Tiny Bones Tell Big Stories: How Fish Ear Bones Inform Great Lakes Fishery Science & Management

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Much like how the growth of trees is chronicled in rings on trunks, fish growth is recorded in layers on otoliths. Fish accumulate layers of calcium carbonate on their otoliths throughout their lives, with the rate of accumulation depending on how fast a fish is growing.

Take a look at the photo below – do you know what the circular object is with the concentric rings? Though it may look like a cross-section of a tree or a thumbprint, the object is actually an ear bone from a fish, called an otolith.

Bloater Otolith Used to Determine Fish Age

An otolith, sometimes called “earstone,” of a bloater, a small prey fish in the Great Lakes. Fishery scientists interpret the age of bloater and other fishes by counting annual growth rings on their otoliths. 

To find out how old this fish is, count the dark rings and double check your answer in the text below!

(Public domain.)

Otoliths are hard, calcium carbonate structures located near the brain of bony fishes. Otoliths vary in size depending on the species of fish, but most are the size of a fingernail or smaller. Fish have three different types of otoliths which aid the fish in balance and hearing, similar to structures of the inner ear in humans. To fishery scientists, though, otoliths have a different use: they can reveal the age of the fish!

Alewife Indicating Otolith Location

Otoliths are located directly behind the brain of bony fishes. On the young alewife shown here, the approximate location of the otoliths is noted with a yellow arrow.

(Public domain.)

Much like how the growth of trees is chronicled in rings on trunks, fish growth is recorded in layers on otoliths. Fish accumulate layers of calcium carbonate on their otoliths throughout their lives, with the rate of accumulation depending on how fast a fish is growing. When a fish grows quickly, more material is laid down and a wider band results; when a fish grows slowly, less material is laid down and a thinner band results. Fish growth varies both on a daily and an annual basis, with faster growth occurring during the summer for most fish in northern temperate regions. Thus, alternating layered bands accumulate on otoliths on a daily and annual basis, though typically as a fish ages, the daily rings become less apparent and only the annual rings are visible. To determine the age of a fish from its otoliths, simply count the number of rings. If you haven’t done it yet, try counting the rings (dark bands) on the otolith above. How many did you count? If you counted eight rings, you would be correct! In this case, the eight rings represent eight years of growth (not days).

Scientists at the USGS Great Lakes Science Center (GLSC) interpret the ages of thousands of fish using otoliths every year. It’s a tedious process counting all of those rings, but the results are extremely valuable. If fishery managers know the ages of the fish in a population, they can better manage that population. Much like how human censuses determine whether populations are growing, shrinking, or holding steady, fishery managers regularly survey fish populations to understand how they are changing and adjust management actions accordingly.

Viewing an Otolith Under a Microscope

Lynn Ogilvie, a USGS biological science technician, adjusts an otolith under a microscope. Photographs of otoliths are taken using a camera mounted on top of the microscope and a video feed on the computer. Notice the magnified otolith on the screen in the background. This photo was taken in 2018, prior to the COVID-19 pandemic.

(Public domain.)

 

Scientists at GLSC age a variety of fish species using otoliths, but most work is focused on three species: alewife, bloater, and cisco. These fish species are important prey for larger, predator fish in the Great Lakes, such as lake trout, salmon, walleye, and others. Much attention is given to prey fish because the abundance of these fish strongly influences populations and sizes of predator fish, many of which are important species in recreational and commercial fisheries.

Graduate Students Mounting and Reading Otoliths

State University of New York-Brockport graduate student, Tom Bianchi, interprets alewife otolith ages (foreground) while student contractor Scott Minihkiem (background) mounts otoliths in epoxy. Bianchi is evaluating how alewife age influences reproductive success and was trained in otolith techniques at the GLSC’s Lake Ontario Biological Station. This photo was taken in 2018, prior to the COVID-19 pandemic.

(Public domain.)

For example, in Lake Ontario, all sport and native fish depend heavily on alewife as prey. Scientists at GLSC’s Lake Ontario Biological Station (Oswego, New York) have aged 500-800 alewife otoliths annually since 1982. Along with the age-information, scientists also measure the length and weight of the fish they collect. The combined data are used to plot relationships such as how alewife average weight at a particular age changes through time.

 

Otoliths in Vials

Hundreds of alewife otoliths, all stored in separate vials, wait to be aged at the GLSC Lake Ontario Biological Station.

(Public domain.)

GLSC scientists working on Lake Ontario have made a few surprising discoveries doing this work. Alewife growth has increased in recent years, so much so that three- and four-year-old alewife now weigh as much as eight-year-old alewife did previously (see figure below). Looking back in time, the change in growth rate occurred during the early 2000s, about the time two important prey items for alewife, the invasive invertebrates Cercopagis and Bythotrephes, became abundant in the lake.

Graph of Mean Weight-at-Age of Alewife by year

Mean (average) weight-at-age for alewife in Lake Ontario from 1984-2017. Age 3 (green line) and age 4 (blue line) alewife now weigh as much as age 8 (gray line) alewife in the early 2000s (compare lines in gray box on right to line in gray box toward middle of figure). Alewife growth increased coincident with increased prevalence of Cercopagis and Bythotrephes in the early 2000s (blue arrow).

(Public domain.)

GLSC staff also work to ensure methods used to age fish otoliths are consistent across the basin. In December 2018, GLSC coordinated the first of two workshops on Great Lakes prey fish age estimation in Traverse City, Michigan. Attendees came from across the Great Lakes region and included over 60 participants from 23 state, tribal, and federal agencies and academic institutions. The purpose of the workshop was to coordinate methods for collecting age information on fish species of conservation and management importance, particularly prey fish. Harmonizing the collection of key data on fish populations, such as age, allows for more relevant comparison of life-history characteristics across the Great Lakes. Additionally, the workshop facilitated communication among regional experts and provided a training opportunity for Great Lakes natural resource agencies.

Great Lakes Otolith Aging Workshop

Twenty-three state, tribal, and federal agencies and academic institutions throughout the Great Lakes region gathered in Traverse City, Michigan, in December 2018 for a prey fish age estimation workshop, co-hosted by GLSC.

(Public domain.)

Across the Great Lakes, prey fish age-structure, size-at-age, and abundance are some of the key factors used by fishery managers to manage predator fish, including determining how many salmon to stock in the Great Lakes. The decision-making process involves input from federal, state, tribal, and provincial agencies as well as universities across the Great Lakes region. It’s a large, collaborative effort in the Great Lakes, relying on big data from little bones.

Holding a Chinook Salmon

Carson Pritchard (left), a former GLSC contractor and 4-H camp counselor, shows off a Great Lakes Chinook salmon with a 4-H camper. Alewife are the primary forage of Chinook salmon. Consequently, understanding alewife age dynamics helps fishery managers balance predator numbers with the available prey. This photo was taken in 2018, prior to the COVID-19 pandemic.

(Public domain.)