Building a trap out of pillars in shallow water on the shoreline with a behemoth gate entrance on the waterside worked wonders for me.

I used a wild tame baryonyx to get the mantas aggro and led it into the trap, jumped of when on land and swam around the trap (while keeping an eye out for any of its friends) and closed the trap.

After that, it was as simple as swimming between the pillars on the opposite side of the gate and feeding it anglergel until it was mine. Got a breeding pair of them now.

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