- Have the angler follow you in to an underwater cave. Hop off your mount and use tranq darts/arrows to know out
- build under water taming pen (Stone foundations/stone doorframes for walls/ stone ceilings) bring extra stone doorframe to close the walls behind you when the angler goes in to the 2x2 foundations/doorframes/ceilings
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.