The Life History and Distribution of Neomysis mercedis in Lower Granite Reservoir

Neomysis mercedis, an estuarine opossum shrimp, became established in Lower Granite Reservoir about 15 years ago; however little is known about its ecology, distribution, or role in the Snake River food web.  These investigations are especially important because this system supports many federally listed salmonids.  We collected Neomysis from deep, main-channel (monthly) and shallow, shoreline habitats (biweekly) with a beam trawl to examine spatio-temporal trends in abundance and collect biological data. Densities of Neomysis were highly variable by season, depth, and location throughout the reservoir.  Maximum densities approached 400 mysids/m² at deep, main-channel sites, but sometimes exceeded 1,000 mysids/m² in shallow, shoreline habitats.  Length-frequency data suggest that mysids live for about one year, and reproduction in shallow, shoreline areas occurs in late May and early June.  Diet analysis indicated that mysids are mainly detritivores but also prey upon cladoceran zooplankton.  Neomysis were abundant in shoreline areas during the same time juvenile salmon were rearing there.  Neomysis are a profitable and abundant prey for juvenile salmon and other fishes, but may also be a competitor for zooplankton at times.  Further study is needed to more fully understand the role of Neomysis in the food web of Lower Granite Reservoir.