Bigheaded Carp Spatial Reproductive Dynamics in Illinois and Wabash River Tributaries

Fisheries managers are devoting considerable effort, time, and funding toward limiting the spread of invasive Bighead Carp Hypophthalmichthys nobilis and Silver Carp H. molitrix (hereafter, “bigheaded carps”). A better understanding of factors that influence spatial patterns of bigheaded carp reproduction can help fisheries managers to prevent their spread. To examine factors that influence spatial patterns of reproduction, we sampled ichthyoplankton by using drift nets and push nets in three tributaries of the Illinois and Wabash rivers during March–September 2016–2018. We compared relative abundances of bigheaded carp eggs and larvae among tributaries by using CPUE. Abiotic data were analyzed from individual study tributaries and were pooled among tributaries with substantial versus minimal evidence of bigheaded carp reproduction. In total, 5,689 larval bigheaded carp were captured during three sampling seasons. Of the six study tributaries, we captured the most bigheaded carp larvae in the Sangamon River, followed by the Little Wabash, Embarras, Spoon, Mackinaw, and Vermilion rivers. Based on previous literature, we investigated differences in watershed area, discharge, water temperature, turbidity, and free-flowing stretches of river in our study tributaries. In general, tributaries with greater relative abundances of bigheaded carp eggs and larvae had larger watersheds, greater discharge, lower Secchi depths (higher turbidity), and longer free-flowing stretches than those with minimal evidence of reproduction. This study provides a thorough investigation of bigheaded carp reproduction in six large-river tributaries and will help fisheries managers to better monitor and mediate the spread of these invaders by facilitating the prioritization of potential monitoring locations.