I finally figured it out.
spear fishing around 11, 35. the sunken Junk

With truth of oceans active
keep catching Ichthyosaur and you will notice a message stating you sense movement in the shadows. keep doing this until the swimming shadows spawns. i managed to get 2 on my first spawn.

I did all of these with the large gig head

edit: i should've refreshed this page before responding lol