Patterns of Age-0 Gizzard Shad Abundance and Food Habits in a Nebraska Irrigation Reservoir

Gizzard shad (Dorosoma cepedianum) are prolific spawners that can influence reservoir communities. Larval gizzard shad may compete with larval recreational fish for zooplankton resources. Therefore, it is necessary to determine larval gizzard shad dynamics and food habits to better understand their potential for competition with larval recreational fish. Our study examined age-0 gizzard shad abundance in Harlan County Reservoir during late spring/summer from 2002–2010 and food habits and prey electivity of age-0 gizzard shad during late spring/summer 2008 and 2009. The annual peak age-0 gizzard shad density 3 from 2002–2010 ranged from 50 to 380/100 m , which falls within the range of values reported in other studies, but all years were considerably lower than densities reported in other studies that documented deleterious effects on zooplankton populations from gizzard shad grazing. Total length of gizzard shad was positively correlated with the number of zooplankton consumed per fish in 2008 (r224 = 0.33, P < 0.001) and 2009 (r225 = 0.84, P < 0.001) when considering shad < 30 mm total length (TL). Gizzard shad TL was also positively correlated to the size of zooplankton consumed in 2008 (r224 = 0.25, P < 0.001) and 2009 (r225 = 0.64, P < 0.001). Small gizzard shad (30 mm TL reduced their consumption of zooplankton and increased consumption of algae and detritus throughout the summer. The results of this study suggest the relatively low densities of larval gizzard shad coupled with their preference for small copepods may reduce the potential for competition with larval recreational fish in Harlan County Reservoir.