California sea lion presence and predation activity in the waters below the lower Columbia River’s Bonneville Dam is at its lowest ebb of any year since 2002 when U.S. Army Corps of Engineers researchers began monitoring the big marine mammals to evaluate their impact on migrating salmon.
Information compiled this year through Wednesday by observers atop the dam indicates that 32 different CSLs (two more were identified toward week’s end) have been identified as visiting the dam this year. And while the season is ongoing, that total would be the lowest since 30 were identified in the first year of the research. Next low was 54 last year and in 2009. The peak count was 104 individual California sea lions in 2003.
After showing a relatively sparse presence at the dam historically, more of the animals began turning up since the turn of the century. They have to large extent preyed on spring chinook salmon searching for fish ladders that facilitate the spawners’ upstream passage.
The salmon and steelhead runs in springtime include five different wild salmon and steelhead stocks that are listed under the Endangered Species Act. The male sea lion contingent typically begins arriving at the dam, which is 146 river miles inland, in late winter and exits by the end of May for a mission south to their breeding grounds in northern Mexico and off the coast of Southern California.
“The average CSL numbers present per day this year is lower than last year, which is the lowest for CSL since 2002,” according to a weekly research status report released today. That average is about 2 ½ California sea lions, compared to peak daily average of more than 15 in 2008.
Since daytime, weekday observations began Jan. 6, a total of 264 chinook and 58 steelhead have been seen taken by California sea lions.
“Predation on salmonids by CSL in particular, continues to be far lower than any previous year monitored,” the weekly report says. “On the other hand, SSL predation on salmonids is still high, but lower than the past two years and SSL predation on salmonids continues to be higher than that for CSLs, which is not surprising when looking at the abundance figures for each species.”
This year’s low CSL numbers, according to the weekly report compiled by Robert Stansell, Bjorn van der Leeuw, and Karrie Gibbons,“ is undoubtedly influenced by the removal of several CSL which would otherwise be adding to the daily abundance estimates.”
A California sea lion removal effort being conducted by the states has resulted in 10 of the animals being taken from the scene year so far this year. Nine were lethally removed and one shipped to a Midwest aquarium. The state project aims to reduce predation on listed salmon. A total of 38 California sea lions were removed in 2008-2010 with 10 going to zoos aquariums and the rest being euthanized.
The 10 removed this year were on a list of 23 (out of the total of 32 seen at the dam this year) that are known to have visited the dam in previous years and had been observed taking salmon.
Observers have documented 68 different Steller sea lions at the dam this year which is similar to last year’s total. They have been seen gulping 465 chinook and 148 steelhead, as well as 1,341 white sturgeon. Until this year the California sea lions have consumed (more than 70 percent) the largest percentage of salmon and steelhead observed taken below the dam.
This year’s, and past year’s, observed Steller salmon take may be the tip of the iceberg.
“There’s a higher probability that SSL are eating some prey underwater, and thus undetected by our observers, than with CSL,” the weekly report said of the Stellers, which are generally larger animals than the California sea lions. “Besides observing many steelhead and smaller chinook swallowed whole by SSL, we are also observing SSL just breaking the surface with larger chinook prey that are already missing the head or tail section, and SSL are usually finished with a large chinook in less than a minute,” the weekly report says.
For more information on Stellar sea lions see CBB, April 20, 2012, “ NOAA Fisheries Proposes Delisting Eastern Stellar Sea Lions; Growing Numbers In Columbia River” http://www.cbbulletin.com/419833.aspx