Using Growth/Mortality Trade-offs to Explore a Crayfish Species Replacement in Stream Riffles and Pools

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Abstract
A crayfish species replacement is underway in Ohio streams. We explore how growth, mortality, habitat use, and size influence the replacement of Orconectes sanborni by O. rusticus. In field cages we assessed growth of crayfish. Both species grew faster when confined to riffles and O. rusticus grew faster than O. sanborni in this fast-water habitat. In another experiment, we tethered crayfish in riffles, shallow pools (< 40 cm), and deep pools (> 40 cm) to assess mortality. After 8 d, more crayfish were eaten in deep pools than in riffles or shallow pools. Given this finding, we predicted that crayfish should preferentially occupy high-growth low-mortality riffles. To test this hypothesis, we quantified crayfish habitat use in sympatric streams, with and without fish predators. Crayfish did not prefer riffles. Apparently, juvenile crayfish do not assess specific benefits and costs using only food and fish predators. Other costs associated with riffles (e.g. passive drift, terrestrial predators, summer desiccation, physiological cost of position maintenance, etc.) must reduce riffle use. In addition, O. rusticus grows larger than O. sanborni. Because size confers advantages in biotic interactions, this critical finding may be the key to the replacement.