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.