Captive Rearing Strategies Could Impact Lake Sturgeon Recovery Success

by guest contributor Derek Smith

Flowing between the kitschy, Bavarian-style architecture of Frankenmuth, Michigan is the Cass River, where biologists released 104 young lake sturgeon in September 2022. Before releasing them, the biologists equipped the sturgeon with tracking and identification devices to follow the fish’s movements and survival after their release. That knowledge is critical to assess the success of sturgeon release events because there is no guarantee the sturgeon will remain where they are released.

A small lake sturgeon being held in a person's hands.
A young lake sturgeon being released into the Cass River. The fish will leave the river after its first year, but scientists are hoping that they return when they are old enough to spawn. Photo courtesy Derek Smith.

Once numerous in North American freshwaters, lake sturgeon are considered globally endangered due in part to severe overfishing in the late 1800s and early 1900s and are absent from many rivers where they historically lived, such as the Saginaw River and its tributaries, including the Cass River. According to US Fish and Wildlife Service biologist Justin Chiotti, the goal of releasing captive-raised sturgeon is to “restore historic populations to self-sustaining levels.”

But sturgeon don’t hunker down in one place. After one year, many sturgeon leave their birth rivers to live in lakes, bays, and river mouths. When they reach sexual maturity between twelve and twenty-seven years later, the sturgeon return to their natal waters, which are better suited to spawning. There, fertilized eggs and larval fish take shelter from predators between the rock crevices in cobblestone substrate and thrive on the fast-flowing, oxygen-rich water.

How sturgeon navigate back to their rivers of origin is still largely unknown, but scientists think that larval sturgeon imprint to chemical signatures in their natal waters. Later in life, the sturgeon “sniff” their way back to their natal rivers to spawn by following those chemical queues.

Imprinting captive-reared sturgeon to rivers where they are now extirpated presents a challenge to conservation biologists When sturgeon are not raised in the waters in which biologists plan to restore self-sustaining populations, the re-introduced fish might not imprint to the historic habitat, potentially reducing the return rates when the fish are old enough to spawn. ,

A lake sturgeon fish hatchery in Genoa, WI. Photo courtesy Derek Smith.

Stream-side rearing facilities could be the solution to the imprinting challenge. At these facilities, biologists raise sturgeon directly in the river water where the sturgeon will be released. The Toledo Zoo has one such facility along the Maumee River, where they have been raising fish for release into the Maumee River since 2018. Similar facilities are located in Michigan, such as the Black River facility run by Michigan State University and the Big Manistee River facility run by the Little River Band of Ottawa Indians.

According to Matt Cross, the Toledo Zoo’s Conservation Coordinator,“the idea with the stream-side facility is that the fish will pick up on the different chemical cues from the river, which they will use to return to the river to spawn.” By raising sturgeon in the waters to which they will later be released, the Zoo’s biologists are increasing the chances that the sturgeon are in the river water during the developmental stage when imprinting occurs, which Cross says is “a big question mark.”

But the stream-side facilities are costly and have water quality issues that off-site hatchery facilities optimized to grow large amounts of fish are better equipped to manage. The National Fish Hatchery in Genoa, Wisconsin is one such facility. There, U.S. Fish & Wildlife biologists rear sturgeon from eggs in Genoa well water. The facility raises hundreds to thousands of sturgeon for release into rivers each year, but there is a chance that hatchery-raised fish miss the crucial period at which imprinting occurs before they are large enough to be released. The unknown timing of when sturgeon can imprint to rivers obscures the relative effectiveness of these two rearing strategies.

Larval lake sturgeon swim around a tank filled with well water at the Genoa National Fish Hatchery. Biologists refer to the fish at this age as “free-living embryos” because they still have a yolk sac that provides all their nutrition. Photo courtesy Derek Smith.

According to Cross, “it’s a cost-benefit problem.” If sturgeon raised in well water still reliably return to the rivers where they are released, the large number of fish raised at the Genoa Hatchery could be a boon to sturgeon conservation across the Great Lakes region. On the other hand, if the hatchery-raised sturgeon wander, additional streamside facilities may prove necessary to re-introduce self-sustaining sturgeon populations to target rivers. The Cass River, for example, lacks a stream-side rearing facility and may potentially benefit from one.

To better assess the costs and benefits of each rearing strategy, biologists at the Toledo Zoo, US Fish and Wildlife Service, Ohio Division of Wildlife, Michigan State University, and the University of Toledo will test whether the two rearing-strategies impact how sturgeon return to rivers. In the summer of 2022, the biologists collected eggs and sperm from the same spawning adults in the St. Clair River, where healthy, self-sustaining populations exist today. Fertilized eggs were created by mixing sperm and egg, then half of the eggs were raised at the Genoa Hatchery while the other half were raised at the Toledo Zoo’s stream-side facility.

Once the fish were fingerling size, the biologists equipped them with PIT tags. The tags are similar to the microchips implanted into pet dogs and cats and encode an identification number linked to information about where an individual fish was released and in which facility it was raised. Whenever a released sturgeon is recaptured, scanning its PIT tag will allow biologists to identify how many fish from each rearing facility returned to the Maumee River to spawn

Both institutions have partnered to release sturgeon into the Maumee River in this manner since 2018, so the oldest fish released in this program are only 4 years old. The team must wait until the first of the released sturgeon mature eight to twenty-three years from now before they get their answers.

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Michigan Nature Monday: Poweshiek skipperling

One of the rarest butterflies, the Poweshiek skipperling, is truly on the brink of extinction. Once abundant in the tall prairie grasslands and the prairie fens of several states and provinces in the upper Midwest, the tiny butterfly is now found only in a handful of sites in Manitoba and northern Oakland County, including an MNA nature sanctuary. Loss of habitat and other factors contributed to a decades-long—and now a relatively recent and rapid—population decline that has scientists scratching their heads and worried about what their disappearance may mean for other pollinators.

The globally endangered Poweshiek is now so rare that only 100 individual butterflies were counted in a 2021 census. Recovery plans—aided by an international partnership that includes MNA—call for captive breeding efforts to headstart individuals and increase survival to adulthood in order to build a reserve population that can be reintroduced to the wild. The Minnesota Zoo, John Ball Zoo in Grand Rapids, Michigan State University’s Haddad Lab, and the Michigan Nature Association are specifically collaborating within the Poweshiek Skipperling International Partnership to annually produce more individuals for wild releases in 2022 and beyond in what is known as ex situ or “offsite” conservation.

Poweshiek skipperling photo by Cale Nordmeyer, Minnesota Zoo

MNA is proud to protect habitat critical for the Poweshiek skipperling’s survival, and to be part of the important partnership that is working to save this species from extinction. We look forward to continuing participation in this partnership effort to increase the Poweshiek skipperling population in the wild in the coming years.

Learn more about our conservation work and how you can contribute at michigannature.org.

Michigan Nature Monday: Vernal Pools

This week’s Michigan Nature Monday features the amazing natural habitat of vernal pools. The Michigan Nature Association has been a lead partner in the Michigan Vernal Pools Partnership for several years, and we are proud to have added to that commitment this year with a dedicated staff member – our Michigan Vernal Pools Partnership Coordinator–and the production of the Ephemeral video, produced in partnership with the award-winning videography team of Fauna Creative.

ephemeral video thumbnail with play button

If you have spent time exploring Michigan’s forests in the spring, you may have come across small, shallow pools of water scattered throughout the landscape. These small wetlands are called vernal pools because they are typically filled with water in the spring (“vernal” means spring) but they usually dry up and “disappear” during the summer. Vernal pools are special types of wetlands– because they regularly dry up and are usually isolated from other wetlands and waterbodies, vernal pools cannot support permanent fish populations. Due to the lack of fish predators, vernal pools provide critical habitat for certain animal species that rely on these fishless habitats for their survival and/or reproduction. These include a number of invertebrate and amphibian species, such as fairy shrimp, wood frogs, spotted salamanders, and blue-spotted salamanders. Vernal pools also provide habitat for many other animal and plant species, including rare, threatened, and endangered species. 

Vernal pools play an important role in maintaining healthy forest ecosystems. Because they provide habitat for diverse and unique animal species and provide other important ecological functions, some have referred to vernal pools as the “coral reefs of Northeastern forests.” However, these vernal pools are vulnerable to a number of threats and are not well-protected under current wetland laws and regulations.

If you are interested in learning more about vernal pools and efforts to protect them, visit mivernalpools.com

Michigan’s Winter Wonders: Green Frog

Winter weather has arrived in most parts of Michigan by now, bringing dread for some, and excitement for others. But have you ever wondered what’s happening under the snow? Many of Michigan’s reptiles and amphibians search for tunnels or other underground cavities to wait out the cold winter temperatures. Still others might remain underwater, remarkably surviving the long winter months with little to no oxygen!

Such is the case with the Green Frog (Lithobates clamitans), one of Michigan’s most abundant frog species. These semi-aquatic amphibians spend their winters hibernating at the bottom of permanent pools and rivers, “breathing” in oxygen through their skin, occasionally nestled among piles of leaf litter that give off some amount of heat as they decompose, and sometimes can be seen swimming before the water has frozen over, or through clear ice. *A common misconception of hibernating animals is that they spend the entire time ‘sleeping’, when in fact hibernation is rather a period of reduced activity.

a green frog sits on a rock in the water
A green frog rests on a rock at MNA’s Joan Rodman Memorial Nature Sanctuary. Photo by Jodi Louth.

So, why spend the winter underwater rather than on land? Other frogs in Michigan like the wood frog, spring peeper, as well as toads, hibernate in tunnels and under leaf piles. These frogs are able to survive the freezing temperatures because they produce excess glucose which helps prevent freezing of the cells in their bodies, acting as a sort of antifreeze. Green frogs on the other hand, are not able to function in this way, and so they must stay in above-freezing temperatures through the winter.

Adequate winter habitat for these species is a critical part of ensuring their survival to the next season, and threats to that habitat exist – even as the snow flies! For many of us, fall is a time for “cleaning up”, for raking up all the leaves that have fallen (and jumping in the pile), then bagging them up and shipping them off to a community dump site. But this practice robs our yards and natural areas of needed habitat, not only for the insects that overwinter in leaf piles like caterpillars and firefly larvae, but also for the many reptiles and amphibians that call our state home. The leaves provide a “blanket” between the ground and the snow, they provide heat as they decompose in place which also returns essential nutrients to the soil.

There are so many benefits for Michigan nature from leaving the leaves, so skip the fall “cleanup” and enjoy reap the rewards come springtime. Learn more about Michigan’s many reptiles and amphibians, and their conservation needs, with Herpetological Resource & Management at herprman.com, and learn more about the benefits of leaving leaves, visit healthyyards.org or leaveleavesalone.org