Intra-Annual Variability of Silver Carp Populations in the Des Moines River, USA

Abstract

Since their introduction in the 1970s, Silver Carp Hypophthalmichthys molitrix have spread throughout the Mississippi River basin. Management of any species relies on an accurate understanding of population characteristics and dynamics. However, Silver Carp seasonal sampling variation is unknown. Sampling during periods of peak catch rates would facilitate Silver Carp assessment and management, improving monitoring and removal techniques. The objective of this study was to evaluate adult Silver Carp seasonal sampling variation with boat electroshocking and trammel nets. Silver Carp were collected monthly (April–October) during 2014 and 2015 from four locations in the Des Moines River, Iowa. Trammel nets rarely captured Silver Carp (mean ± SE = 4.9 ± 1.6 fish/net; 60% of fish were captured in 6.3% of net sets) and therefore were not included in analyses. Electroshocking catch rates (CPUEs) exhibited a bimodal distribution, with peak CPUEs generally occurring in May, June, and September and lower catch rates observed during July and August. Catch rates were positively related to river discharge at upstream sites but not at downstream sites. Silver Carp size structure was similar among months and sites except at Cliffland, where fish were smaller during August and October compared to earlier in the year. Finally, Silver Carp condition peaked during April and May and decreased throughout the year except at Keokuk, where peaks were observed during both May and August. Although spatiotemporal variability was substantial, these results suggest that sampling of Silver Carp via electroshocking in May–June and September–October generally produces higher catch rates compared to July–August sampling and generates a more representative size structure. Using site-specific knowledge, monitoring and surveillance programs could more effectively sample during these periods of high vulnerability and densities in order to manage the spread and impacts of Silver Carp at statewide and regionwide scales.

Received December 7, 2016; accepted May 11, 2017Published online June 30, 2017

ACKNOWLEDGMENTS

We thank several technicians who were involved with data collection, and we thank Jason Euchner and Kim Bogenschutz (IDNR) for providing helpful insights about Silver Carp. We are especially grateful to Lendie Follet and Geoffrey Thompson for providing statistical consulting and to Alison Coulter and two anonymous reviewers for providing comments on earlier versions of the manuscript. This study was funded through the Fish and Wildlife Trust Fund to the IDNR. Work was performed under the auspices of Protocol Number 7-13-7599-I approved by the Institutional Animal Care and Use Committee at Iowa State University, and animals were collected under Permit SC1037 from the state of Iowa. Use of trade, product, or firm names is descriptive and does not imply endorsement by the U.S. Government.