Through the Ears of a Fish

by Ed Kluender, guest blogger
Ask a fish its age or where it’s been and what it ate for lunch, and chances are it couldn’t tell you.  It could probably hear your question, though, with the help of its inner ear bones, called otoliths.  Otoliths are one of the most useful tools in a fish biologist’s toolbox.  When removed and studied, otoliths can reveal a fish’s age, growth rate, and some history of where it’s been.
Otoliths are similar to the bones in human inner ears.  They are three pairs of small stone-like structures, called the lapilli, sagittae, and asterisci (singular: lapillus, sagitta, asteriscus).  Otoliths are suspended in fluid-filled sacs, and as in humans, help with balance and orientation.
Inner ear structure and otolith location in typical teleost fish.  The sagittae and lapilli are the two typically used for aging fish.  Source: David Secor, University of Maryland. 

Otoliths grow throughout a fish’s life, and are formed by layers of calcium carbonate that are laid down at different rates, depending on metabolic rate.  Slower metabolic rates during the winter form denser layers (the opaque zone), while high metabolic rates in spring and summer form less dense layers (the translucent zone).  This makes the otoliths look like an onion, with the opaque bands corresponding to slower growth appearing as dark rings. 

Because each opaque band represents a year of growth, scientists can use otoliths to estimate a fish’s age. Then, using the fish’s total length and proportional diameters of each opaque zone to the total diameter of the otolith, we can estimate the fish’s length at any age and thus its growth rate.
Left: Alternating opaque and translucent zones reveal a fish’s age. Right: To determine the age of a fish in years, otoliths are cut into thin wafers, mounted on a slide, and the rings are counted like tree rings.  This cross-section from a Freshwater Drum reveals the fish to be about 74 years old.

But  age and growth is just one of many uses of otoliths. Because different types of food items and aquatic environments contain unique isotopic signatures, the chemical structure of otoliths can reveal where a fish has been and what it has eaten at any point in its life. This field of study is called microchemistry, and can also reveal other environmental conditions at any life stage, like salinity and temperature.

The size and shape, or morphology, of otoliths  can reveal more information about population structure and environmental conditions.  Otoliths have even been useful in recent climate change research . Some research even shows that fish can hear quite well with their otoliths, and sense the direction and nature of the sound.
Otoliths contain all of this information, even in larval fishes.  The metabolism of a larval fish fluctuates on a daily basis in response to water temperature and daylight.  At night, metabolism slows, which causes the formation of a minute opaque zone.  With the aid of a microscope, we can look at the tiny otoliths – some as small as 0.1 mm – and count the number of daily rings to estimate the day at which the larvae hatched.  After that, we can estimate the fish’s daily growth rate since hatching.
A larval Flannelmouth Sucker before otolith extraction. Precise total length measurements are necessary to calculate the most accurate growth rate possible. In fish this small, the tiny otoliths must be extracted with very fine tools under a microscope. Source: Colorado State University. 

From there, based upon the knowledge of how long different species must incubate as eggs, we can back-calculate the exact day the eggs were spawned and fertilized and how fast they have developed since then.  Daily records of weather, temperature, and hydrology create a context that tells us quite a lot about the needs of different fish species, and the effects they have had on growth.

Otolith extracted from the larval Flannelmouth Sucker pictured above.  At 17 days old, a total length of 15.96 mm tells us that this fish was growing 0.94 mm per day during this critical stage in development.  This fish was part of a sample collected from a stream in which water is diverted for irrigation. Source: Colorado State University
This information is very valuable to scientists and land managers.  Because the habitats and water conditions in which fish spawn, incubate, and rear their young are very important to successful recruitment into the population, care must be taken to keep that habitat suitable for optimum growth rate.

Seemingly simple actions like diverting water for agriculture or power plant cooling result in the water returning to its source with a very different temperature, turbidity, and chemistry that can prevent successful reproduction.Please see an earlier post by The Fisheries Blog for more information about otoliths.Be sure to like The Fisheries Blog on Facebook, follow us on Twitter (@FisheriesBlog), and feed the fish at the bottom of the page.

References
Campana, S. E. 1999.   Chemistry and composition of fish otoliths: pathways, mechanisms and applications. Marine Ecology Progress Series. 188:263-297
Popper, A. N. and Z. Lu. 2000. Structure-function relationships in fish otolith organs. Fisheries Research. 46:15-25
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