Date of Award
Master of Science
Invasive prey species pose a threat to ecosystems and can alter food web and community dynamics. Populations of silver carp (Hypophthalmichthys molitrix), recognized with bighead carp (H. nobilis) as “Asian carp,” are growing rapidly in Illinois and may make up a large fraction of available prey for river otters (Lontra canadensis) in larger waterbodies. Asian carp occupy a considerably lower trophic level than most commonly recognized otter prey. My goals were 1) to assess the influence of consuming silver carp on the trophic position of Illinois otters using stable isotopes of nitrogen (δ15N) and carbon (δ13C), and 2) to assess the frequency of Asian carp in otter diets. I also compared the frequency of occurrence of prey groups (fish, crayfish, and amphibians) between land cover types and seasons. For my first goal, trappers collected tissue samples (n = 30) from harvested otters during November−April 2012−14, and I compared δ15N and δ13C values between otters from waterbodies with and without silver carp. I also measured δ15N and δ13C values of various prey types (silver carp, other fishes, crayfish, and amphibians) collected from otter harvest locations and used 2 common isotope mixing model programs, IsoSource and MixSIAR, to estimate relative contributions of prey types to otter isotopic signatures. Silver carp were primary prey for the Carlyle Lake (CL) otters (n = 6) based on mean MixSIAR (25.7 ± 18.7%) and IsoSource (73 ± 4.1%) contribution results, which constitute 6 of 8 otters harvested from an area containing silver carp. The other 2 otters from the Carlyle Lake Area (CLA) had similar MixSIAR contribution results but considerably lower IsoSource contribution results. MixSIAR provided a more evenly distributed contribution across all sources, whereas IsoSource assigned high contribution estimates to select sources with signatures closest to the consumer signature. However, MixSIAR provides a useful tool to handle additional information and uncertainties, which are naively disregarded with IsoSource. I predicted otters at locations where silver carp were present would have a lower δ15N value, but instead δ15N values were higher for the CL otters than the otters at locations without silver carp present. However, the increased δ15N signatures seem to be a result of elevated δ15N of primary producers and potential otter prey in that system. I used sunfish [i.e., longear sunfish (Lepomis megalotis) and bluegill (Lepomis macrochirus)] as respective indicators of the isotopic baseline. Compared to local sunfish as a baseline, otters at the CLA did not show elevated or reduced δ15N values compared to other sites. For the second goal, I estimated the frequency of occurrence of Asian carp otoliths and pharyngeal teeth in otter scat collected from 43 stream sites in central and southern Illinois during sign surveys in January−April 2013 and 2014. Consistent with previous studies, fish and crayfish were primary prey items for otters, followed by amphibians. Frequency of occurrence of crayfish increased from January−February to March−April, but frequency of occurrence of the other prey types remained similar between those periods. Land cover type did not seem to influence frequency of occurrence of prey types. Asian carp pharyngeal teeth and otoliths occurred in 2.6% of scat samples. However, I collected scat samples at only 6 of my 18 sites confirmed to have Asian carp present.
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