An experimental study on the effects of crayfish on the predator-prey interaction between bass and sculpin.

1. We examined the hypothesis that in a one predator-two prey system, prey that share a common refuge might have indirect interactions mediated by their reciprocal effects on each other's refuge use. Our experiments were done in laboratory pools with plexiglas refuges, using predatory smallmouth bass, Micropterus dolemieui, that can consume crayfish, Orconectes putnami and mottled sculpin, Cottus bairdi. 2. Bass consumed 35% of the sculpin in 36 h. Bass predation rates on sculpin were significantly reduced in the presence of crayfish; i.e., crayfish had an indirect positive effect on sculpin. Predation rates on cray fish were low (1.7% in 36 h) and were not affected by the presence of sculpin. 3. The indirect effect of crayfish on sculpin can be explained in terms of patterns of sculpin refuge use. When bass were present, but crayfish were absent, only 14% of the sculpin were in refuge. Instead of going into refuge, sculpin responded to bass by "freezing" in place, with most sculpin outside of refuge; that is, sculpin responded to bass presence by decreasing overall movement, including entry rate into refuge and exit rate out of refuge. In contrast, in the presence of crayfish, sculpin responded to bass by increasing their use of refuge. The presence of crayfish allowed sculpin to have a 10 fold increase in entry rate into refuge and a 2.5 fold increase in refuge use, relative to when crayfish were absent. We suggest that these increases occurred because crayfish drew the attention of the bass away from sculpin. 4. Crayfish did not alter their refuge use or movement patterns in response to the presence of either bass or sculpin. In retrospect, this is not surprising, given the low bass predation rates on crayfish.