Habitat use and population characteristics of potentially spawning shovelnose sturgeon Scaphirhynchus platorynchus (Rafinesque, 1820), blue sucker (Cycleptus elongatus (Lesueur, 1817), and associated species in the lower Wisconsin River, USA

Summary The goal of this study was to compare the possible locations, timing, and characteristics of potentially spawning shovelnose sturgeon (Scaphirhynchus platorynchus), blue sucker (Cycleptus elongatus), and associated species during the spring of 2007–2015 in the 149-km-long lower Wisconsin River, Wisconsin, USA, a large, shallow, sand-dominated Mississippi River tributary. A 5-km index station of two pairs of rocky shoals surrounded by sandy areas was electrofished for shovelnose sturgeon and blue sucker in a standardized fashion a total of 40 times from late March through mid-June, the presumed spawning period. On one date in 2008 and two dates in 2012, all rocky shoals and adjacent sandy areas in the lowermost 149 km of the river were also electrofished for both species. Shovelnose sturgeon and blue sucker appeared to spawn in the limited rocky areas of the river along with at least four other species: mooneye (Hiodon tergisus), quillback (Carpiodes cyprinus), smallmouth buffalo (Ictiobus bubalus), and shorthead redhorse (Moxostoma macrolepidotum), usually at depths of 0.8–2.0 m and surface velocities of 0.4–1.0 m/s. However, apparently spawning shovelnose sturgeon were found only on mid-channel cobble and coarse gravel shoals within a single 7-km segment that included the 5-km index station, whereas apparently spawning blue suckers were encountered on these same shoals but also more widely throughout the river on eroding bluff shorelines of bedrock and boulder and on artificial boulder wing dams and shoreline rip-rap. Both species showed evidence of homing to the same mid-channel shoal complexes across years. Blue sucker tended to concentrate on the shoals earlier in the spring than shovelnose sturgeon, usually from late April through mid-May at water temperatures of 8.0–15.5°C along with quillback and shorthead redhorse. In comparison, shovelnose sturgeon usually concentrated on the shoals from mid-May through early June at 13.5–21.8°C along with mooneye and smallmouth buffalo. Based on recaptures of tagged fish, at least some shovelnose sturgeon and blue sucker returned to the shoals at one-year intervals, although there was evidence that female blue sucker may have been more likely to return at two-year intervals. Most shovelnose sturgeon could not be reliably sexed based on external characteristics. Spawning shovelnose sturgeon ranged from 487 to 788 mm fork length, 500–2400 g weight, and 5–20 years of age, whereas spawning blue sucker ranged from 495 to 822 mm total length, 900–5100 g weight, and 5–34 years of age, although age estimates were uncertain. Females were significantly larger than males for both species although there was overlap. Growth in length was negligible for tagged and recaptured presumably spawning shovelnose sturgeon and low (3.5 mm/y) for blue sucker, suggesting that nearly all growth may have occurred prior to maturity and that fish may have matured at a wide range of sizes.