Survival of Razorback Sucker Stocked into the lower Colorado River
Razorback sucker have been stocked into the lower Colorado River for nearly 30 years, but stocking was accelerated in recent years to meet requirements of a 1997 U.S. Fish and Wildlife Service Biological Opinion of lower river operations. The purpose of this project was to assess survival of stocked fish by performing an intensive, opportunistic survey that targeted razorback sucker in approximately 282 km of river from Parker Dam downstream to Yuma. Multiple samples from the main channel and all watercraft-accessible backwaters and side channels were collected to assess post-stocking survival, distribution, and abundance of the target species.
Sampling efforts yielded more than 2,000 razorback sucker captures out of more than 100,000 fish collected. Razorback sucker was consistently encountered near the stocking site and the stockings appeared to be supporting small populations in this reach. Primary mortality factors were piscine and avian predation on released fish. Observations indicated that piscine predation was ubiquitous in all stocked sites. Estimated predator abundances, though low in certain localities, were sufficient to impact stocked populations. Overwhelming evidence of avian predation on stocked fish indicated it was a significant threat to survival. Fish were captured with wounds from failed avian predation as early as two days post-release. Regardless of release size, estimated annual survivorship was less than 30% for stocked razorback sucker. Evidence of long-term survival was rare, indicating that population augmentation through stocking failed to replace lost wild populations or establish new ones of significant size. Continued stocking to the lower Colorado River was not recommended because mortality threats were substantial and conditions were not conducive to long-term survival of stocked fish. Instead, population augmentation was recommended for habitats depleted or devoid of predators. Present and proposed predator free habitats did not mitigate avian predation, and so appropriate control measures were also advocated. In case further stocking into the lower Colorado River was mandated, additional recommendations were provided for enhancement of survival of stocked fish including pre-release training, natural feed delivery, and temperature acclimation.