A patch approach to understanding the effect of habitat degradation on food web and population structure of endemic fishes of the Colorado River basin

The native fishes of the Colorado River basin have experienced dramatic reductions in range due to habitat degradation, hydrograph alteration, and species introductions.  Three species of special concern, the flannelmouth sucker (Catostomus latipinnis), bluehead sucker (Catostomus discobolus), and roundtail chub (Gila robusta) typically co-occur and are managed as an ecological unit.  The San Rafael River in southeastern Utah is home to populations of each of these species and is a system representative of tributaries of the Green and Colorado Rivers in terms of physical geomorphology and anthropogenic impacts.  We investigated how habitat degradation is affecting both the food web and population structure of these native fishes.  We sampled physical habitat and biological indicators of food web structure among patches designated a priori to represent 3 different levels of habitat degradation.  Overall, CPUE was greatest (mean = 67.38 fishes/hour) in patches designated as having medium habitat degradation.  However, CPUE of native fishes was greatest (mean = 37.75 fishes/hour) in reaches experiencing little habitat degradation.  Invertebrate data and stable isotope analyses indicated that food availability and trophic links are spatially dynamic.  These interactions between physical habitat degradation and biotic invasions are reflected in population structure, characterized by source-sink dynamics.