Spear Fishing Trophy in Ghost of Yōtei

The sensei tale The Way of the Yari in Ishikari Plain gives you the yari and has you spear four fish as part of it. For the fifth, find shallow water where a white circle shows a fish, and press with the yari equipped as it jumps. Fish are often in the water by the entrance of Amber Respite Shrine in north-west Tokachi Range.