While court battles continue to be waged – sea lions vs. salmon lawyers – the interaction at Bonneville Dam on the lower Columbia River has reached its seasonal ebb apparently.

“Only a couple” of California sea lions were seen last week and this week exploring the waters below the dam in search of spawning chinook salmon. And no Steller sea lions, listed under the Endangered Species Act, were seen during the past two weeks below the dam

“Very little predation occurred the past week,” according to the U.S. Army Corps of Engineers’ Robert Stansell, who heads research at the dam aimed at evaluating the effect of pinniped (sea lions and seals) predation on migrating adult salmon.

Chinook predation last week dropped dramatically from the previous week, according to the “final” weekly research of the season from the Corps issued May 25.

“Our most comprehensive estimate of total salmonid catch by sea lions through May 23 (2,347 expanded by interpolating for weekends and hours missed and adjusted for unidentified prey and night predation factors) is lower than any year after 2002,” the report said.

When the research was launched in 2002, 30 individual California sea lions, and zero Stellers, were observed at the dam and the expanded take of chinook salmon was 808 fish.

Annual springtime California sea lion presence has been up and down since the start of the study, with a peak of 104 in 2003, and lows previously of 54 individuals in 2009 and 2011.

The peak take of salmon was estimated to be 5,757 salmon when 89 California sea lions and 75 individual Steller sea lions were identified as visiting the dam. That 166 pinniped total in 2010 (including two harbor seals) was the most recorded over the course of the study.

Last year California sea lion observed consumption of chinook salmon below Bonneville was more than double that of Stellers. This year the Stellers may have the edge. Observed Steller predation through May 23 includes 498 positively identified catches, and 284 prey events involving unidentified species. The California sea lions were seen catching 508 chinook, as well as 30 unidentified species.

“Predation on salmonids, by CSL in particular, continues to be far lower than any previous year monitored,” according to the May 25 weekly report. “On the other hand, SSL predation on salmonids is higher than for any previous year and SSL predation on salmonids was higher than that for CSL for the first year ever, which is not surprising when looking at the abundance figures for each species,” the report says.

The maximum number of pinnipeds seen any day this year was 38 (on April 25). The maximum number of SSL seen any day so far this year was 29 and 14 for CSL,” according to the report. “Average CSL numbers present per day this year is lower than last year, which is the lowest for CSL since 2002.

“This is undoubtedly influenced by the removal of several CSL which would otherwise be adding to the daily abundance estimates and by up to four CSL in the forebay,” the report says.

Since California sea lion removals began in 2008, more than 50 animals have either been trapped and euthanized or moved to zoos or aquariums.

“We have documented about 70 different SSL’s visiting the dam so far and 40 CSL” during the 2012 season, the report says. “At least 34 of the SSL are confirmed as seen in past years, and 25 of the CSL. SSL outnumbered CSL almost every day this year until the past two weeks. Surprisingly, no harbor seals were observed this year at Bonneville Dam.

“Although the data is still preliminary, it appears the overall predation expanded estimate will be about 1.3 percent of the January 1 through May 31 salmonid run, with CSL contributing 0.6 percent and SSL contributing 0.7 percent. The final adjusted estimate (for unidentified prey and night time predation) will be slightly higher.

No sea lions were trapped by the states during the past two weeks.

Observations made by researchers based on preliminary 2012 data include:

-- There were more Steller sea lions present than California sea lions, both total individuals and average daily abundance.

-- Steller sea lions consumed more salmonids than California sea lions for the first year since we began observing.

-- Steller sea lion predation on both salmonid and sturgeon was higher than for any previous year.

-- The observations showed the lowest number of California sea lions present, both total and average daily, since the first year of observations, as well as the lowest salmonid predation estimates for California sea lions.

-- As for most years observed, about two-thirds of the California sea lions were identified as seen in previous years at the dam, and half of those were removed from the population by the states this year (12).

For more information see CBB, April 20, 2012, ‘NOAA Fisheries Proposes Delisting Eastern Stellar Sea Lions; Growing Numbers In Columbia River’ http://www.cbbulletin.com/419833.aspx