Critical swimming speeds of sturgeon are presumably lower in vertically uniform, rectilinear flow than in heterogeneous boundary-layer flow. Movement in rectilinear flow of the water column necessitates frequent high-energy free-swimming, while movement in boundary-layers near the bottom of the river permits a variety of lower energy behaviours, and presumably, negotiation of greater mid-column water velocities. Comparative studies of sturgeon swimming performance in rectilinear and boundary layer flows, however, are lacking. Using a 1200-l laboratory swim tunnel, swimming performance was measured for 12 adult shovelnose sturgeon (Scaphirhynchus platorynchus), 494-705 mm standard length. Tests were conducted at 20-25°C in rectilinear (N = 4) and boundary-layer flows (N = 8). Fifteen minute critical swimming speeds ranged from 89.3 to 112.6 cm s )1 in rectilinear flow, 129.5-172.1 cm s )1 in boundary-layer flow. Higher critical swimming speeds observed in boundary-layer flows was a combination of behavioural adaptations (body appression to flat, horizontal substrate) and availability of a low velocity refugium (water velocities at bottom of tank 21-65 cm s )1 slower than those in mid and upper levels of tank). Results support the idea that sturgeon exploit boundary-layers to effectively move or hold position in fast-flowing rivers. Data may be applied to man-made river structures to facilitate fish passage and to reduce risk of sturgeon entrainment.

Summary Tested was the hypothesis that juvenile Atlantic sturgeon (Acipenser oxyrinchus oxyrinchus) would exhibit no response in short‐term survival or swimming performance when exposed to varying concentrations of suspended sediment simulating dredge plumes in waterways where this species may be impacted by dredging operations. Sediment collected from Savannah Harbor, South Carolina, USA was used to simulate a worst‐case scenario. Juvenile sturgeon were contained for a 3‐day period in flow‐through aquaria, with limited opportunity for movement, in sediment of varying concentrations (100, 250 and 500 mg L−1 total suspended solids [TSS]) mimicking prolonged exposure to suspended sediment plumes near an operating dredge. Of the 90 fish exposed, 86 (96%) survived the test. Of the four fish that died, one was exposed to 250 TSS and three to 500 TSS. Swimming performance results indicated that nearly all fish were positively rheotactic. Critical swim speeds (Ucrits) were moderate, whether measured as absolute values (21–31 cm s−1) or as relative values (1.4–2.1 body lengths s−1), with no significant differences among treatments (F < 0.83, P ≥ 0.4874). Behavior was dominated by contact‐based locomotion and station‐holding. Absence of substantial or significant immediate effects on survival and swimming performance suggest that impacts of sediment plumes in nature, where fish have freedom of movement and the power to escape rapidly, are minimal.

Dredging is considered a major threat/impedance to anadromous fish migrating to spawning habitat. Due to the perceived threat caused by dredging, environmental windows that restrict dredge operations are enforced within many rivers along the east coast. However, it is generally unknown how anadromous fish react to encountering an active dredge during spawning migrations. Atlantic sturgeon (ATS) are an endangered, anadromous species along the Atlantic slope of North America. To determine if and how an active dredge may affect ATS spawning migration, a Vemco Positioning System array was deployed around an active hydraulic-cutterhead dredge that adult ATS must traverse to reach spawning habitat in the James River, VA. Telemetry data showed that all ATS that entered the study area survived. ATS that migrated upstream during dredge operations (N = 103) traversed the dredge area and continued upstream to spawning habitat. Many ATS made multiple trips through the study area during dredge operations. There was no noticeable difference in swim behavior regardless of whether the dredge was absent or working within the study area. We suggest that dredging in the lower James River does not create a barrier for adult ATS migrating to spawning habitat or cause adults to significantly modify swim behavior. This is the first study to utilize fine-scale telemetry data to describe how an organism moves in relation to an active dredge. This methodology could be used to describe dredge-sturgeon interactions on different life stages and in other locations and could be expanded to other aquatic organisms of concern.