It’s more than just gravel and sand!

By Aaron Delonay, Robert Jacobson, and Casey Hickcox

Scientists hypothesize that pallid sturgeon (Scaphirhynchus albus), like other sturgeon species, spawn over hard or coarse substrate such as bedrock, cobble or gravel. The type, size, or composition of coarse substrate that adult pallid sturgeon select when spawning  is currently a focus of research for the Comprehensive Sturgeon Research Project (CSRP). To investigate, researchers refine their knowledge of substrate use by following telemetry-tagged adults to identify spawning locations.  Once spawning locations are identified sediment samples and hydraulic data are collected to characterize the selected habitat. Studies of pallid sturgeon spawning in the Yellowstone River provide research teams the opportunity to characterize spawning substrate in what is believed to be the least altered pallid sturgeon habitat within the species’ range.

USGS hydrologists prepare to deploy a BM-54 bed-material sampler on the Yellowstone River.

Figure 1. Scientists lower a bed-material sampler to the bed of the Yellowstone River to sample sediment.

A sidescan sonar image sample shows highlighted areas of suspected gravel substrate.

Figure 2. Side-scan sonar imagery of the bottom of the Yellowstone River collected at a frequency of 400 kHz and a range of 50 meters. Researchers identify areas of exposed gravel (outlined in red) by looking at the shape, texture and brightness of the deposits on the sandy bottom.

During the last week in June, researchers sampled the substrate of a 500 meter x 150 meter section of the Yellowstone River (between river mile 5.4-5.7) where pallid sturgeon had been documented to spawn days earlier.  Researchers used a heavy torpedo-shaped, bed-material sampler (see Figure 1) and then a 15-cm pipe dredge to systematically sample the river bottom.  Sampling of bed material was supplemented with bathymetric and velocity mapping, and sonar imagery using side-scan sonar (see Figure 2) and DIDSON.   Two other sites (near river mile 5.3 and river mile 6.3-6.5) were also sampled where spawning was documented in 2013. Scientists hope that quantification of spawning conditions in the least-altered reaches of the Yellowstone River will provide a model of functional spawning habitat that can be translated to other, more altered areas of the species’ range.  Researchers from the CSRP working in the Lower Missouri have also documented spawning habitat, but the river there is highly altered.  Scientists in the Lower Missouri River are concerned that the habitats selected by pallid sturgeon there represent only the best available in a highly engineered river, and may not be necessarily suitable for successful reproduction.  Over 130 sediment samples collected from the Yellowstone River will be analyzed using special graduated sieves to document grain-size distributions.  The samples collected will be compared with substrates known to be available in the mainstem Missouri River.

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