Effects of Selective Removal of Eurasian Watermilfoil on Age-0 Largemouth Bass Piscivory and Growth in Southern Michigan Lakes

Controlling exotic aquatic weeds such as Eurasian watermilfoil ( Myriophyllum spicatum L.) is a high priority for many aquatic plant management programs. Yet, the effects of Eurasian watermilfoil invasion and subsequent removal on sportfish production and food webs are insufficiently understood. Changes in aquatic plant abundance or architecture, caused by invasion and/or removal of exotic plants, may affect age-0 largemouth bass ( Micropterus salmoides Lacepede) growth and recruitment. We hypothesized that age-0 largemouth bass piscivory and growth: 1) decreases along a gradient of increasing Eurasian watermilfoil coverage and 2) benefits from lowdose applications of fluridone herbicide intended to shift Eurasian watermilfoil dominated plant communities to native dominated plant communities. In a multiple lake evaluation of the effects of selective removal of Eurasian watermilfoil on age-0 largemouth bass, percent coverage of Eurasian watermilfoil ranged 8% to 50% in lakes treated with 5 to 7 ppb fluridone and 43% to 91% in reference lakes. Coverage of native plants ranged from 62% to 95%. Selective removal of Eurasian watermilfoil did not have a significant positive effect on age-0 largemouth bass growth. Rather, age0 bluegill ( Lepomis macrochirus Rafinesque) prey availability, which varied considerably among lakes, had a strong positive effect on age-0 largemouth bass piscivory and growth. Factors influencing age-0 bluegill availability to age-0 largemouth bass appear more related to size structure of largemouth bass and bluegill populations than to plant cover.