An ongoing bloom of golden alga has continued to kill fish in Roosevelt Lake, biologists with the Arizona Game and Fish Department confirmed this week.
Fish have been floating ashore in clumps, apparently victims of a toxin produced by the algae, which normally lives at low levels in the reservoir but which underwent a population explosion this summer in the wake of drought conditions in the reservoir that supplies much of the Valley’s drinking water.
Officials said they’re not sure what’s causing the increase in the deadly algae and fear it may spread downstream to other lakes, like Apache, Saguaro and Canyon.
“Golden alga can produce a toxin that affects gill breathing organisms,” said Marc Dahlberg, acting Arizona Game and Fish Fisheries Branch chief. “This toxin is not known to be a health threat to humans.”
This most recent fish kill follows on the heels of a golden alga caused fish kill on approximately 20 miles of the Salt River just upstream from Roosevelt Lake during July.
The current fish die off at Roosevelt has apparently affected the entire lake, with gizzard shad accounting for most of the observed deaths. Sensitive to the effects of the algae, some 30 to 40 large (13- to 15-inch) dead gizzard shad in various stages of decay can be found throughout the lake on a regular basis.
The shad form the basis of the food chain in the massive reservoir, providing the primary food for bass and other predatory fish.
“We suspect that threadfin shad, a fish that is also very sensitive to golden alga toxin, are also being impacted,” Dahlberg said. “However, because threadfin are so much smaller they are probably being rapidly consumed by birds and are not as readily observed as the larger bodied gizzard shad.”
Although golden alga fish kills have occurred in a number of states, scientists are still not sure what environmental conditions actually result in golden alga producing toxins.
Game and Fish biologists said that if environmental conditions do not improve, there is a possibility that the golden alga kills could extend to other fish species on Roosevelt and possibly downstream to other lakes as well.