Post-stocking condition of endangered pallid sturgeon released at age 0

Abstract

Condition indices are commonly used to assess fish population status and are well suited for endangered species given that length and weight data collection is noninvasive. In the Missouri River, endangered age-0 pallid sturgeon Scaphirhynchus albus are typically released during autumn, but 2 experimental summer stockings of 53-56 or 41-42 day-old individuals occurred during consecutive weeks in July 2018. Captures during subsequent field seasons provided a rare opportunity to assess condition of known-age individuals that spent most of their lives in the wild. By the end of 2022, 115 unique pallid sturgeon were captured at age 1 or older, which is a high capture rate (1.35%) compared to autumn age-0 stockings. Most individuals yielded condition values in the normal category with median values for both stockings near 1. Despite different mean fork lengths at stocking (118 or 80 mm), condition values of captured individuals were not significantly different between stockings. Additionally, the percentage of individuals in low, normal, and robust condition were not significantly different compared to published long-term datasets from the lower and upper Missouri River basins. We conclude that pallid sturgeon captured from these experimental stockings have fared well in the wild, which highlights the potential benefits of stocking relatively young individuals during summer. Similar experimental stockings in the future would provide additional data for evaluating a potential shift in age-0 stocking strategy.