They spawn south on the island. Between the land and the barrier, when found, use a crossbow to get its attention and use your mount as bait (Recommend a basilo or another mosa for their high health). Once it agros on your mount, swim up and start tranqing it. Their torpor drains fast so wait a while until it reaches 40% and spam some bio toxin or narcotics in. I recommend starve taming as you could easily lose your kibble if you go away for a while.(Make sure your mount is fully healed for an emergency getaway)

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