Scavenger or predator? Examining a potential predator-prey relationship between crayfish and benthic fish in stream food webs

Commonly referred to as keystone species, crayfish are ecologically important for their ability to transfer energy efficiently to higher trophic levels in aquatic food webs. They also are ecosystem engineers, modifying substrate and influencing distribution of other benthic invertebrates. However, crayfish have yet to be examined as potential predators of benthic fish in lotic systems. We examined a possible relationship between crayfish and benthic fish populations by quantifying natural densities of both groups and by using enclosure/exclosure and laboratory experiments to test for crayfish predation on benthic fish. Data obtained by sampling Illinois streams showed that increased crayfish density was correlated with decreased fish density, and this relationship was not influenced by habitat variables. Data from in-stream enclosure/exclosure experiments showed predation by crayfish (Orconectes sp.) on darters (Etheostoma sp.) in all treatments. The highest overall fish mortality occurred in the low-density crayfish treatment. We ran controlled tank experiments to seek formal evidence that crayfish are capable of killing and consuming benthic fish. Our study provides evidence that crayfish predation on benthic fish may have an influence on benthic fish populations, and this interaction could be of particular importance in streams with the invasive crayfish Orconectes rusticus.