By Colt Holley
September 3, 2019
The USGS recently collaborated with Montana Fish, Wildlife and Parks, U.S. Army Corps of Engineers, and U.S. Fish and Wildlife Service personnel in a trawling effort to search for settled pallid sturgeon larvae after nearly 1,000,000 free embryos were released 150 miles upstream. This effort was part of the 2019 larvae drift experiment which began July 1st (see previous blog Unprecedented Pallid Sturgeon Larval Drift Experiment) on the Missouri River from river mile 1701 near Wolf Point, MT downstream to the headwaters of Lake Sakakawea at river mile 1550 near Williston, ND. The trawling followed an intensive 8-day nearly around-the-clock sampling effort to capture drifting free embryos that were released upstream (See previous blog Down to the Bottom, and Back Again…and Again).
From July 16 through September 11, a beam trawl was used to sample benthic habitats for settled pallid sturgeon larvae. The setup consists of a pair of tapered nets (outer coarse net, inner fine-meshed net) attached to a steel beam and frame. The beam trawl is lowered over the bow before the boat reverses downstream, dragging the net along the substrate. This process agitates the substrate and is used to target young-of-year benthic fishes. Sampling occurred at several river bends of the Missouri River upstream and downstream from the confluence of the Yellowstone River in western North Dakota and eastern Montana. At each bend, the trawl was deployed in various macrohabitats and at various depths to determine locations where settled pallid sturgeon larvae were present.
More than 700 sturgeon larvae were sampled over the 9-week sampling period. At very small sizes, pallid sturgeon and shovelnose sturgeon are visually indistinguishable. As the larvae grow, some characteristics such as the length and positioning of the barbels start to differ between young pallid and shovelnose sturgeon. Given the similarity between larvae of the two sturgeon species, genetic analysis is conducted on tissue from all collected specimen to differentiate them as pallid or shovelnose sturgeon.
This experiment is part of an ongoing study on the federally endangered pallid sturgeon to determine factors associated with decades of recruitment failure. It is hypothesized that pallid sturgeon free embryos (the first life stage after hatching) disperse in the river currents for several days and require many miles of free-flowing river habitat prior to developing into benthic-oriented larvae.