The functional response of drift-feeding Arctic grayling: the effects of prey density, water velocity, and location efficiency

An important aspect of a predator–prey system is the functional response of the predator to changing prey densities. We studied the feeding rate response of drift-feeding Arctic grayling (Thymallus arcticus) on a small invertebrate prey, Daphnia middendorffiana, at densities ranging from 0.01 L–1 to 1.8 L–1 and current velocities of 25, 32, and 40 cm·s–1. We videotaped the feeding of grayling to determine the duration of the search and pursuit components of the feeding cycle and the location efficiency of grayling feeding at different current velocities. Feeding rate increased approximately as the prey density to the 0.4 power from 0.01 to 1.25 prey·L–1, above which the feeding rate dropped. Current velocity had no significant effect on feeding rate. Search and pursuit times dropped with increasing prey density, but neither was affected by current velocity. However, current velocity reduced both maximum location distance and location efficiency. The lack of increase in feeding rate with increasing current velocity may be due to a trade-off between the increasing likelihood of encounter and decreasing location efficiency as current velocity increases. These data suggest that grayling could effectively feed in a variety of stream habitats with different current velocity.