Comparison of Active and Passive Larval Sampling Gears in Monitoring Reproduction of Invasive Bigheaded Carps in Large‐River Tributaries

Abstract Silver Carp Hypophthalmichthys molitrix and Bighead Carp H. nobilis , collectively known as bigheaded carps (BHC), are invasive fishes in the Mississippi River and surrounding basins. Increasing evidence suggests harmful impacts of BHC on native fisheries (e.g., competition). Monitoring the abundance of BHC is difficult with traditional fisheries gears, and few studies have evaluated early life stage sampling. Identifying the spawning locations of BHC through early life stage sampling has the ability to enhance management efforts by applying focus to areas that have potential to serve as population sources. We evaluated the performance of three gears in sampling larval BHC and native taxa within large‐river tributaries in terms of abundance, community assemblage, and size structure. We sampled ichthyoplankton in tributaries with active (push net) and passive (drift net, light trap) gears from March through September 2016. Relative abundance of BHC was greatest in push nets followed by drift nets and was lowest in light traps. Native cyprinids and catostomids as well as BHC comprised a large portion of the total catch. Environmental and habitat characteristics (stream velocity, dissolved oxygen, and temperature) related to BHC reproduction influenced each gear’s ability to capture larval BHC, although relationships in final models were not significant. Taxonomic size selectivity existed among gears; in particular, larger BHC were collected in push nets relative to the other gears. Push nets were the most effective in sampling BHC, and remaining gears exhibited unique strengths. Although less effective than active push nets, drift nets proved useful for monitoring BHC in tributaries and may be advantageous in shallow systems with adequate flow. Light traps were ineffective at capturing BHC larvae in tributaries but may offer utility in lentic habitats or for native cyprinids. Our comparison serves as a guide for monitoring larval BHC in their invaded range and for detection in new areas, such as tributaries of the Great Lakes.