Once the otoliths are collected they must be prepared to reveal the growth rings. To comply with Brazilian bio-piracy laws the otoliths are baked before shipment to ensure that there free of useable genetic material (There is a very cool history here involving cloak-and-dagger theft of rubber seeds from the heart of the Amazon, untold elegance and luxury and excess in the most remote city in the world, and English colonialism that make up the roots of these laws).
In the lab the otoliths are embedded in epoxy, then cut using a diamond saw to reveal the internal ring structure. Then they are polished to make the rings clear. Each otolith is then photographed, like the picture above, before chemical analysis so that growth analysis can be completed. The video below shows a process similar to ours for otolith preparation.