Crayfish are affected by predators directly through consumption and indirectly through behavioral changes, altered activity patterns, or shifts in habitat use. Abiotic variables, such as the availability of refugia (often a function of substrate size), may strongly impact crayfish predation risk. In this study, predation risk for small [carapace length (CL) < 20 mm], medium (CL ranges 20.0-29.9 mm), and large (CL ³ 30 mm) crayfish (Orconectes obscurus) was assessed in deep pool, shallow pool, and riffle habitats during Summer 2005 and 2006. Crayfish of all three size classes were tethered to the streambed for five days at random locations in all three habitats (N = 12) and were checked daily. Abiotic variables including water depth, current velocity, and average grain size were measured at each tethering location. During Summer 2007, a third tethering experiment manipulating substrate assessed the habitat-specific effects of substrate size (sand, gravel, and cobble) on small crayfish predation risk.
Results/Conclusions
There was a significant effect of crayfish size (2005: P = 0.0015, 2006: P = 0.006) and habitat type (2005: P < 0.0001, 2006: P =0.0024) during Summer 2005 and 2006. Small crayfish had highest mortality in deep and shallow pools and had a significantly higher mortality rate than medium and large crayfish. In contrast, predation risk for medium and large crayfish was similar across habitat types. Although abiotic variables are significantly different across habitat types (water depth: P < 0.0001, current velocity: P < 0.001, average grain size: P = 0.0072), these differences did not significantly affect predation risk (P > 0.05) with the exception of current velocity during Summer 2005 (P = 0.0377). During Summer 2007, mortality was lowest in riffles, however, substrate size did not affect habitat-specific mortality rate (P > 0.05). While effects of predation risk on crayfish were size- and habitat-dependent, abiotic variables did not mediate the effects predation risk in any habitat type.
