It's a limited tame idk how long but apparently you won't be able to have them forever, also they will teleport away if they feel fear, they will also teleport if they get stuck in an area for too long
Don't use mek because the astrocetus takes first the torpor and then the damage. The effectiveness will go down.
We tamed a 130 with mek and Kibble and get a 136 -.-
I think the stronger the weapon the more you loose the effectiveness, so the "best" way is to knock out with rocket launcher.
It's the same effect with rock elementals.
Wildcard should repair that.