Go to the Sunken Junk and spearfish for ichthyosaurs (large gig) with Truth of the Oceans active. As you catch more ichthyosaurs, the game will do some emotes about shadows gathering and intensifying, which will result in a swimming shadows pool forming (similar to an unspoiled botany/mining node). Truth of the Oceans will point you in the direction of that. There you can catch the quest fish. I still used a large gig and that worked fine. I'm not sure if it spawns in random spots, but while the Sunken Junk is at roughly (9, 34), my dafangshi node was at (38, 36).