Habitat Use by Hatchery-Reared Adult Razorback Suckers Released into the Lower Colorado River, California–Arizona

Abstract

Designation of the Imperial Division, Colorado River, California–Arizona, as critical habitat for the endangered razorback sucker Xyrauchen texanus by the U.S. Fish and Wildlife Service in 1994 prompted the need for information on current habitat use by the species. Although information on habitat use by razorback suckers has been widely reported for the upper Colorado River basin, little applicable information existed for the Imperial Division. Between December 1995 and Jan 1998, we released 87 hatchery-reared adult razorback suckers (413–592 mm total length) implanted with ultrasonic transmitters into the lower 9.6 km of the Imperial Division and monitored their use of habitat types. Available aquatic habitat within the study area was designated as main channel, backwater, side channel, or impoundment and was described in terms of surface area. Relative availability of each habitat type (percent of available habitat) was used to compute the percent number of observations expected within each habitat type. We monitored the study fish at 2–3-week intervals and recorded their distribution among the habitat types. Use of habitat and movement by individual fish were variable. The fish used all habitat types but did not use them in proportion to their availability (χ2 = 616.61; df = 3; P < 0.01). Backwaters and the impoundment were selected more frequently than expected, based on their relative availability (Jacobs' D = 0.42 and 0.63, respectively). The main channel was used less frequently than it was available (Jacobs' D = −0.72), and side channels were used approximately in proportion to their relative availability (Jacobs' D = −0.12). Seasonal selection of habitat was not evident as overall use of habitat remained consistent throughout the year. Although side-channel habitats were not selected for by razorback suckers, they are important corridors between the main channel and many backwaters. From a management perspective, the results of this research suggest that habitat types that are selected for should be given greater emphasis in programs focused on conservation, restoration, or enhancement of habitat. Our research indicates that in the Imperial Division, hatchery-reared adult razorback suckers actively select off-channel (i.e., backwater) habitats. Therefore, the loss of those habitats could affect recovery of the species.