Extirpation of red shiner in direct tributaries of lake texoma (Oklahoma-Texas): a cautionary case history from a fragmented river-reservoir system

The effects of reservoirs on composition of fish assemblages are well documented in and downstream of reservoirs but are less well known upstream, especially in small tributaries. Here we report that a historically very abundant native minnow species (red shiner Cyprinella lutrensis) has been extirpated from several direct tributaries of Lake Texoma, a large impoundment of the Red and Washita rivers, Oklahoma-Texas. Using historical and recent data, we document the recent loss of or sharp declines in the red shiner from six of seven creeks that are direct tributaries of Lake Texoma. The red shiner is widespread, tolerant of harsh conditions, and highly invasive, so its demise is particularly noteworthy. The species remains common in direct tributaries of free-flowing reaches of the rivers that we sampled recently. Centrarchids that are potential predators of or competitors with the red shiner are now more abundant in Texoma-direct creeks than in tributaries to the rivers. Loss of the red shiner occurred within the past 20 years in Texoma-direct creeks, although the reservoir is more than 60 years old. We suspect that recent severe droughts initiated loss of the species and that recolonization has been inhibited by the reservoir, which fragments the former river-creek system. The lower reaches of the Texoma-direct creeks appear to have undergone habitat modification during recent high-water episodes, resulting in the formation of deep pools that contain large numbers of centrarchids or other piscivores and are poor habitat for the red shiner. Reservoir effects on the red shiner in these tributary streams did not appear until decades after impoundment, after particular combinations of extreme drought and flood events. The decline of the red shiner in the Lake Texoma system suggests that biologists should be alert for similar changes in native fish populations in direct tributaries to other reservoirs.