The number of invasive American shad counted passing over Bonneville Dam's fish ladders this year is the lowest since 2000 and continues a downward trend that started following 2004's record count of nearly 5.4 million fish.

The 1.4 million shad counted at the lower Columbia River dam this year is the lowest total since 2000 (1.2 million). The species, which is native to the East Coast, starts showing up in dam counts in late April and are at peak abundance in early summer. The run peters out by the end of August.

The fish counted at Bonneville Dam and upstream dams spawn in reservoirs. It has been estimated that as many as 20 million may enter the lower Columbia River during April-June with most staying in the 145 miles of river below Bonneville.

U.S. Geological Survey researcher Mike Parsley speculates that perhaps the cool ocean conditions recently that help to boost salmon survival may negatively affect shad.

"The Atlantic tends to be warmer," Parsley said of the American shad's native ocean. "Improving ocean conditions for salmon may not be good for shad."

And/or it's possible that "Ichthyophonus" caught up with them. Ichthyophonus hoferi is a protozoan parasite that has repeatedly caused massive outbreaks of disease among species such as herring. Researchers have found that the parasite infects 72 percent of adult, pre-spawn American shad in the lower Columbia River. The shad are in the same genetic family as herring so potentially a disease could have swept through the population.

"They are similar types of fish," said Parsley who is engaged in research intended to better understand the role of shad in the Columbia River and how their presence may affect salmon and steelhead stocks.

The fish were brought west in 1871 by fish culturist Seth Green at the request of the California Fish Commission. Green loaded four eight-gallon milk cans with 12,000 shad fry from a Hudson River hatchery aboard the transcontinental railroad. Stopping in Illinois, Nebraska and Utah for fresh water and cooling ice, he arrived in Sacramento with 10,000 of the young fish still alive.

Then he dumped the fish in the Sacramento River. The population grew and soon sought out new range. The first recorded sighting in the Columbia was in 1876. The American shad can now be found from Baja California, Mexico, to Alaska and has even been spotted across the Bering Strait in Russia.

Rapid shad population growth since the 1950s coincides with the further hydro development, and creation of reservoirs, in the Columbia/Snake system -- The Dalles in 1960, John Day in 1971 and McNary in 1957. The Dalles reservoir submerged Celilo Falls, which had blocked the shad's upriver passage.

There was a big rise in shad numbers in 1961 and1962 when 265,000 and 436,000 were counted at Bonneville. No count previously had been more than 100,000. The counts have been ongoing since 1938.

The shad counts stayed in that range for the most part but spiked again in 1978 and 1979 to 861,000 and 1 million, respectively, and topped 2 million for the first time in 1990. The new millennium witnessed the rise and then the fall of shad numbers.

American shad have been found on the mainstem as far up as Priest Rapids Dam on the Columbia and Lower Granite on the Snake, though concentrations are by far the greatest in the Bonneville and The Dalles pools on the lower Columbia.

"They've basically expanded to the extent that they can," Parsley said.

The research led by Parsley hopes to document effects of American shad on salmon and then point the way for further research that might be needed to evaluate overall impacts.

"We hope to highlight some positives and some of the negatives," Parsley said. Bioenergetics modeling -- an examination what the fish eat -- shows that earlier migrating subyearling fall chinook can enjoy a rich and abundant source of food in larval shad.

But then again shad grow up, changing into juvenile fish. "At that point they could compete with salmon for food," Parsley said.

Another negative could be that juvenile shad "are providing a forage base for predators on salmon" such as northern pikeminnow and bass, Parsley said.

The research has documented that not all shad migrate to the ocean at 6 to 9 months of age. Some linger for a year or even two. That means that those fish have a year round food source on which to build into "nice, big predators" to await migrating juvenile salmon.

On the other hand, recent research has found that Columbia River shad contained elevated amounts of thiaminase and could be a threat to predators which utilize shad as a primary food source. Thiaminase is an enzyme that degrades the essential vitamin thiamine -- vitamin B1.

The research is also exploring whether consumption of American shad is causing a thiamine deficiency in white sturgeon, which may result in reduced reproductive potential.

The shad that die after spawning do provide a source of marine-derived nutrients for the mainstem, though the benefit is relatively small compared to that of salmon that can swim far up into the headwaters. The shad contribution could, however, prompt higher production of plankton, which would be a positive benefit to juvenile salmon.

Parsley said that salmon managers need to consider the impacts of the sum total of American shad on the Columbia River native inhabitants such as salmon.

Parsley said that he and the former lead on the shad research project, James Petersen, have been "trying to get the region to acknowledge the presence of American shad" and that the fact that they could be affecting efforts to revive salmon stocks in the basin.