No problem with the answer but I do question it. The meaning of oda is movement in a direction leading to a place. If we say, the fish in the river swim there, we aren't saying that the fish are swimming to the river because, presumably, they're already in it. In this question, what is it that the fish are swimming to? I would have thought ott was more appropriate in the sense of the fish being located there and swimming rather than swimming to a particular place.