By Patrick Braaten
Figure 1. Researchers prepare to release pallid sturgeon code 39 following a post-spawn analysis to confirm a successful release of eggs during the spawning event.
She was initially captured in September 1993, but at the time, the sex and reproductive status of the 1325 mm, 15.9 kg pallid sturgeon were not determined. She was implanted with a passive integrated transponder (PIT) tag for future identification and released. Six years later in April 1999, she was captured again and identified by her unique PIT number. She measured 1356 mm and weighed 17.7 kg though her sex and reproductive state remained undetermined. Her history over the course of the next 15 years is unknown as she eluded collection efforts implemented as part of pallid sturgeon propagation and research efforts in the Upper Missouri and Yellowstone rivers. Her 15-year period of at-large anonymity ended on June 11, 2014, when she was unexpectedly encountered in a trammel net drifted in the lower Yellowstone River. Reproductive assessment indicated that the unknown pallid sturgeon was indeed a female, in reproductive condition, and likely to spawn soon. Measuring 1405 mm (approximately 4 feet and 7 inches) and weighing 17.5 kg (38.6 pounds), the spawn-ready female was implanted with radio transmitter code 39 and released. Female code 39 joined the research population of three other female pallid sturgeon (codes 30, 35, 36) that were already identified as reproductively ready to spawn.
With a cast of four spawn-ready females and numerous telemetered male pallid sturgeon, crews from Montana Fish, Wildlife and Parks (MFWP) and the U.S. Geological Survey Columbia Environmental Research Center (CERC) focused efforts to maintain contact with the spawners and identify spawning locations. Tracking teams were accompanied by habitat crews and crews equipped with a DIDSON acoustic “camera” – in an attempt to visualize spawning behavior (see previous post “Northbound”).
Figure 2. A USGS research vessel uses DIDSON sonar imaging on the Yellowstone River to document pallid sturgeon spawning activities on June 27, 2014.
For the next several days after being implanted with the telemetry transmitter, pallid sturgeon female code 39 exhibited the pre-spawn “roaming” behavior frequently observed in female pallid sturgeon in the Yellowstone River – alternating sequences of upstream and downstream migrations of 15-30 miles. Code 39 skirted around and occasionally through a large aggregation of male pallid sturgeon located between rivermile 5.1 – 5.5 of the Yellowstone River while exhibiting her up- and downstream roaming behavior. A quick recapture of code 39 on June 22 upstream from the male aggregation indicated that she was still carrying her eggs following her most recent passage through the male aggregation. The roaming behavior of code 39 changed on the evening of June 26 as her upstream migration slowed when the male aggregation was encountered at rivermile 5.3 – 5.5. By morning on June 27, code 39 and numerous males were engaged in spawning activity that persisted through late evening. DIDSON imaging was deployed during the prolonged spawning event to document spawning activities and fish interactions at the spawning patch (see aggregation and spawning location videos below). Spawning activities were completed by the morning of June 28 as code 39 was solitary downstream from the spawning patch; her recapture indicated a 20% loss of body weight due to egg deposition. Habitat crews quantified depth, velocity and substrate conditions in the spawning patch (see previous post “Yellowstone River Habitat Update”).
Videos (click to view in new window):
Video 1: ARIS (Adaptive Resolution Imaging Sonar) imagery showing aggregations of pallid sturgeon and other fishes over coarse substrate deposits in the Yellowstone River, North Dakota.
Video 2: ARIS (Adaptive Resolution Imaging Sonar) imagery showing telemetry-tagged pallid sturgeon over spawning substrate in the Yellowstone River, North Dakota. Spawning locations in the Yellowstone River appear to be characterized by sand dunes interspersed with deposits of sorted, coarse gravels.
Code 39 was the last of the four telemetered females to spawn in 2014. Spawning by female codes 30 and 35 in the lower Yellowstone River was verified earlier, in mid-June. Crews from MFWP verified spawning by female code 36 also in mid-June; however, in contrast to the other three telemetered females, spawning by code 36 occurred in the upper Yellowstone River system – most likely in the Powder River. Codes 30, 35 and 36 were the focus of spawning investigations in 2012, and if the 2-year reproductive periodicity is maintained, these three females will again be the focus of spawning events in 2016. Fitted with a long-term transmitter, reproductive progression in code 39 will also be assessed through the next several years to determine her reproductive periodicity. However, unlike female codes 30, 35, and 36, female code 39 will likely be used in the pallid sturgeon propagation program upon reaching spawn-readiness in the next 2-3 years as her genetics have not yet been incorporated into the conservation propagation program.