IDFG points to possible factors in Hells Canyon white sturgeon population decline

White sturgeon numbers have been declining in the Snake River reach between Hells Canyon and Lower Granite dams for decades. Research by Idaho Department of Fish and Game (IDFG), Idaho Power, the Nez Perce Tribe, the Army Corps of Engineers, and the University of Idaho points to several factors, including slow growth, entrainment, and predation of juvenile fish by invasive species. In this case, entrainment means sturgeon that pass over the dam and then become trapped in the reservoir below, unable to move back upriver. At the October 15 Council meeting, Joe DuPont, Clearwater Regional Fisheries Manager with Idaho Fish and Game, presented updates on this research, including new studies on the potential benefits of trapping and transporting fish up- and downstream (watch video | see slides).

Studies on the Hells Canyon population date back to the 1970s. In early studies, 91% of the fish sampled were juveniles because larger fish were being harvested. In 1970, Idaho instituted a catch-and-release program, resulting in more sub-adults and adults surviving. However, researchers noted a serious downward decline in juveniles being sampled over the next few decades.

In 2014, a concerted effort began to determine the causes of decline. DuPont noted that since 2014, no age 0-1 sturgeon had been seen in the free-flowing reach of the river. And while the reservoir showed slightly improved survival rates, only one year in the last decade, 2017, showed significant recruitment (fish who survive to year 1).

Several factors may explain the decline

DuPont stepped through several possible contributing factors. First, he discussed flow. In the past decade, years with some level of juvenile recruitment overlapped with years that had average or above flows in the Snake River. However, when historical data was compared, it showed significant recruitment even in years with lower-than-average flows. DuPont also said that the earlier change in harvest regulations likely masked some of the effects on juveniles, leading managers to assume the population was more stable than it really was. In the last 10 years, the population of the entire reach has declined by 33%, with the largest decline (51%) seen in the reservoir.

Another factor DuPont discussed was growth rates in this population being among the slowest in the Columbia River Basin. There's a pronounced difference between reservoir and river fish, with reservoir fish growing at almost 20 times the rate of sturgeon in the river. DuPont attributed the slow growth rates to a lack of prey. Since the Hells Canyon Dam complex went in, there is less sediment transport and fewer areas with fine sediment where the insects live that juvenile sturgeon like to eat. There are also fewer lamprey and mussels, also important food sources, and fewer salmon returning, reducing overall nutrients.

Next, DuPont discussed the issue of entrainment, or fish becoming trapped below Lower Granite Dam. Data from PIT tags, acoustic tags, and fish separators showed that between 50-80% of the 2017 year class were entrained at Lower Granite Dam. Entrainment, together with the very slow growth and survival rates for juvenile sturgeon, is contributing to the long-term declining trend in juvenile abundance.

Finally, DuPont pointed to an increase in certain invasive species as cause for the more recent declines. Opossum Shrimp, an invasive crustacean named for its brood pouch, first showed up in Lower Granite in 1994. Less than an inch long, they now make up over 90% of the bottom-dwelling biomass in the reservoir.

Opossum shrimp are a food source for another invasive species, the Siberian Prawn. They were first detected at Lower Granite Dam in 1998, are about 3.5 inches long, and prefer the slower, deeper water that juvenile sturgeon also like to use. The population has skyrocketed to number in the millions in Lower Granite Pool. While there isn't direct evidence of the Siberian prawns consuming juvenile sturgeon, the larval sturgeon are approximately the same size as known prey. Another invasive species, the Sand Roller, first seen at Lower Granite in 2003, are also known to consume opossum shrimp and juvenile fish and are likely to be consuming larval sturgeon as well.

Researchers are investigating translocation (trapping and transporting fish) as a way to address the decline. One study is evaluating the effects of moving sturgeon from the free-flowing river to the reservoir. Results have shown that the relocated fish are growing 20-40 times faster in the reservoir than in the free-flowing river. However, some of the relocated fish continue migrating downstream, passing over Lower Granite Dam and getting stuck there. Another new study will analyze capturing fish downstream of Lower Granite Dam in the Little Goose Pool and moving them to two different locations upstream. A hatchery program may also be recommended for a sustainable population if recruitment does not improve.