PHOENIX — The significant fish die-off last week on the Verde River below Horseshoe Dam was the result of a lack of oxygen, and not because of anything found in the river, according to a water-quality investigation compiled by the Arizona Game and Fish Department.
On Thursday and Friday, the Department collected water-quality samples, analyzing fish for bacteria, viruses and parasites. The fish die-off was in an area approximately 100 yards downstream from Horseshoe Dam, which is operated and managed by Salt River Project for the Bureau of Reclamation.
It is estimated that approximately 20,000 fish were killed. About 60 to 70 percent were goldfish and carp. The remaining fish species were comprised of largemouth bass, flathead catfish, channel catfish, and bluegill. The Department believes that most of the fish killed were from Horseshoe Lake and that only a small fraction may have been from the Verde River below Horseshoe Dam.
On June 1, SRP began releasing approximately 1,000 cubic feet of water per second from Horseshoe Lake. SRP empties Horseshoe Lake each year to comply with the Horseshoe-Bartlett Habitat Conservation Plan (H-BHCP), which was established with the U.S. Fish and Wildlife Service under the authority of the Endangered Species Act (ESA) to protect endangered species from potential impacts of SRP’s operation of Bartlett and Horseshoe dams.
Mitigation activities in the H-BHCP, including the annual rapid draw-down of Horseshoe Lake, were developed to benefit native birds, native fish and reptiles in the upper Verde River that are listed as threatened or endangered under the ESA. Specifically, the draw-down of the reservoir increases riparian nesting habitat availability for the southwestern willow flycatcher and yellow-billed cuckoo, while decreasing the numbers of non-native fish species that may compete with, or prey upon, native fish in the upper Verde River.
By June 28, the reservoir was nearly empty. As the reservoir level decreased, the fish in Horseshoe were concentrated in an increasingly smaller area. Sediment that was disturbed as the water was being drawn down consumed dissolved oxygen, resulting the low oxygen levels and causing the die-off.
As the last of the water was released from Horseshoe, the dead and dying fish were passed through the dam into the Verde River. Some of these fish may eventually drift down to Bartlett Lake.
In the meantime, it is important for anglers to remember they should not be taking these dead or dying fish.