Me and my buddy just tamed a 145 by building a trap against our north tropical bubble. From there you can actually reach the plesiosaur from the inside and tame it using darts to speed up the torpor process. From there it's just a waiting game. You can pretty much use any trap that you can use for wyvernd. Behemoth gate frames and one gate or conservatively use one gateway and door and pillars and ceilings staggering it down so that if it swims more towards the bubble it'll be way easier to hit
These guys are great and super fast and also very strong. Warning when your gonna go tame these guys bring a sarco or an icthy , they also spawn In 2,s or 3,s so be careful because these guys do a crap ton of damage .
They can't fit through the resource caves' entrances. Just lure it into one and pull out your crossbow/harpoon gun for the knock out. Just be aware of any nearby megalodons, as they can fit into the cave easily and ruin your swimming session.