Discovery Of Larval Pallid Sturgeon Unexpected By Researchers

Andrew McKean, FWP Information Officer

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After a couple of decades of watching wild pallid sturgeon populations dwindle, biologists in the upper Missouri River have discovered the first evidence of natural reproduction of the endangered species.  

The discovery last fall of two tiny pallid sturgeon larvae in the Missouri River provides evidence that pallid sturgeon spawned in 2002.

T he pair of larvae, which resemble inch-long tadpoles with goatees, were sampled in early September near the headwaters of Lake Sakakawea, downstream of the confluence of the Yellowstone and Missouri rivers . Because the pallid sturgeon closely resembles the abundant shovelnose sturgeon, the larvae were sent to an expert for conclusive identification and the positive results were received this March.

This is the first evidence in the past 40 years that natural spawning produced   pallid larvae in the Missouri or Yellowstone rivers and biologists are happy to find evidence that spawning occurred.

However, they believe flows in the lower Missouri River are not consistently adequate to trigger spawning and that any larvae that are produced probably drift downstream into Lake Sakakawea, a reservoir environment in which the larvae cannot survive.   Larvae tend to drift downstream for at least a week before finding a niche where they’ll stop to grow.

Without natural reproduction, the species’ recovery depends on a hatchery program that has only slowed the sturgeon’s slide toward extirpation. Since its listing, biologists have documented the continuing decline of the species.   The wild population below Fort Peck Dam and the lower Yellowstone River is estimated at less than 166 fish, while fewer than 25 fish exist in the Missouri upstream of Fort Peck Lake Reservoir.   So few pallid sturgeon remain in Montana that biologists say nearly every adult is known to them.

The pallid, an armor-plated, bewhiskered throwback to the Cretaceous Period (200 million years ago), can grow to 80 pounds and more than 5 feet in length.   The fish lives 40 or more years and doesn’t begin to reproduce until around 15 years of age. It evolved in the once warm, muddy spring runoff of the Missouri and Yellowstone as the rivers churned across the prairies.

Fort Peck Dam, constructed in the 1930’s, blocked the Missouri’s channel and replaced its dynamic, turbid flow with consistent cool, clear water. Below Fort Peck, just as below other dams on the Missouri and Mississippi rivers, pallid populations have dwindled over the past half-century with the loss of spawning and rearing habitat and conditions.

Larval pallid sturgeon have been found only a few times in the last half-century on the Missouri downstream of Kansas City, after floods in1993 made the river behave as it did before dams tamed it. That’s why the discovery of pallid sturgeon larvae last fall is so remarkable, raising more questions than it answers.

Research being conducted by Pat Braaten, a fisheries biologist with the U.S. Geologic Survey and Dave Fuller, a FWP technician whose crew found the young pallids, focuses on habitat and how habitat changes might improve pallid sturgeon spawning success. Working in cooperation with the U.S. Army Corps of Engineers that controls the flows from Fort Peck Dam, the researchers are collecting baseline data in the lower Missouri before a planned release of warm water over Fort Peck Dam’s spillway occurs.  

This experimental release will mimic the river’s natural seasonal flow. Fish movement will be measured in response to the increased flows, but the release cannot take place until Fort Peck Reservoir levels reach the spillway. Researchers hope the water flow will cue the pallid sturgeon to spawn and help the larvae to drift into safe, natural habitats where they might grow to reproductive age.

The pallid sturgeon is the premier indicator species for the Missouri, one researcher said. With declines observable in blue suckers, paddlefish, sauger and ling populations, what we learn about recovering the pallid sturgeon may help us conserve these other species as well.