As the upcoming winter holiday season approaches, many of us are getting ready to embark on trips back to our hometowns. But while airports and train stations may be stressful, spare a thought for the travel tribulations of migratory fishes.
As highlighted in the recent WWF Living Planet Report, man-made barriers such as dams represent a major threat to migratory fish species across the planet because they block fish movement between critical habitats and limit the recruitment potential of species that can no longer access riverine habitats located upstream. Globally, only 37% of rivers longer than 1000 km remain free-flowing over their entire length (Grill et al. 2015).
“Over the river and through the woods” takes on a literal meaning for some migratory fish species blocked by dams. When dams can’t be removed (e.g., they provide hydropower), humans have devised many ways to help fish get around. You may have automatically thought of fish ladders and the internet-famous Salmon Cannon. However, understanding what happens to fish after passage through or transfer around dams is critical to determining success of these projects but is generally understudied (Roscoe and Hinch 2010).
Post-transfer behavior is particularly understudied for non-salmonid migratory species including the Lake Sturgeon (Acipenser fulvescens). Lake Sturgeon were once abundant throughout the North American Great Lakes and historically served as an important food source for Native American and First Nations peoples. However, sturgeon numbers were dramatically reduced during the 19th and 20th centuries by a combination of overharvest, pollution, and habitat loss. In recent times, efforts have been undertaken across the Great Lakes Basin to restore populations, including through releases of juveniles raised in streamside rearing facilities.
The hope is that these released fish will grow up and return to these rivers to reproduce, but unfortunately, Lake Sturgeon have reproductive traits that make their rehabilitation difficult. They are slow to mature (sexual maturity is reached between 14-33 years for females and 12-17 years for males) and have periodic interrupted spawning cycles (females typically spawn once every 3-7 years while males may spawn every 1-4 years).
To increase the likelihood of successful spawning, scientists and managers are looking for new ways to help sturgeon access good spawning habitat located upstream of dams.
Enter: the Sturgeon Elevator.
The first elevator specifically designed to capture Lake Sturgeon began operation in 2015 on the lowermost hydroelectric dam of northern Wisconsin’s Menominee River.
The Menominee River is a tributary to Lake Michigan’s Green Bay that supports one of the largest remaining populations of Lake Sturgeon in the Great Lakes. Historically, fish could travel up the river for nearly 130 river kilometers (rkm), but now encounter the Menominee Dam only 4 rkm upstream from Green Bay. As a result, over 90% of what is considered high-quality sturgeon spawning habitat is located upstream of the dam and therefore unavailable to fish (Daugherty et al. 2009). The lack of access to habitat has likely contributed to sturgeon population decline (Coscarelli et al. 2011) and is considered an impediment to the species’ recovery.
The Sturgeon Elevator is a rectangular metal hopper that can be lowered to the bottom of the river. Water from above the dam flows through the hopper area, creating a current that leads sturgeon into entering. The hopper is then lifted to the upper floor of the powerhouse and a door on the side of the hopper opens, emptying the fish into a sorting tank.
From there, non-sturgeon species that are unintentionally captured are sorted out and released back downstream. Lake Sturgeon are inspected for injuries or disease, measured, and then tagged. Sturgeon that are not ready to spawn during the next spawning period (late April to mid-May) are also released back downstream. Adult fish that are ready to spawn are loaded into a trailer and driven above a second nearby dam and released to continue their migration upstream.
In its first four years of operation, the elevator moved about 400 adult sturgeon upstream. A recent study by Isermann et al. (2022) found that transferred sturgeon remained upstream of both dams for at least one spawning opportunity. While it’s still unknown whether these opportunities will result in increased recruitment for sturgeon the Lake Michigan basin, studies like this represent first steps in determining whether over-dam transfers can be used to increase abundance of non-salmonid migratory fishes.
How’s that for giving migratory fish a lift?