To get stuff frot their dams place a sleeping bag, transfer the whole dam inventory then let them kill you. After respawning retake your stuff again, they dont attack you anymore.
Thumb up pls,for help.
Make sure no other Castoroides are around then bola it and shoot it with tranqs until it knocks out. Make sure you bring loads of bio toxin, because their torpor drains fast. I used this method and tamed one at level 42