Age‐0 Silver Carp Otolith Microchemistry and Microstructure Reveal Multiple Early Life Environments and Protracted Spawning in the Upper Mississippi River

Abstract

Silver Carp Hypophthalmichthys molitrix are highly mobile and fecund planktivorous cyprinids that have invaded much of the Mississippi River and are known to alter food webs and compete with native planktivores. In 2016, for the first time, an abundance of age‐0 Silver Carp (n = 12,208; 16–231 mm) were captured at many (n = 11) sites upstream of Lock and Dam 19 on the upper Mississippi River. Previous reports were of a few individuals at a few locations; however, effort to capture juveniles of this size was likely less in previous years. Determining the origin, frequency, and timing of the reproductive events that led to this large year‐class is important for determining control strategies. We used otolith microstructure and microchemistry from age‐0 Silver Carp to estimate timing and frequency of spawning and early life environments of these fish. Hatch dates were determined from the lapillus otoliths of 190 age‐0 Silver Carp (16–231 mm), and early life environments were identified from otolith microchemistry for 124 of these fish (64–231 mm). Age‐0 Silver Carp were collected from Pools 18 and 19 during July–October 2016 by using a variety of sampling gears. We identified 10 cohorts with hatch dates ranging from May to August 2016 and with main‐stem Mississippi River (75%) and tributary (23%) early life signatures. Tributary otolith chemistry signatures were present in all cohorts between May and July (n = 8) but were absent from the August cohorts (n = 2). Our results indicate that tributaries and small tributary streams, in addition to the main‐stem river, play an important role in Silver Carp recruitment in areas near the reproductive front, where management actions (e.g., contract removal and deterrents) are often targeted.

Light Trapping Reveals Multiple Bigheaded Carp Spawns Upstream of Lock and Dam 19 in the Upper Mississippi River

“…Juvenile Silver Carp (16–238 mm) were captured in Pools 18 and 19 of the upper Mississippi River in 2016 but were not detected until late in the season in Pool 18 (Williams et al. 2021). Light trapping was able to detect larval and juvenile bigheaded carp in many of the same Pool 19 locations but failed to detect bigheaded carp in Pool 18.…”

Using Otolith Chemistry to Determine Early Life Environments and Movement of the Emerging Bigheaded Carp Population in Pools 16–19 of the Upper Mississippi River

“…In addition, few age-0 bigheaded carp have been documented above LD19, an indication of poor recruitment from the larval stages. Only in 2016 did substantial collections of age-0 bigheaded carp occur upstream of LD19 (La Hood et al 2021;Williams et al 2021; both this special section).…”

Examination of Bigheaded Carp Ovaries Indicates Batch Spawning

“…In the UMR and main tributaries, collections of drifting eggs and larval bigheaded carp support a protracted spawning period and multiple spawning events within a year (Larson et al 2017;Camacho et al 2021). During 2016, hatch dates of age-0 Silver Carp indicated 10 distinct cohorts (Williams et al 2021), and larval bigheaded carp captured in backwaters revealed seven distinct spawning events over 4 months in the UMR (La Hood et al 2021). Therefore, bigheaded carp may display flexible spawning strategies (Vilizzi 2011), including protracted or multiple spawning that may increase the likelihood of their successful reproduction in invaded ecosystems.…”

“…Silver Carp in the middle Mississippi River have a bimodal distribution of egg size, indicating that individuals are able to spawn multiple times (Williamson and Garvey 2005). Bigheaded carp are known to have multiple spawning events or an extended spawning period within a year based on egg collections (Wabash River; Coulter et al 2013Coulter et al , 2016Lenaerts et al 2015), the appearance of multiple young-ofthe-year size-classes (Illinois River; Gibson-Reinemer et al 2017), and larval fish collections (upper Mississippi River [UMR]; Larson et al 2017;La Hood et al 2021, this special section;Williams et al 2021, this special section). Bigheaded carp egg, larval, and young-of-the-year collections in the UMR have back-calculated hatch dates between May and August (Larson et al 2017;Camacho et al 2021;Williams et al 2021) and up to seven distinct spawning events (La Hood et al 2021), supporting the potential for batch or protracted spawning.…”

“…Bigheaded carp are known to have multiple spawning events or an extended spawning period within a year based on egg collections (Wabash River; Coulter et al 2013Coulter et al , 2016Lenaerts et al 2015), the appearance of multiple young-ofthe-year size-classes (Illinois River; Gibson-Reinemer et al 2017), and larval fish collections (upper Mississippi River [UMR]; Larson et al 2017;La Hood et al 2021, this special section;Williams et al 2021, this special section). Bigheaded carp egg, larval, and young-of-the-year collections in the UMR have back-calculated hatch dates between May and August (Larson et al 2017;Camacho et al 2021;Williams et al 2021) and up to seven distinct spawning events (La Hood et al 2021), supporting the potential for batch or protracted spawning. In previous years, water temperature and hydrograph fluctuations in the UMR and major tributaries provided adequate spawning cues between late spring (April or May) through early fall (September or October; Camacho et al 2021).…”