Down to the Bottom, and Back Again…and Again.
July 20, 2019
When the sturgeon larvae went into the water on July 1st, the drift experiment was underway. There was no undo button, do-overs, or alternate endings. Each step in the experiment needed to be executed with precision. The results were dependent upon the ability of sampling crews comprised of university, and state and federal agency biologists to track and then sample those groups of free embryo pallid sturgeon as they moved downstream through 150 miles of river. Drift models that had been in development for years were dependent upon it – and the crews were up to the task.
Through cold rainy nights and hot sunny days, crews spent 8 days sampling the Upper Missouri River from the release point near Wolf Point, Montana, to the headwaters of Lake Sakakawea near Williston, North Dakota. Working in 12-hour shifts, multiple alternating crews provided round-the-clock sampling to map the movement of the sturgeon larvae as they traversed the stretches of river. Using water velocities and hydraulic models developed from previous drift studies (See previous blog post, 700,000 Baby Fish), Comprehensive Sturgeon Research Project (CSRP) scientists provided projections of drift times to guide the deployment of larval sampling boats to track the mass of sturgeon as they moved downriver. Those models were crucial to ensure that sampling crews were in place and sampling before the sturgeon free embryos passed each of the six designated sampling sites along the 150-mile trek. Boats were in their designated sampling locations before the first larvae were projected to arrive and left only after sturgeon free embryos were no longer detected. Sampling before arrival allows scientist to estimate any background natural reproduction of shovelnose sturgeon or paddlefish. Sampling after the mass of sturgeon passed allows scientists to model how the mass of free embryos spreads out through time (dispersion).
Boats sampled for the drifting free embryos using fine-mesh ichthyoplankton nets suspended just above the river bottom where most pallid sturgeon are believed to drift. Biologists also sampled at the surface and in the middle of the water column each hour to validate that assumption. The objective was to collect as many samples as possible during the 12-hour shift. All boats were equipped with a pair of nets; one suspended off either side. Biologists would lower their nets with lead weights using a winch near the bottom, allow them to sample for a designated period, then retrieve the nets and sift through the nets’ contents using forceps. Free-embryo sturgeon are only about 6 to 20 millimeters. Biologists needed to maintain focus during the long hours to give the painstaking attention needed to every leaf and twig to ensure each specimen was counted and preserved.
The preserved free embryos will be sent back to CERC to be measured and their developmental stages determined. Then they will be prepared for genetic analyses to verify species and to confirm that the parentage of each specimen can be traced back to the adult broodstock parents spawned for this experiment in the hatchery. Scientists expect the data from this experiment to be invaluable to efforts to continue advancing their knowledge and understanding of larval drift and refining critical multi-dimensional hydraulic models to test hypotheses and guide management actions (see previous blog post, Fine-scale Mapping of the Complex Upper Missouri River - Phase Two).