Arowana

The arowana is a fish you catch in rivers. Look for a large shadow. In the northern hemisphere look for it in Jun – Sep, from 4 PM – 9 AM. It can only appear after you have caught 50 fish. Nook's Cranny pays 10,000 Bells, C.J. pays 15,000 Bells.

A Large river shadow that bites from late afternoon until morning once you reach 50 catches. Black bass share the same shadow size, so expect plenty of those before this 10,000 Bell prize shows up.