Interspecific forage fish competition in a resource limited reservoir

In many reservoirs where productivity is low and primary forage for pelagic piscivores has historically been gizzard and threadfin shad, the addition of a third or even forth pelagic prey fish could cause potentially serious, yet unknown, consequences. Over the last ten years Lake Norman, located in the piedmont region of North Carolina, has been subject to accidental and intentional fish introductions. Previous studies suggested that forage availability was the primary driver of striped bass condition, so anglers introduced alewife and blueback herring, intending to augment the forage base. Because the outcome of these introductions is unclear, we assessed the species composition, population structure and competitive interactions among pelagic forage fish using purse seine samples collected four times (spring, summer, fall, and winter) during 2009. Stomach contents were analyzed from threadfin shad, alewife, gizzard shad and blueback herring, and diet overlap was estimated using Morisita’s Index.  Because resource overlap can occur when exploitative competition does not, we then combined resource availability, fish condition, relative abundance and diet overlap information to infer the extent to which competition was plausible. Strong competitive interactions could prove detrimental for one or more pelagic forage fish and have further consequences up the food web.