How old is a fish? Tree rings in fish bones (Ep. 51)

We’ve all heard of tree rings that allow us to determine the age of trees. Counting the lines on a cut-down tree allows us to figure out their age pretty accurately. Unfortunately, this only holds true for ring-forming trees, and even among those, it is necessary to damage the live tree for such age determination.

There is a similar method used to determine the age of fishes—did you know that fish is the correct plural form of fish for same-species groups only? Unfortunately, this method requires cutting open the skull of the fishes, which is why other methods are frequently used instead.

A common live-specimen method is measuring the length of the fish and comparing it to species-specific guides. This method isn’t very precise, as there are a lot of factors playing into the growth rates of fishes, even within the same population or species.

In some clown fish, such as Amphiprion ocellaris, all organisms are born male, and depending on where they end up in the hierarchy, they change their sex to female later down the road. In breeding pairs, the female then goes into a rapid growth period while the male stays smaller. In general, the largest organism is often the alpha of a group, but the cause and effect of this differ between species: in some species, the largest becomes the alpha, in other species, the alpha grows fastest, partially because they get the most access to food. Add in random variation of size (just like in humans, some individuals are larger or smaller), and this method seems almost foolishly unreliable.

This is particularly important as length is frequently connected to catch prohibitions for both private and commercial fishermen. Individuals below a certain length threshold are returned to the ocean (frequently with damaged mouths or hyperbaric issues, but that’s a topic for another day), thus granting an evolutionary advantage to smaller individuals—contrary to the evolutionary pull where bigger means a higher attractiveness to partners.

With the most common method to determine the age of a fish so unreliable, it is beneficial to have a more precise method, even if it can only be used to determine the age of dead fish. This method is very like that of tree rings, but on a much smaller scale. These “age rings” appear not on the entire body, but in small calcium carbonate structures found inside the inner ear of most vertebrates—yes, including Homo sapiens.

Otoliths are small structures similar to growing crystals used for balance and audio processing. These structures exist in a primitive form even in invertebrates such as sea jellies, but are not present in shark, lampreys, or rays. These otoliths are also present in humans and develop even before our sight. Fetuses use the otoliths for spacial awareness in the womb. The evolutionary reasoning for this counterintuitive development where humans and most fishes share an organ that other fishes don’t is something I am still researching. I haven’t found a satisfying answer yet, but I am reading a book on the evolutionary biology of fishes, where I will hopefully find my answers.

For those fishes that have them, otoliths are a group of three calcium carbonate structures that float in the endolymphatic liquid of the inner ear. As they aren’t connected to the rest of the body, this floating allows interpretation of spacial information.

Otoliths in an upright posture and affected by gravity. Drawn by Kate Hildenbrand.

These otoliths grow faster or slower depending on the season and other stress factors. The otoliths are essentially just a weight for gravity to work on. The otolilth membrane gets distorted by their movement, which both we humans and other animals with otoliths can use for balance and to aid in hearing.

The otoliths grow with age, and thanks to a slower rate of growth during certain periods of the year, they form alternating rings of translucent and opaque material. During times of slower growth, less material is deposited in the otolith, and they appear translucent, while times of faster growth lead to opaque areas. In most fishes, scientists have found that the translucent zones are formed during winter, when there is less nutrient availability, and the temperatures are lowered, but this is not a general rule.

Three studies of Norway (Dannevig, 1956), the North Sea ****(Pilling et al., 2007), and the Skagerrak (Gjøsæter and Danielssen, 2011) found that the inverse is true for Atlantic cod, Gadus morhua. Atlantic cod, and other cold-water fishes, seem to actually prefer the cold times of the year and their translucent zones form during summer instead.

In either case, it is possible to translate the number of rings into the age (Williams and Bedford, 1974; Kalish et al., 1995). Much like with tree rings where you have lighter and darker areas, four translucent rings would translate to four years of age. Even in tropical fish, it is possible to perform similar procedures, as growth rates are different during breeding seasons. So, even though there are no cold and warm periods, other factors have similar effects.

For sharks, a very similar method is used, but instead of the ear bones, the vertebrae are examined. This isn’t possible for all sharks, as some sharks such as the Greenland shark, Somniosus microcephalus, don’t form these bands. In those cases, different forms of carbon dating have to be used (Save Our Seas, 2022).

Tree-ring like dating of fish is certainly a valid method with a higher reliability than more frequently used methods. However, the necessity of a dead organism is certainly a limitation to not be taken lightly. It will be necessary to weigh the benefit of potential knowledge gained with the certain price the organism has to pay: their life.

In already dead organisms, using this “black box” of information is certainly worth it, and can lead to valuable information about fishes, both on an individual and a species level.

Kate Hildenbrand

Kate Hildenbrand

Kate Hildenbrand is the writer behind the essays here, author of fiction novels, the creator of the Kate Hildenbrand podcast, and a student of marine ecology. At least, that's her on the surface.