The Influence of Trophic Guild Composition On the Body Size Distributions and Trophic Structure of Mid-Order River Fish Communities

The size and trophic structure of fluvial fish communities are driven by energy availability, competition, energy loss through trophic transfers, predation, and predator gape limitations.  Size and trophic analyses can provide important information about overall food web structure and function of fish communities.  Fish community assemblages were sampled in three tributaries of the Saginaw Bay watershed, Lake Huron in 2011 and 2012.  We used a combination of community size-spectra (CSS, log abundance x log body-size regression), stable isotope, and trophic guild analyses to test the hypotheses that fluvial fish communities will be (a) strongly size-structured and (b) that variation in trophic guild composition will influence the community size-structure.  Preliminary analyses indicate that the fish communities are strongly size-structured (mean CSS slope = -2.24), and there is a negative correlation between piscivores and Cyprinids (r = -0.64) and piscivores and large bodied low trophic position species, such as Catostomids (r = -0.68), suggesting that piscivores may be exerting strong top-down control on fish community structure.  Trophic guilds were not, however, related to community size-spectra parameters in contrast to our original predictions.