The overall goal of this method is to provide an effective alternative technique for extracting undamaged otoliths from threatened fish, such as Lake Sturgeon. This otolith extraction method can help answer key questions about fish age and growth, the placement of ear bones in relation to the brain morphology, and habitat correlated microchemistry. The advantage of this technique is that in fish with thick skulls and fragile otoliths, opening the brain cavity doesn't harm the otoliths or disrupt other critical structures.
The idea for this method was developed during research on the restoration ecology of threatened Sturgeon. In remnant populations, specimens for necropsy and laboratory analysis are rarely obtained. So successfully collecting age and other information from individual animals is critical for filling species'data gaps.
Mark will be preforming the procedure of removing the otoliths from the Lake Sturgeon. To extract the otoliths, first lay the Sturgeon on its belly and use a measuring tape to determine the total length from the snout to the tip of the tail to the nearest one millimeter. Next, weigh the animal on a tared scale to the nearest one gram and evaluate the body for any external tags or marks.
After recording these data, use a large fillet knife to make a vertical incision through the body between the base of the skull and the first dorsal scute to separate the head. After its removal, turn the head so the mouth is facing up. Then use the fillet knife to cut along the ventral side of the skull to remove the soft tissue.
Followed by an electric cast cutter incision from the tip of the snout to the base of the skull to completely bisect the head. Separate the two halves by hand to expose the brain cavity. Then use forceps to remove and discard the brain matter to expose the semi-circular canals within the brain cavity.
Remove the semi-circular canal cartilage from the brain cavity. Then use the tip of the forceps to carefully puncture the semi-circular canal cartilage. After identifying the hard bony structure within the cartilate, withdraw the otolith and dip the ear bone into 92%ethanol for four seconds to remove any remaining cartilage.
Finally, dry the otolith in an open 25 milliliter scintillation vial for 24 hours. The adult size of Sturgeon across the world varies greatly, but the location of the otoliths is consistent. As demonstrated, a sharp fillet knife easily removes the soft tissue of the mouth.
An electric cast cutter is the preferred tool for the lateral incision to expose the brain cavity however. Careful attention to the bisection of the mid-line of the skull is needed. Otherwise, the otoliths may be crushed during the process.
Forceps facilitate the straightforward removal of the brain matter, allowing access to the semi-circular canal cartilage which contains the otolith at the base of the brain cavity. The cartilage in the semi-circular canal can then be removed from the otolith with little trouble using forceps for further ear bone analysis. Once mastered, this technique can be completed in less than five minutes if it is performed properly.
While attempting this procedure, it's important to remember to take your time making the incisions. Don't forget that working with dissecting instruments and dead fish can be dangerous and smelly. So precautions such as proper use of the surgical instruments and suitable PPE should always be taken while performing this procedure.
After watching this video, you should have a good understanding of how to open the skull of the Sturgeon from the ventral side and remove the otoliths with no damage to the ear bones. After its development, this technique paved the way for researchers in the field of fisheries science to explore alternative ways to successfully remove otoliths in fish species where standard techniques proved difficult.
