Unraveling the ecological impacts of alewife in a large Midwest reservoir decades after introduction

Alewives Alosa pseudoharengus are native to the Atlantic Coast but have been introduced to inland waters as forage for sportfish populations. Alewife introductions have benefitted sportfish growth and body condition but have also negatively influenced larval fish survival through predation or competition for zooplankton. In 1986, alewives were stocked into Lake McConaughy, Nebraska, and became abundant and self-sustaining shortly thereafter. It has since been hypothesized that the alewife introduction altered zooplankton size and species composition and potentially negatively influenced Walleye recruitment. However, no studies on alewife distribution or food habits have been conducted on Lake McConaughy to date to investigate the potential influences of alewife on the fish and zooplankton communities. We collected alewives in the nearshore areas of Lake McConaughy during May–July 2022 and 2023 using short-term (about 1 h) small mesh horizontal gill net sets. We spatially assessed alewives by sampling the north and side south sides in addition to four classified quadrants of the reservoir. Total lengths (mm) were recorded, and stomachs were collected from up to 20 alewife per net. Prey were identified and food habits were summarized as percent composition by number. Zooplankton consumption was also paired with vertical Wisconsin zooplankton net tows taken at each site to assess prey selectivity. There was no evidence of piscivory suggesting that alewife predation is not influencing fish recruitment in Lake McConaughy. Alewives almost exclusively consumed zooplankton throughout the study period, suggesting the potential for interspecific competition with young-of-year fish. Our research provides a greater understanding of how introducing a non-native prey species to benefit sport fishes may have unintended negative consequences.