Ecology of the Opossum Shrimp (Neomysis mercedis) in a Lower Snake River Reservoir, Washington

Northwest Science
By: , and 



The opossum shrimp Neomysis mercedis has expanded its range from the lower Columbia River upstream 695 kilometers into Lower Granite Reservoir where it is now very abundant. We studied Neomysis ecology in the reservoir during 2011–2015 to better understand the physical and biological factors that shape their distribution as well as their potential role in the food web. Benthic densities in offshore habitats ranged from 19 to 145 mysids m-2 in shallow (2–12 m) water and from 3 to 48 mysids m-2 in deep (> 12 m) water. Water velocity, depth, substrate, and seasonal interactions were important variables for explaining variation in Neomysis densities in offshore habitats. During spring, daytime densities in shoreline habitats (< 2 m deep) were variable, but nighttime densities generally decreased in summer following reproduction and as temperatures approached 23 °C. Neomysis were mainly collected from the water column during nighttime vertical tows in the downstream end of the reservoir when water velocities were low during summer and autumn. Reproduction occurred mainly in spring and early summer, but a second, smaller reproductive event was observed during autumn. The diet of Neomysis consisted primarily of detritus, rotifers, and copepods, but cladocerans were more prominent during summer and autumn. Physical factors like water velocity may have limited vertical migrations of Neomysis to feed in the water column and influenced use of different habitats in the reservoir. Neomysis are prey for a number of species, including juvenile salmon, but their relations are still largely unknown, and continued monitoring and research is warranted.

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Additional publication details

Publication type Article
Publication Subtype Journal Article
Title Ecology of the Opossum Shrimp (Neomysis mercedis) in a Lower Snake River Reservoir, Washington
Series title Northwest Science
DOI 10.3955/046.091.0205
Volume 91
Issue 2
Year Published 2017
Language English
Publisher Northwest Scientific Association
Contributing office(s) Western Fisheries Research Center
Description 21 p.
First page 124
Last page 139
Country United States
State Washington
Other Geospatial Snake River
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