Washington and Oregon biologists say the sturgeon population in the Bonneville pool of the Columbia River is less than half of what it was estimated to be three years ago.
State and tribal biologists measure the sturgeon populations of the mid-Columbia reservoirs every third year.
In 2003, the estimate was 6,900 legal-size (38 inches to 54 inches fork length) sturgeon between Bonneville and The Dalles dams.
In 2006, the estimate was a similar 6,250.
But in 2009, the estimate ballooned to 29,650, then dropped last year to 14,200.
Tony Nigro of the Oregon Department of Fish and Wildlife said Bonneville pool has good spawning habitat at its upper end near The Dalles Dam.
The fish like a strong current plus turbulence for spawning.
The 2009 research found improved growth rates in the pool and projected a larger number of fish would grow into the legal-size slot by 2012.
But sampling in 2012 showed the fish are growing at their more typical rate, thus fewer reached legal size, he said.
Nigro said the 2009 estimate of 29,600 probably was too high and the 14,200 estimate from 2012 is more realistic.
Sturgeon grow slowly in Bonneville pool.
Tucker Jones, a research biologist for the Oregon Department of Fish and Wildlife, said Bonneville pool sturgeon grow slower than in The Dalles or John Day pools, or downstream of Bonneville Dam.
Jones said it is hard to pinpoint a precise cause for the slower growth.
“We don’t know if the cause for this slower growth is environmental, for example water temperature related, biological, for example, competition for food with other sturgeon, or a combination of factors,’’ he said.
Washington and Oregon started 2013 with a 2,000-sturgeon recreational harvest guideline for Bonneville pool.
The states have reduced the catch guideline to 1,100.
The states also estimate a retention of 335 sturgeon in the first portion of the Bonneville pool season, which ended Feb. 10.
Additional days of sturgeon retention in Bonneville pool are likely in mid- to late June.
In The Dalles pool, the estimated catch through March is 54 sturgeon on a guideline of 300. In John Day pool, the catch is estimated at 132 on a guideline of 500.
Both The Dalles and John Day pools remain open for fishing daily.