciousness after making it to the island. He sits up, and he sees Uri holding the crab in his paws, cracking its shell, and eating it. We cannot know for certain that this is the first thing he has eaten. For all we know, he could have been eating the whole time Mafatu was unconscious.
As far as we know, though, the robber crab is the first thing Uri eats on the island in Chapter 3.