Finding what isn't lost: The search for catfish otoliths

If you read an earlier post on ageing catfish (click here), you may have been curious about otoliths. Otoliths are calcium-based structures in the heads of ray-finned fishes. Their function is to aid in balance and hearing, similar to our inner ear bones. Otoliths sit in fluid-filled sacs where they move in response to vibrations from sound or movement of the fish. The small movements of the otoliths are detected by sensory cells and transmitted to the nervous system. Because of their size and location relative to other fishes, catfish otoliths can be hard to find. Consequently, it’s not uncommon for trained and experienced scientists to lose an otolith during their extraction. Even when you know exactly where to look, you may not always find them. Otoliths provide an effective means to determine ages of many fishes, but unfortunately they require the fish to be euthanized first. Thus, scientists only collect otoliths when age information is critical for population assessments or can be collected opportunistically from harvested fish.

To extract otoliths from catfish:

1) Make an incision with a hacksaw (or sharp knife for smaller fish) about 1/8-1/4 in. in front of an imaginary line connecting the pectoral spines (yellow dashed line). You want to make sure the cut is straight, both up to down and right to left. Cutting at an angle can cause you to cut through the otoliths. Once you cut through the brain case you will feel less resistance and can stop cutting.

A Channel Catfish skull provides an example of where to cut to extract otoliths (yellow dashed line). Note one pectoral spine is missing.

2) Now you can begin looking for the otoliths. If we pull away the front part of the skull we can see the braincase (blue oval). You should remove the brain to see the otoliths clearly and reduce the likelihood of losing them. The otoliths sit in small “pockets” in the bone (red ovals) in the bottom/sides of the braincase. You can grab the otoliths with forceps and remove them. The otoliths will usually have a membrane and possibly a nerve attached that you can rub off with your fingers. The membrane is harder to remove after drying and must be removed before processing. The cleaned otolith looks like a tiny ear.

Channel Catfish skull with the section anterior of the incision removed. Brain case (blue oval) and pockets containing otoliths (red ovals) shown.

Pocket where the right otolith would be located (red oval).

Otolith extracted from a 19 inch Blue Catfish from the James River, Virginia.

Then what? Often scientists have different preferences on how to process catfish otoliths, but due to the opaqueness of the otoliths they must be sanded or sectioned to view growth rings. Once the otoliths have dried for a while and are glued upright on a microscope slide, I sand them down with sand paper to reveal growth rings to determine age.

How can it go wrong? In my experience, the most common mistake during otolith extraction is cutting in the wrong place. If you cut too far forward (toward the snout), the otoliths are further back in the brain case than you can see or reach with the forceps. If you think this is the case, you can remove some of the top of the brain case with wire cutters to provide a window to look down into. If you cut too far back (toward the dorsal fin), you can damage the otolith or push it down into the gills. Another common issue is that the otoliths come out of their pocket when you remove the brain. Sometimes removing the brain will also remove the otoliths as they are connected by nerves. Consequently, the otolith is no longer in the pocket. Keeping an eye on the pocket when removing the brain can help avoid this issue, although I’ve noticed it being more of a problem in other fishes (Grass Carp and suckers).

After you clean your next cooler of catfish for the frying pan, see if you can find these important little structures. Otoliths and age data are critical to help us understand the age distribution and growth of fishes to support the development of fisheries management strategies.

Look for a post in early April summarizing a soon to be published study on Flathead Catfish diet and distribution in Virginia tidal rivers!