ECOLOGY OF AN ESTUARINE MYSID SHRIMP IN THE COLUMBIA RIVER (USA) CRAIG A. HASKELL a and JACK A. STANFORD b * a United States Geological Survey, Columbia River Research Laboratory, 5501A Cook Underwood Road, Cook, WA 98605-9008, USA b Flathead Lake Biological Station, Division of Biological Sciences, The University of Montana, 311 Bio Station Lane, Polson, MT 59860-9659, USA ABSTRACT The estuarine mysid, Neomysis mercedis, has colonized John Day and other run-of-the-river Reservoirs of the Columbia River, over 400 km from the estuary. In John Day Reservoir N. mercedis numbers peaked (2 m À3 ) in August in areas near the dam in association with lower water velocity and softer bottom than at the upstream sampling sites. Neomysis broods were primarily released in late spring and early fall. Gut content analysis showed that Neomysis feeds mostly on cladoceran zooplankton and rotifers in John Day Reservoir. Diel vertical migration was documented, with daytime distribution restricted to the bottom and preferentially to the soft-textured sediments in the deepest areas. Common pelagic fishes in the reservoir, especially juvenile American shad (Alosa sapidissima) and chinook salmon (Oncorhynchus tshawytscha), are daytime zooplankton feeders that cannot prey on Neomysis owing to mysid diel vertical migration. Thus, Neomysis has become an important food web component in John Day Reservoir. We also collected N. mercedis further upstream in Lower Granite Reservoir, where another estuarine crustacean, Corophium salmonis, also is reported, underscoring the need to better understand the role of these estuarine invertebrates in the trophic ecology of the Columbia River. Copyright # 2006 John Wiley & Sons, Ltd. key words: Neomysis mercedis; mysid; John Day Reservoir; zooplankton; food web; water quality; exotic invertebrates; reservoir ecology INTRODUCTION The estuarine mysid shrimp (Crustacea:Mysidacea), Neomysis mercedis (Holmes) is native to the western coast of North America from Alaska to California. N. mercedis occurs naturally in the Columbia River estuary (Haertel and Osterberg, 1967; Daly and Damkaer, 1986; Kirn et al., 1986) and in the lower Columbia below Bonneville Dam, where the river is tidally influenced (Muir and Emmett, 1988). In 1982, N. mercedis was found in the gut contents of northern pikeminnow (Ptychocheilus oregonensis), walleye (Stizostedion vitreum), small mouth bass (Micropterus dolomieu) and channel catfish (Ictalurus punctatus) in John Day Reservoir (Gray et al., 1984). In 1994, many N. mercedis were observed in smolt sampling facilities at John Day Dam, obviously entrained from the reservoir; subsequently, on 16 August, 1994, a research team working for Stanford found N. mercedes in six vertical hauls in John Day Reservoir; numbers ranged from 2.5 to 25.8 m À3 . On 13 March, 2003, Haskell collected N. mercedis in Lower Granite Reservoir, the most upstream impoundment on the Snake River and upstream from John Day and McNary reservoirs. These observations strongly suggest that N. mercedis currently exists in all mainstem impoundments of the lower Columbia and is moving upstream on the Snake. This pattern is coherent with commercial and juvenile salmon transport barge traffic on the river; barges go through the dams to Lewiston, Idaho, on the Snake River, but do not travel above McNary Reservoir on the Columbia River. We infer that mysids are able to move upstream through lock operations or are transported upstream via barge bilge water. RIVER RESEARCH AND APPLICATIONS River Res. Applic. 22: 739–753 (2006) Published online 11 April 2006 in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/rra.927 *Correspondence to: Jack A. Stanford, Flathead Lake Biological Station, The University of Montana, 311 Bio Station Lane, Polson, MT, 59860- 9659, USA. E-mail: jack.stanford@umontana.edu Copyright # 2006 John Wiley & Sons, Ltd. Received 15 October 2002 Revised 19 October 2005 Accepted 3 November 2005