There are no official Opeth tab books for bass. Thus, you can either follow the (most of the time, shitty) tabs on the internet, or use your ear. eos.
To elaborate upon this further: tabs online are 99% of the time only peoples' interpretations of how to play something (unless you're looking at tabs from Dan Mongrain or someone else who has tabbed their music out before for fans.). A lot of times, they are wrong, especially when the production on an album is poor or the person tabbing it simply has no ear training other than picking out a few notes here and there. Therefore, using your ear to learn bass parts would be a lot wiser than using tabs if you wish to play an accurate version of the song. When it's difficult to tell what the bass is playing, especially if it's a bass fill or some other such thing, make up your own fill - or try to play the fill as best as possible, and add your own spice to it. That's what Mendez does with DeFarfalla's basslines, and that's what Niemann did when he filled in for Mendez on the Nova Rock gig this year.
