According to the dossier, now available on Discord, the method of taming is as follows. This hulking leviathan is a passive tame, however, to initiate the taming process you need a ‘school’ of Ichthyosaurs ( you’ll need a certain number of Ichtys to create the ‘school’), these will act as a distraction that enables you to do the next stage. Next, using a sickle, you swim to barnacle nodes on the Shastasaurus and harvest them. Assumably (since it doesn’t sate otherwise in the dossier) you have to repeat this several times.