Feeding ecology of pallid sturgeon in the Missouri River above Fort Peck Reservoir, Montana

Abstract

Age and size at sexual maturity of juvenile Pallid Sturgeon (Scaphirhynchus albus) in the Missouri River above Fort Peck Reservoir is driven by growth; therefore, understanding the mechanisms for the variation in growth is crucial for conserving Pallid Sturgeon. For this research, we addressed the following questions: 1) "Does the level of piscivory differ among year-classes," 2) "Has the level of piscivory changed from 2003 and 2004 to 2023 and 2024," and 3) "Is trophic position related to age, length, and weight?" Pallid Sturgeon were sampled for stable isotopes (i.e., delta13C and delta15N), gut contents, and fecal material (i.e., DNA metabarcoding of prey species) in the Missouri River above Fort Peck Reservoir. Data were analyzed by year- class, size (length and weight), and age to better understand the factors influencing the use of food resources in the Missouri River. For all year-classes pooled, piscivory only accounted for 11.84 mean percent by weight and benthic macroinvertebrates accounted for 88.16 mean percent by weight. Simuliidae was the most common diet item and had the greatest mean percent by weight of all fish sampled. DNA metabarcoding of fecal material detected eleven additional fish species that were not observed in the gastric lavage samples, including Carpiodes carpio, Ictiobus bubalus, Polyodon spathula, Sander canadensis, and Cycleptus elongatus, which have not been recorded in Pallid Sturgeon diets previous to the present study. The 1997 year-class (oldest year-class; 28 years old) had the highest mean trophic position of 4.17, which is indicative of an apex predator. Conversely, the 2021 year-class (youngest; 4 years old) had the lowest mean trophic position of 3.18, which is consistent with an insectivore. Pallid Sturgeon were observed introducing fish into their diet at 439 mm, 308 g, and 3.5 years old and transitioning exclusively to fish at 674 mm, 1,450 g, and 18.4 years old. Trophic position was strongly associated with Pallid Sturgeon length, suggesting that the switch to piscivory increases growth rate. This dietary transition ultimately shapes when individuals reach sexual maturity, which has implications for Pallid Sturgeon conservation.