February 1, 2024

It’s All In Their Heads – What We Can Learn From Fish Otoliths

Photos courtesy of the authors.

Fisheries biologists need to know a lot about the fish populations they manage in order to design effective management plans. How fast are the fish growing? How many fish are surviving to older ages and larger sizes? How many new fish are being added to the population and where are they coming from? How can a fisheries biologist get this information? Fortunately, much of this information can be derived from clues contained within the heads of the fish themselves. Within each fish’s head are ear stones called otoliths, and some of these otoliths can provide a record of that fish’s life experiences in terms of its growth history, number of days or years lived, and sometimes even which waterbodies or habitats that fish lived in during its lifetime.

Otoliths are calcium carbonate structures within the inner ear of bony fishes (cartilaginous fishes, such as sharks, do not have otoliths) that aid in balance and hearing. Fishes have three sets of paired otoliths within their cranial cavity – the sagitta, lapillus, and asteriscus otoliths. Of these three, the sagitta are the otoliths most commonly used by fishery biologists because this type of otolith is typically the largest and easiest to work with. Like other calcium carbonate structures used for aging fish (e.g., spines, fin rays, scales, etc.; Box 1), otoliths are formed by alternating layers of calcium-rich and protein-rich deposits that are visible as opaque and translucent bands (Figure 1). Otolith structures are particularly useful for aging fish that live in temperate climates with distinct growing seasons, as this seasonality produces distinct annual rings (annuli) from the extremely slow growth that occurs during the winter season compared to the summer. For some types of fish less than a year old, daily growth increments are also visible as rings. Biologists can dissect captured fish for their otoliths (Figure 2) and use a microscope to count and measure otolith annuli or daily rings (Box 2). All of the age and growth information derived from otoliths relies on previous studies that have validated that the otoliths of the fish species being studied form bands on an annual basis and that growth of the otolith in diameter is proportional to fish growth in length.

Two photos collaged together. On the left is a light blue micrograph of a bone of a fish. The image reveals rings that ages a fish. The image on the right is of a fish dissection. The head is cut and bent up to show a section of the fish behind its head.
Figure 1 (left). Black crappie sagitta otolith from Evergreen Lake (McLean County) in October 2022. Dark, translucent bands are periods of fast growth and white, opaque bands are periods of slow growth (e.g., winter). Dots and numbers identify the annular rings on this structure. Figure 2 (right). White crappie head that was cut and bent, with the otic capsule broken to expose the paired sagittal otoliths (identified by red arrows) found within the cranial cavity behind the gill rakers.

Fisheries biologists rely on accurate estimates of fish ages and growth to assess the status of a fish population, diagnose problems (e.g., overharvest, low natural recruitment), and design appropriate management actions (e.g., harvest regulations, stocking priorities, and habitat management). Using age and length data from collected fish, biologists can estimate the average rate of fish growth within a population by plotting ages versus lengths and statistically fitting a curved line to the points in the graph (Figure 3). The length at which the curved line flattens out indicates the average maximum size attained by fish in that population. Biologists can also estimate mortality (or, conversely, survival) rate by examining the catch curve, a plot of the declining trend in fish abundance with increasing age. Growth rates, maximum sizes, and mortality rates estimated from otoliths can be compared between different fish populations and used to diagnose signs of over-harvest or stunted growth and to inform the proper design of regulations to address these issues. Additionally, a fish population’s age structure – the relative abundance of individuals of different ages – gives insight into a population’s relative success in recruiting new generations of fish each year. Similar to counting annual rings, fish biologists can even count daily rings on the otoliths of young fish to back-calculate their hatch dates and use this information to direct management actions at critical times for successful reproduction. Along with the information contained within otoliths on fish age and growth, natural and artificial chemicals taken up during otolith formation can provide answers to questions such as which rivers did a fish use and was this fish raised in a hatchery.

A black and white graph showing the age-length relationships for a bluegill fish population at a lake. The graph shows  a quick growth rate, and then the growth rate begins to plateau as the fish get older.
Figure 3. The age-length relationship for a bluegill population in Forbes Lake, (Marion County). Points depict the measured lengths and otolith-derived age estimates of Bluegill collected from 2018 to 2020, and the curved line models the average growth rate for the population.

Throughout a fish’s life, layers of calcium carbonate and various elements are added to the otoliths and these layers reflect the chemical signatures of the water the fish inhabited. Otolith microchemistry techniques usually involve mounting the otolith in a resin, cutting a cross section of the otolith through the center, and then using laser ablation technology to burn a small section at the nucleus (i.e., center) of the otolith and chemically analyze the emissions. The chemical signature can then be compared to the chemical signatures of different waterbodies or hatcheries to look for a match that would indicate the fish originated or spent part of its lifetime within the matching environment. These techniques can be especially useful for invasive species such as invasive carps by helping reveal movement patterns, habitat usage, and origins. Fish otoliths can also be artificially marked by immersing fish in oxytetracycline (OTC), which leaves a fluorescent band on the otolith annulus corresponding to the year when it was marked. One application of this marking method is to assess the contribution of stocked fish to a population by batch marking fish with OTC prior to stocking. From a subsample of fish captured during subsequent surveys at the stocked waterbody, biologists can examine otoliths under UV light for an OTC mark to determine if they were from the hatchery.

Though it is regrettable that some fish need to be sacrificed to provide otoliths for study, judicious collection of otoliths provides valuable information used to tailor management actions for sustainable and quality fisheries in Illinois.

Joe Parkos, Dakota Radford, and David Yff are Illinois Natural History Survey research scientists at the Kaskaskia Biological Station, a Prairie Research Institute and University of Illinois at Urbana-Champaign (UIUC) field station in Sullivan, IL. Parkos is also affiliate faculty in the Department of Natural Resources and Environmental Sciences at UIUC.

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