Habitat overlap and resource partitioning between a lotic fish assemblage and crayfish (Orconectes obscurus)

Background/Question/Methods

In streams, competitive interactions between crayfish and fish for food resources have received little attention. Since many stream fishes are relatively small, they are unable to consume crayfish due to body size and gape limitations. However, they often occupy similar habitats and consume similar resources as crayfish, leading to the potential for fish-crayfish competition. In this study, we assessed habitat overlap among three size classes of crayfish (Orconectes obscurus) [small: carapace length (CL) ≤ 19.9 mm, medium: CL = 20.0-29.9 mm, and large: CL ≥ 30.0 mm] and multiple species of young-of-year (YOY) and adult fishes using field surveys conducted during five seasons (Summer 2005, Fall 2005, Spring 2006, Summer 2006, Fall 2006). To gain insight into resource partitioning, stable isotope analysis (natural abundances of δ15N and δ13C) was used to distinguish trophic levels (δ15N) and resource partitioning/overlap (δ13C) among groups.

Results/Conclusions

Two-way cluster analysis revealed two distinct groups that possessed substantive within-group overlap in habitat use. Group 1 (shallow-water taxa) included all three size classes of crayfish and most small-bodied fishes. Group 2 (deep-water taxa) included large crayfish, large-bodied fishes, YOY Etheostoma nigrum, and YOY Catostomus commersoni. δ15N signatures revealed, that all size classes of crayfish belonged to the same trophic level. Furthermore, crayfish displayed ontogenetic diet switching with larger individuals foraging on more enriched carbon sources than small crayfish. Medium and large crayfish, however, did not partition resources and may compete for food resources in the field. All fishes, with the exception of Semotilus atromaculatus (which fed at the same trophic level as crayfish), were at a higher trophic level than all sizes of crayfish. Large crayfish overlapped in habitat use and partitioned resources with adult S. atromaculatus. Small, medium, and large crayfish overlapped in both habitat and resource use with YOY S. atromaculatus and may be competing for food resources in the field. Overall, competition for food resources appears to be a weak interaction among crayfish and stream fishes and does not appear to be a major factor affecting habitat use of crayfish and fish in this system.