A specially designed net was used to study fish entrainment and injury through towboat propellers in 13 pools of the Upper Mississippi and Illinois rivers. The net was attached to the stern of a 48.8‐m‐long towboat with twin propellers (in Kort propulsion nozzles), and sampling typically took place while the towboat pushed 15 loaded barges upstream at a time. In total, 254 entrainment samples over 894 km of the 13 study pools were collected. The sampling efforts produced 16,005 fish representing 15 families and at least 44 species; fish ranged in total length from 3 to 123 cm, but only 12.5‐cm or longer fish were analyzed because smaller fish could escape through the mesh of the trawl. Clupeidae (68% of total catch) and Sciaenidae (21%) were the dominant families. We detected no effects of towboat operation variables (speed and engine [i.e., propeller] revolutions per minute [RPM]) on entrainment rate (i.e., fish/km), but entrainment rate showed a wedge‐shaped distribution relative to hydraulic and geomorphic characteristics of the channel. Entrainment rate was low (<1 fish/km) in wide sections of the river, deep water, and swift current (or time periods characterized by faster flow); however, entrainment in narrow sections with shallow, slow water was highly variable and occasionally reached high levels (>30 fish/km). Although total entrainment rate was not related to engine RPM, the probability of being struck by a propeller increased with fish length and engine RPM. Limits on engine RPM in narrow, shallow, and sluggish reaches could reduce entrainment impact, particularly for large‐bodied fish.
During early life stages, fish in the Mississippi River system may become stranded by shoreline drawdowns induced by the passage of commercial vessels. We examined the stranding of larval shovelnose sturgeon Scaphirhynchus platorynchus, paddlefish Polyodon spathula, and bigmouth buffalo Ictiobus cyprinellus and of juvenile blue catfish Ictalurus furcatus, largemouth bass Micropterus salmoides, and bluegill Lepomis macrochirus in a laboratory flume. Stranding was measured at three vertical drawdown rates (0.76, 0.46, and 0.21 cm/s) and two bank slopes (1:5 and 1:10). Blue catfish, shovelnose sturgeon, and paddlefish were not tested at both bank slopes. Susceptibility to stranding varied among species and was independent of drawdown rate. At a slope of 1:5, shovelnose sturgeons had the highest stranding percentage (66.7%), followed by paddlefish (38.0%), bluegills (20.0%), bigmouth buffalo (2.2%), and largemouth bass (0.0%). At 1:10, blue catfish had the highest stranding percentage (26.7%), followed by largemouth bass (15.3%), bluegills (5.3%), and bigmouth buffalo (0.0%). The likelihood of stranding was related to the behavioral response of fishes to receding water levels. Species that typically occur in littoral and backwater areas swam with the current or passively drifted, whereas the young of main-channel fishes, such as sturgeons and paddlefish, exhibited positive rheotaxis and were more likely to become stranded.
Summary
The gut contents of 12 museum specimens of the federally (USA) endangered Alabama sturgeon were analyzed. This collective series represents 32% of the known museum specimens. Gut contents were dominated by aquatic insects and fishes. At the taxonomic level of insect orders, Dipterans (174.3/fish) were the most numerically abundant food item consumed, followed by Ephemeroptera (19.0/fish), Trichoptera (4.0/fish), Coleoptera (2.4/fish), and Odonata (2.0/fish). Weight, which better reflects the caloric value of food items consumed, showed a similar consumption pattern. Dipterans (12.5%) made up the largest percentage of the total weight of insect prey, followed by Ephemeroptera (8.9%), Trichoptera (6.0%), Odonata, (4.5%) and Coleoptera (0.9%). Volumetric values were substantially higher than weights for Odonata (13.4%) and Ephemeroptera (14.3%), but were comparable for Diptera (10.6%), Trichoptera (4.7%), and Coleoptera (2.2%). The diet of the Alabama sturgeon is similar to the pallid and shovelnose sturgeon, eating both aquatic invertebrates and fish. In terms of piscivory, the volume of fish in the diet of the Alabama sturgeon (34.5%) is intermediate between that of the shovelnose (0% fish) and the pallid sturgeon (60% fish). Based upon the habitat preference of the prey family assemblages present from the 12 guts examined, probable feeding habitats of this sturgeon include sandy to rocky bottoms, slow to swift water velocities, and the water column. Conservation of fish prey may be important to the survival and recovery of the Alabama sturgeon.
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