White sturgeon, Acipenser transmontanus (Richardson), are at risk of entrainment from dredging, with young-of-the-year fish at greatest risk. To evaluate this entrainment risk, swimming performance trials were conducted in a laboratory swim tunnel with hatchery-reared juvenile white sturgeon with varying experience levels including: naı¨ve (only tested once), tested (retested after being kept in no flow) and trained (re-tested after kept in flow for nearly three weeks). Individuals of various sizes (80-100 mm TL) and all experience levels were strongly rheotactic (> 80%), but endurance was highly variable among fish. Small juveniles [< 82 mm total length (TL)] had lower escape speeds (< 40 cm s )1 ) than medium (82-92 mm TL) and large (> 93 mm TL) naı¨ve fish (42-45 cm s )1 ), all of which had lower escape speeds than trained fish (72 cm s )1 ). Behavior was also highly variable among fish. Overall, benthic station-holding behaviors were least frequent in small fish, intermediate in medium and large fish, and most frequent in trained large fish. Probability of entrainment of juvenile white sturgeon can be reduced by maintaining dredge head flow fields at less than 45 cm s )1 for wild-spawned fish or by rearing hatchery fish to > 93 mm TL and exposing the fish to moderate flow velocities (10-12 cm s )1 ) prior to their release.

Thirty-six sturgeon (Scaphirhynchus spp.) were captured from the lower Mississippi River using baited trot-lines, anesthetized using tricaine methanesulfonate, and subjected to endoscopic evaluation of their coelom (coelioscopy) on the river bank. Using a 2.7 mm rod-lens telescope and gravity-fed sterile saline infusion, gender and reproductive stage were determined in the field, and later re-evaluted and confirmed by histology of endoscopic biopsies in the laboratory. Gender and reproductive stage were determined endoscopically in all fish without complication, and proved to correlate 100% with biopsy histology. Modern rod-lens telescopes and sterile saline insufflation can be used under field conditions, provide superior visualization of sturgeon gonads, and offer a high degree of accuracy even with inactive gonads. Rigid endoscopy is recommended as a minimally-invasive means of assessing the reproductive organs of sturgeon and other fish.

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.

Sturgeon are threatened by dredging, but there is no established protocol for determining risk of entrainment to different populations of wide-ranging species. We demonstrate that laboratory evaluations of swimming performance for individual populations are an effective way to describe susceptibility of entrainment. Using a Blazka-type swim tunnel, we quantified positive rheotaxis (head-first orientation into flowing water), endurance (time to fatigue), and behaviour (method of movement) of juvenile sturgeon in water velocities ranging from 10 to 90 cm s )1 . Sturgeon representing four different populations of the United States were tested: two populations of lake sturgeon (Acipenser fulvescens) and two populations of pallid sturgeon (Scaphirhynchus albus). Lake sturgeon from Lake Winnebago were weaker swimmers than those from the Wisconsin River, and pallid sturgeon from the Yellowstone River were weaker swimmers than those from the Atchafalaya River. Rheotaxis, endurance, and behavioural data were used to calculate an index of entrainment risk, ranging from 0 (unlikely) to 1.00 (inevitable), which was applied to hydraulic models of dredge flow fields. Risk of entrainment varied among populations but for all groups tested, substantial entrainment risk occurred only within a 1.25 m radius of the draghead and this risk could be significantly reduced or eliminated by reducing the diameter of the dredge pipe.