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Last modified
7/14/2009 5:02:33 PM
Creation date
5/24/2009 7:27:30 AM
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UCREFRP
UCREFRP Catalog Number
8133
Author
Collins, K. and D. K. Shiozawa.
Title
The Effects of Fish Predation on Backwater Invertebrate Communities of the Green River, Utah\
USFW Year
1996.
Copyright Material
NO
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few corixids caught in control areas were early instars as well. Whether increased densities of <br />corixidae in caged areas is a result of increased survivorship or immigration is unclear. On <br />the other hand, while the zooplankton could also fit through the mesh, they are known to avoid <br />unnatural structures, such as walls (Zaret 1980). This leads us to believe that higher densities <br />of adult copepoda and, in particular, E. s eratus, during the first weeks of the experiment are <br />a result of increased survivorship or of an increased proportion of the copepods entering the <br />water column from the benthos. <br />The fact that adult copepods and E. e ratus were significantly lower in density in the <br />plankton, but not in the benthos, suggests that they are more susceptible to predation while in <br />the water column than in the benthos. This is also supported by the observation that the <br />smaller copepod life stages (nauplii and copepodites) were the most numerous planktonic <br />groups, yet showed no significant differences in density in the controls and the treatments. <br />Size-selective predation could generate this pattern (Brooks and Dodson 1965). Since the <br />water was quite turbid (secchi depth averaged 22 cm on a calm day) the "reactive distance" for <br />a visual predator (such as fish) would be quite short, especially for something as small as a <br />nauplius or copepodite (O'Brien 1979). <br />Indirect effects <br />The benthic densities of the naupliar and copepodite stages of copepods show a <br />significant decrease in full exclosure treatment. This is probably due to increased densities of <br />corixidae and T us. Both Tanypus and the corixid genus Tricorixa are known to feed on <br />benthic organisms (e.g. chironomidae and oligochaeta, see Merritt and Cummins 1984). <br />While the density of E. s eratus (an herbivore) significantly decreased in the open and <br />10
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