Pallid Sturgeon Spawning in the Yellowstone River

Pallid sturgeon in the upper Missouri and Yellowstone rivers can live a long time.  Many are older than most of the biologists who study them.  Over the years, scientists have become very familiar with the few remaining wild sturgeon. One female pallid sturgeon in particular, currently known as code 30, has a long and storied relationship with sturgeon biologists.  Biologists met her for the first time in 1995 when she appeared in their nets during the early days of pallid sturgeon work.  Since that initial meeting, she was re-captured and released in 1996, 2002, and 2003.  After a few years of avoiding their nets, she was re-captured again in 2008.  Close examination in 2008 revealed that she was ready to spawn and biologists surgically implanted her with a radio transmitter.  Although she was tracked by biologists and finally determined to have spawned in the river in 2008, they were unable to determine precisely where spawning took place.

In 2010, code 30 was caught again in reproductive condition and ready to spawn.  This time, she was loaded into a large tank on a truck and taken to the hatchery where she contributed to the pallid sturgeon propagation effort.  Biologists spawned her in the hatchery and fertilized her eggs with milt from males captured in the river.  The eggs hatched and her offspring survived to be released back into the Missouri and Yellowstone rivers.  As biologists moved her from the hatchery back into the river they suspected that she was on a two-year reproductive cycle, spawning every other year in even years.  The stage was set for her to spawn again in 2012.

In April 2012, the reproductive status of female pallid sturgeon, code 30, was confirmed.  She had mature eggs and was ready to spawn.  Researchers from USGS and Montana Fish, Wildlife, and Parks (MFWP) kept close tabs on code 30 during May and June.  Radio-tracking efforts indicated that she was capable of rapid upstream and downstream movements.  After a 50-mile downstream migration, code 30 was recaptured for reproductive assessment on June 12.  Biologists used ultrasound to examine her ovaries and a hypodermic syringe to conduct an egg biopsy to determine how close she was to spawning, or if she had already spawned (see previous post “Did She or Didn’t  She” for a description of ultrasound).  The results clearly showed that she was still carrying all her eggs.

Biologists prepare to perform ultrasound and an egg biopsy on female pallid sturgeon, radio code 30, to determine how close to spawning she is or whether spawning may already have occurred. Biologists from Montana Fish, Wildlife and Parks determined that code 30 eventually spawned in the Yellowstone River on July 19-20, 2012.

During the next several days, the tracking boats followed her as she moved upstream and downstream past numerous ready-to-spawn male pallid sturgeon in the lower Yellowstone River (see previous post “Just One of the Guys”).  The female, code 30, and the males seemed to show little interest in each other.  All that changed dramatically one afternoon in the middle of June.  On June 19, pallid sturgeon female, code 30, joined a group of 11 telemetry-tagged, pallid sturgeon males in the Yellowstone River, 6.7-6.9 miles upstream of the confluence with the Missouri River. This tight aggregation of adults in a deep swift trough-like channel with a sand, gravel, and bedrock substrate indicated to biologists that spawning was likely underway.  Spawning was confirmed by drifting a net through the spawning aggregation.  The female, code 30, and several of her male suitors were quickly captured and pulled aboard the tracking boat.  The female had ovulated and was releasing eggs as she was taken from the net.  The males captured in the same net were releasing milt.  The spawning aggregation remained present at the site through noon on June 20.  Seventeen years after first meeting code 30, biologists were finally able to document precisely where and when pallid sturgeon spawn in the Yellowstone River.

Biologists were able to document the spawning of a female pallid sturgeon, radio code 30, as she released her eggs (shown above) during a recapture event targeting a spawning aggregation in the Yellowstone River.

Biologists returned to the site after several days to sample for embryos, free embryos, and larvae to document successful fertilization, hatch, and drift using methods similar to those used in the Lower Missouri River (see previous post “Searching for a needle in a haystack”).  Samples will be sent for genetic testing to determine whether the embryos and larvae captured at the site are pallid sturgeon or the more common shovelnose sturgeon that also spawn in the lower Yellowstone River.  Genetic testing may also determine which and how many males in the spawning aggregation contributed to fertilization.

By Pat Braaten and Aaron DeLonay

 

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