FISH DIET DIFFERENCES BETWEEN TWO DIVERSE RIVER ECOSYSTEMS

Emily M. Eng and Michael D. Delong

Large River Studies Center, Winona State University, Winona, MN 55987.

Stable isotope ratio and gut content analyses can be used together to give 
information on fish diet.  Stable isotopes show what has been assimilated 
into diet and can provide an estimate of trophic position, whereas gut 
contents show what a fish has consumed recently.  The objective of this study 
was to use these two methods to assess trophic status of fish with a possible 
application for examining the relationship between lateral complexity and 
trophic structure in river ecosystems.  We analyzed gut contents and stable 
isotope ratios from samples taken in October 1993 of the Upper Mississippi 
River and lower Missouri River.  These rivers differ significantly in lateral 
complexity.  The lower Missouri River has been altered to become a self-
scouring, single channel to ease navigation.  The Upper Mississippi River 
still retains natural hydrologic conditions despite some modifications.  Only 
the Upper Mississippi seasonally overflows its banks and rejuvenate its 
floodplain.  Percent abundance of each representative food type was compared 
to stable isotope data from previously published work.  The stomachs of many 
fish examined were empty, making it difficult to test the validity of stable 
isotope data.  However, available gut content data corresponded well with 
stable isotope data.  The results of the stable isotope gut comparisons show 
fish in the more laterally complex Upper Mississippi River had greater ?15N 
values while fish in the Missouri River had greater ?13C values.  There was a 
large difference in the trophic position for specific feeding guilds. 
Invertivores, planktivores and omnivores all had substantially higher trophic 
positions in the Upper Mississippi River.  These findings suggest fish living 
in a laterally complex river ecosystem are able to feed at a higher trophic 
level, probably due to the greater availability of resources from the greater 
diversity of habitat patches present in laterally complex riverine 
landscapes.

Keywords:  fish diet, gut content, stable isotope ratio, Upper Mississippi 
River, Missouri River