Crayfish: Scavenger or deadly predator? Examining a potential predator-prey relationship between crayfish and benthic fish in aquatic food webs

Benthic food webs are complex and often poorly understood. Crayfish in particular play a key role in the transfer of energy to higher trophic levels, and can constitute the highest proportion of benthic invertebrate biomass in an aquatic system. Commonly referred to as a keystone species, crayfish are seen as ecologically important for their consumption of detritus and algal material, as well as for their role as a common prey item for over 200 North American animal species. More recently, crayfish have been recognized as obligatory carnivores, but studying crayfish as a potential predator of benthic fish in lotic systems, where they co-occur, has yet to be addressed. Competition exists between crayfish and Percid benthic fish in Illinois streams, for food resources and refuge from larger predators. Previous research, though, has ignored a potential predator-prey interaction between these two groups. I examined a possible relationship between crayfish and benthic fish populations, by both quantifying natural densities of both taxa and using enclosure/exclosure experiments. Data obtained from sampling Illinois streams showed that increased crayfish density correlates with decreased fish density, and this relationship is more pronounced in sites that have established populations of invasive crayfish. Additional data from manipulated in-stream enclosure/exclosure experiments examining the relationship between crayfish density and fish mortality from predation showed an occurrence of predation in all crayfish treatments, with the highest overall fish mortality occurring in the low density crayfish treatment. Finally, controlled tank experiments were performed to provide formal evidence that crayfish are capable of actively killing and consuming benthic fish. The results of this research provide novel insight into the complex and vital role of crayfish in aquatic food webs. This research also has important management and conservation implications, suggesting that streams that contain invasive crayfish might see a decline in native benthic fish biodiversity