Use a high damage crossbow to avoid accidently killing the angler. If you dont have such a crossbow its highly advised to use tranq darts: however you need to lure the angler into a water cave and shooting it within as longneck rifles cant be use in water. Good luck
Be really care when knocking these things out from a distance! Unfortunately they don't look knocked out when they are so continuing to shoot darts at them can be an easy an unfortunate mistake to make.
Everyone’s making it hard on themselves tame them in the blue sone river on abb that way you don’t have to go in the deep dark ocean to get them:) plus there body glows in the dark as well:) hope this helps:)