Barreleye

The barreleye is a fish you catch in the sea. Look for a small shadow. In the northern hemisphere it is around all year, from 9 PM – 4 AM. It can only appear after you have caught 100 fish. Nook's Cranny pays 15,000 Bells, C.J. pays 22,500 Bells.

Only a small sea shadow, out from 9pm to 4am after 100 catches, so it is easy to ignore. Cast at every small night-time shadow, because this one sells for 15,000 Bells.