Compatibility of Alewives and Gizzard Shad as Reservoir Forage Fish

We assessed the compatibility of alewives Alosa pseudoharengus and gizzard shad Dorosoma cepedianum through comparison of their distributional, reproductive, and early life history characteristics in Smith Mountain Lake, Virginia, a hydroelectric impoundment where these clupeids have coexisted for 20 years. Larval gizzard shad (<30 mm total length, TL) were most likely to be affected by alewives through trophic competition and predation. However, gizzard shad larvae were concentrated in uplake, littoral areas when alewives of all ages were downlake and pelagic. Gizzard shad spawning was virtually completed in June but most alewife spawning occurred in July. Gizzard shad larvae grew faster (1.33 mm/d) than alewife larvae (0.84 mm/d) throughout the summer. Alewife larvae 5–30 mm TL exhibited lower mortality (mean, 11.0%/d) than gizzard shad larvae (means: 40.1%/d and 12.9%/d for length-groups of 7.0–12.0 and 12.5–30.0 mm TL, respectively). Earlier spawning and faster growth of gizzard shad provided a high degree of temporal isolation, limiting both interference and exploitative competition with alewife larvae. We concluded that alewives and gizzard shad are compatible in Smith Mountain Lake and will likely be so in other systems of similar morphometry and water quality. However, potential negative impacts on other fish dictate that the alewife should be considered with extreme caution as a forage-species candidate.