From tree rings to sturgeon growth rings: The fish aging process

Left: Sturgeon pectoral fin spine section when viewed under a microscope, USFWS. Right: Tree rings (source: http://ow.ly/G8Zh50yWKoB)
Lake sturgeon to be surveyed by U.S. Fish and Wildlife Service biologists, view of pectoral fin, USFWS.

It’s common knowledge that trees have annual growth rings. The contrast of new, light-colored wood during the growing season and darker, older wood toward the end of the growing season form tree ring patterns. By studying tree rings, scientists can not only determine tree age, but what the weather conditions were like during each year. Thick rings signify rainy conditions good for growth while thin rings signify dry seasons or drought. Scars between rings represent damage from forest fires. By studying tree rings, scientists can observe patterns in climate.

Lesser known is the fact that lake sturgeon (Acipenser fulvescens) have similar growth rings from an accumulation of a form of calcium carbonate, bioapatite, in their pectoral fin spines. Pectoral fins, located on each side of a sturgeon’s body, help the fish control directional movement in the water.

Clipping from a pectoral fin spine bone (above) with sample sections cut by the Isomet saw (below), USFWS.

In order to age lake sturgeon, biologists at the U.S. Fish and Wildlife Service New York Field Office take clippings from the pectoral fin spine bone, slicing them into thin sections using an Isomet saw. Viewed under a microscope, the sections show clearly defined rings or annuli, each representing one year of growth!

Like reading clues from tree rings, biologists can learn about sturgeon growth rates and life cycles by studying intricacies in bioapatite accumulation. Wide rings represent healthy growth, thin rings represent slow growth and for female sturgeon, time of spawning can be determined. Since females only spawn once every 3–5 years, annuli are thinner in years of spawning, signifying more resources have been allocated for egg production than growth.

New York Field Office U.S. Fish and Wildlife biologists surveying lake sturgeon, USFWS.

Fin spine bones also have geochemical signatures showing differences in trace elemental profiles, allowing biologists to track sturgeon movements between different habitats and bodies of water. Biologists can begin to unveil patterns in fish movement, determining the duration a sturgeon spent in various habitats. When biologists are collecting pectoral spine samples for aging, they may also note the length and weight of the fish. Age, weight, and length combined paint a picture of the sturgeon’s overall health and provide clues to the condition of the environment they inhabit.

U.S. Fish and Wildlife biologists at the New York Field Office use age data to keep track of lake sturgeon stocking projects where hatchery-raised sturgeon have been released back into the wild. Aging is critical when it comes to assigning sturgeon to stocking year classes, documenting wild recruitment, and correlating between years spent stocking and numbers of adults in the wild.

Juvenile lake sturgeon. Ear bones (otoliths) are located in the center of the head just behind the eyes, USFWS.

There are other methods used to age fish, but for lake sturgeon, taking cuttings from the pectoral fin spine is the least invasive. Considering sturgeon have a combination of bones and cartilage, bone samples for aging can only come from a few key places.

Sturgeon also have otoliths (ear bones) located behind the eyes that aid in hearing, orientation, and balance. Floating in fluid, otoliths shift with external applied force, guiding fish through changes in movement and direction. Like pectoral spine bones, otoliths also accumulate rings of a form of calcium carbonate as a fish ages, but cannot be extracted without causing fish mortality.

Close-up of embedded scute bones, Ryan Hagerty/USFWS

Fish may also be aged by their scales which, when viewed under a microscope, show annual rings. While scale samples are relatively easy to collect, not all fish species have suitable scales, including lake sturgeon! Lake sturgeon have multiple rows of protective external bony plates called scutes. While scute bones show aging patterns, they are deeply embedded and difficult to remove without causing damage.

While collecting samples from pectoral fin spines is easier on the fish, the work can be grueling for biologists. Working to conserve lake sturgeon, New York Field Office biologist Scott Schlueter remembers days before the Isomet, when pectoral fin spines were sectioned using a jewelry cutting saw. The process was time consuming and slow. Since each spine section had to be polished, they were sanded by hand- a very dusty process!

Isomet saw blade cutting through a pectoral spine bone, USFWS.

The Isomet saw makes the process clean and easy, allowing biologists to gather and organize fin spine samples more efficiently. However, modern fin spine sectioning still requires ample time and care. With the Isomet, cutting a fin spine section takes several minutes and usually multiple cuts are required. Once processed at the New York Field Office, the spine sections are then packaged, labeled, and filed for future reference.

Hatchery-raised lake sturgeon juveniles for stocking, USFWS.

The aging process for lake sturgeon is an essential method for biologists to document the species they are working to conserve. Lake sturgeon, considered threatened not just in New York, but in 19 of the 20 states they inhabit, have been the catalyst for many conservation projects. Starting in 2014, the New York Field Office, Genoa National Fish Hatchery, and New York’s Department of Environmental Conservation (NYSDEC) have been working on restoring lake sturgeon populations in New York. Lake sturgeon raised by the hatcheries are released annually into waterways throughout the state!

Whether it’s the counting of tree rings or sturgeon growth annuli, the aging process is an important tool for understanding and observing long-lived species. For lake sturgeon, age data collected over time reveals larger patterns in population numbers, health, and habitat condition. This data allows biologists to continue collaborating on hatchery and stocking projects, ultimately working toward the goal of seeing self-sustaining populations of lake sturgeon in the wild.

Michele Lockhart/USFWS

Michelle Lockhart is the Outreach Coordinator with the New York and Long Island Field Offices. With her passion for writing and the natural world, she tells stories of the landscape, wildlife, and local conservation efforts.

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U.S. Fish & Wildlife Service Northeast Region

U.S. Fish & Wildlife Service Northeast Region

Conserving wildlife and habitats from Maine to Virginia, New York, and Pennsylvania.