Habitat use by crayfish in stream pools: influence of predators, depth andbody size

1. Crayfish in headwater streams are subjected to predation from two source s: (a) fish; and (b) terrestrial predators including wading birds and mamma ls. Field observations of the mortality of tethered crayfish of two size cl asses were used to examined how predation risks imposed by the two types of predators varied with water depth and crayfish size. We also examined the depth distribution of large and small crayfish in stream pools with and wit hout predatory fish. 2. Predatory fish, mostly green sunfish, Lepomis cyctuellus, and creek chub , Semotilus atromaculatus, rapidly consumed tethered small crayfish (body l ength = 25-30 mm) in deep areas (0.4-0.7 m), but not in shallow areas (0.05 -0.15 m) of stream pools. Tethered large crayfish (body length = 50-75 mm) suffered little mortality from fish at either depth. Terrestrial predators such as racoons, Procyou lotor, and herons, Butorides and Ardea, caused lit tle crayfish mortality in deep areas, but rapidly consumed both small and l arge crayfish exposed in shallow areas. 3. A comparison of habitat use by crayfish in pools with and without fish s uggests that small, but not large, crayfish shift their distributions to sh allow water in the presence of fish. In pools without fish, the distributio n of small crayfish was independent of depth, whereas large crayfish occupi ed deep water.