Density and Distribution of Amphipods in Oneida Lake, New York, after the Introduction of the Exotic Amphipod Echinogammarus ischnus

The exotic amphipod Echinogammarus ischnus, first reported in North America from western Lake Erie in 1995, was recorded in Oneida Lake, NY in 2001. Some North American studies have suggested that E. ischnus was replacing native amphipods, but other studies found no evidence for this. We sampled amphipods at six depths (3.8 m) along six transects in Oneida Lake to quantify variation in densities of amphipod species as a function of depth, substrate (cobble with Dreissena and with or without macroalgae, sand with or without Dreissena, and macroalgae or submersed vascular plants) and density of Dreissena, and compared the present amphipod density to the historical record. Four species of amphipods, Gammarus fasciatus, Hyalella azteca, E. ischnus, and Crangonyx sp., were collected from Oneida Lake. Gammarus fasciatus was 9 to 90 times more abundant (mean = 0.09 individuals/cm2) than other amphipod species and was collected on all substrates and at all depths, as was H. azteca. Statistical comparisons were made with non-parametric tests between mean ranks of density of amphipods and Dreissena and the other variables. Mean ranks of density of G. fasciatus were correlated with depth (Spearman rank = 0.28, P < 0.0001), but mean ranks of density of H. azteca were not, and neither species was correlated with mean ranks of density of Dreissena. Mean ranks of density of G. fasciatus were greater on sand with or without macroalgae or submersed vascular plants (SVP) or Dreissena than on cobble with macroalgae and Dreissena (H = 28.2, P < 0.0001). Mean ranks of density of H. azteca were greater on sand with SVP, with or without Dreissena, than on sand with Dreissena and without SVP (H = 21.8, P = 0.0013). Echinogammarus ischnus was collected only in water less than 1.8 m depth and always with Dreissena. Mean ranks of density of E. ischnus were correlated with depth (Spearman rank = -0.29, P < 0.0001) and with Dreissena mean ranks of density (Spearman rank = 0.14, P = 0.01). Mean ranks of density of E. ischnus was greater on cobble with Dreissena than on sand with Dreissena regardless of the presence or absence of macroalgae or SVP (H = 35.4, P < 0.0001). Although E. ischnus is established in the near-shore zone of Oneida Lake, we found no evidence that it will replace the native amphipods G. fasciatus and H. azteca.