USING GROWTH MORTALITY TRADE-OFFS TO EXPLORE A CRAYFISH SPECIES REPLACEMENT IN STREAM RIFFLES AND POOLS

A crayfish species replacement is underway in Ohio streams. We explore how growth, mortality, habitat use, and size influence the replacemen t of Orconectes sanborni by O. rusticus. In field cages, we assessed g rowth of crayfish. Both species grew faster when confined to riffles a nd O. rusticus grew faster than O. sanborni in this fast-water habitat . In another experiment, we tethered crayfish in riffles, shallow pool s (<40 cm), and deep pools (>40 cm) to assess mortality. After 8 d, mo re 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 withou t fish predators. Crayfish did not prefer riffles. Apparently, juvenil e crayfish do not assess specific benefits and costs using only food a nd fish predators. Other costs associated with riffles (e.g. passive d rift, terrestrial predators, summer desiccation, physiological cost of position maintenance, etc.) must reduce riffle use. In addition, O. r usticus grows larger than O. sanborni. Because size confers advantages in biotic interactions, this critical finding may be the key to the r eplacement.