American shad are not native to the Columbia River basin, but they have largely flourished since being introduced to the West Coast from Pennsylvania in the late 19th century.
The East coast natives have become a relatively popular sport fishery in the Columbia and have been the focus of varying levels of commercial effort over the years. The American shad is the largest member of the herring family. Their average adult size in the Columbia is from 17 to 19 inches and three to four pounds.
Since the extensive development of mainstem hydroelectric projects, shad runs have increased markedly in abundance and have extended their range into the upper Columbia River and into Hells Canyon of the Snake River, according to a joint ODFW-WDFW staff report. Since the late 1970s, all shad runs have exceeded one million fish per year, with a peak of 6.5 million in 2005. Shad run timing extends from mid-May through early August at Bonneville Dam, with peak daily counts occurring in June.
The number of shad has, however, declined each year since that 2005 peak. The 2010 minimum shad run size was 1.3 million, with a minimum spawning escapement exceeding 1.2 million upstream of The Dalles Dam, plus an unknown number of spawners downstream of The Dalles Dam and downstream of Willamette Falls.
The 2010 shad run in the Columbia River was the lowest since 1982.
It is possible that the rapid growth shad population resulted in the “amplification” of the presence of Ichthyophonus, a parasite of wild marine fishes, in the Columbia basin. Researchers believe the Ichthyophonus is endemic to the Pacific Ocean, where shad spend three to four years of their life before returning to freshwater to spawn.
“An Ichthyophonus epizootic occurred among American shad in the Columbia River during 2007, when infection prevalence was 72 percent, and 57 percent of the infections were scored as moderate or heavy intensities,” according to “Amplification and transport of an endemic fish disease by an introduced species, “a research paper published in the April 17 edition of “Biological invasions. Lead author is Paul Hershberger of the U.S. Geologic Survey’s Western Fisheries Research Center. An epizootic is a disease that appears as new cases in a given animal population, during a given period, at a rate that substantially exceeds what is "expected."
“Therefore it is possible that mortality among infected adult shad occurred after completion of spawning and prior to re-entry into the seawater,” the research report says. “However, it should be cautioned that demonstration of any causal relationships between the ichthyophoniasis epizootic and population-level impacts to American shad require further investigation….”
“There appears to be a coastwide decline in American shad,” said USGS researcher Mike Parsley. Shad, originally introduced in the Sacramento River, are now found from southern California to Kodiak Island, Alaska
“We could see shad rebound,” Parsley said, if the disease relents.