Invasive crayfish in a high desert river: Implications of concurrent invaders and climate change

No crayfish species are native to the Colorado River Basin (CRB), including the portion of the state of Colorado west of the Continental Divide. Virile crayfish [Orconectes virilis (Hagen, 1870)], a recent invader in the middle Yampa River in northwestern Colorado, displayed an abrupt increase in abundance in the early 2000s, which coincided with a drought, a severe decline in the abundance of small-bodied and juvenile native fishes, and a dramatic increase in the abundance of nonnative smallmouth bass [Micropterus dolomieu (Lacepede, 1802)]. The annual density of virile crayfish was 6.4/m(2) in 2005 and 9.3/m(2) in 2006. The annual biomass density of virile crayfish was 9.0g/m(2) in 2005 and 15.8 g/m(2) in 2006, representing a riverwide biomass of 122 kg/ha, which equaled that of other macroinvertebrates and fish combined (120.7 kg/ha). Efforts to recover and preserve native fishes in the Upper Colorado River Basin (UCRB), particularly in the Yampa River, have been hampered by nonnative predatory fishes, but the implications of crayfish may have been overlooked and underestimated. Stream conditions during the drought apparently facilitated proliferation by virile crayfish in the middle Yampa River, likely contributing to hyperpredation on native fishes by invasive smallmouth bass. This trophic interaction between virile crayfish and smallmouth bass, in conjunction with regional projections for climate change, will likely make efforts to reduce the abundance and negative ecological impact of smallmouth bass in this ecosystem more difficult and costly. Given the nonnative status of all crayfishes in the CRB, and their invasive capacity and potential to negatively reconfigure native lotic food webs, all states in the UCRB should prohibit the importation, movement, sale, possession and stocking of any live crayfish.