Abundance, Distribution and Biology of the Siberian Prawn Exopalaemon modestus in the Lower Snake River

Siberian prawns Exopalaemon modestus are native to eastern Asia and were first documented in fish collection facilities at Snake River dams in the late 1990’s.  Since then their population has been rapidly increasing, but little is known about their status, distribution or potential impacts on the existing food web.  We collected over 17,000 prawns in monthly beam trawl sampling on Little Goose and Lower Granite Reservoirs from May 2011 through March 2013.  Prawn densities varied both seasonally and spatially with the greatest densities found in deep habitats within the lower portions of the reservoirs.  Densities were consistently about four times higher in Little Goose than in Lower Granite Reservoir with maximum densities exceeding 5 prawns/m².  Length-frequency distributions identified two size classes and suggested a lifespan of a little over one year.  Prawns attain a maximum size of about 76 mm TL.  Reproduction occurs from late July through September with fecundity averaging 189 eggs/female.  Prawn diet consisted primarily of opossum shrimp Neomysis mercedis.  Currently, prawns probably have a benign effect on the food web, and their use of deep, benthic habitats and relatively large size makes them unlikely prey for juvenile salmonids and other fishes.