Research Documents Sea Lions' Impact on Salmonby Barry Espenson
Columbia Basin Bulletin - December 6, 2002
Sightings of ocean-roving pinnipeds such as California sea lions have for decades been reported by fishers and others as far upstream as Bonneville Dam -- more than 140 river miles from the Pacific Ocean.
But their presence seems more in evidence below the dam in recent years, intercepting adult salmon as they approach fish ladders that get them past that first hydrosystem hurdle on the Columbia River.
Since many of the Columbia Basin's salmon and steelhead stocks are listed under the Endangered Species Act, the National Marine Fisheries Service in a 2000 biological opinion on hydrosystem operations called for research to find out just how big a bite the marine mammals were taking below Bonneville. The BiOp says that planned Columbia/Snake river federal hydro operations would jeopardize the existence of eight of the 12 listed stocks and prescribed measures that the agency felt necessary to avoid jeopardy.
After an initial year of research, it appears that the sea lions' impact on salmon at Bonneville Dam is slight -- an estimated 0.54 percent of the salmonid run passing the dam during the study period. That's based on some 733 hours of observations from March 21 through May 17 of this year. The predominant run present at that time of year is spring chinook salmon, heading for upriver hatcheries and tributary spawning grounds.
The sea lions appeared to have found their way to the tailraces below the dam's two powerhouses and spillbays to store calories for their journey southward to breeding and birthing grounds -- most notably islands off the southern California and Baja, Mexico coasts.
Most pups are born in June or July and weigh 13-20 lbs., according to information posted on the Marine Mammal Center web page. They nurse for at least five to six months and sometimes over a year. Breeding takes place a few weeks after birth. Males patrol territories and bark almost continuously during the breeding season.
The sea lions enjoying the Columbia River bounty are almost certainly all males that left the Columbia in early summer for those southern climes, according to the Corps of Engineers' Robert Stansell. He reported on the findings of the first year of the study, "Evaluation of Pinniped Predation in the Bonneville Dam Tailrace," at the agency's annual research review. The evaluation will continue next year, and likely beyond, so that the effect of the sea lions on salmon can be monitored.
While the impact this year was limited, the number of sea lions making the trip to Bonneville appears to have grown.
"It's a group of 30 or so that are coming and going," Stansell said. "In the past it was only two or three that we would see."
The first sighting, by a fisherman, near the dam came on Jan. 2 of this year. But the creatures were not consistently present until March 21, according to an abstract of the 2002 research findings. The sightings promptly decreased to an occasional sighting after late May.
"It pretty much followed the peak" of the spring chinook run up the lower Columbia River, Stansell said. The sea lions are described in literature as "opportunistic feeders."
The most sea lions seen in any one tailrace at any one time was five. The highest number observed at any one time at the Bonneville project was eight. The highest number seen at the project on any one day was 11.
One facet of the study was to identify individual sea lions -- through scars or other markings and by tags or brands (five of the animals had been branded over the past three years at Astoria, Ore., and another in the Puget Sound). That is so researchers can catalogue the mammals' comings and goings, both within and between years.
The sea lions do get around. One was spotted at the dam on March 29, was branded in Astoria near at the river mouth on April 5 and was again spotted at Bonneville on May 9. Another was spotted at the dam on May 26, and at San Francisco's Pier 39 13 days later.
The abstract says the researcher were able to identify through observations and videotape or photographs at least 29 individual sea lions at the dam during the two-month study period.
The sea lions were observed catching and eating 452 salmonids during the study period with the bulk -- 67.5 percent, taken at the second powerhouse tailrace. An estimated 26.3 percent of that observed catch was at the first powerhouse and 2 percent in the spillway.
Observers were in place for nearly 40 percent of the daylight hours possible at the second powerhouse and 29 percent of the possible daylight hours at the first powerhouse during the study period. The researchers estimated that 1,368 salmonids were consumed during the study period.
That total percentage of the run taken by the sea lions, 0.54 percent, is minor, especially when compared to the 42-65 percent impact on the winter steelhead run by the infamous sea lions in the Ballard Locks area of the Puget Sound between 1986 and 1992.
But the Bonneville situation does need to be monitored, Stansell said. This year's upriver spring chinook run was an estimated 294,900 adult fish, second highest on record to the previous year's count of 416,500. The estimates are of the number of adult fish that return to the Columbia mouth. The records date back to 1938, when Bonneville Dam was completed.
But the stock's status has not always been, and may not be in the future, as rosy. As recently as 1995 only 10,200 spring chinook returned to spawn. If the number of sea lions taking advantage of the situation grows, and spring chinook runs shrink, the overall impact on the run could climb rapidly, Stansell said. v The biggest eater amongst the sea lions put away nine salmon in one 15-hour period and ate a total of 51 salmon over the 14 days in which he was observed. He also feasted on five lamprey. The study revealed that the sea lions, on average, took about 15 minutes to consume a salmon and one minute to eat a lamprey.
"They have to take large prey to the surface" and then tear it apart of eat it, Stansell said. He said the researchers estimated that, on average, the sea lions need about 1 ½ to 2 salmon per day just to maintain their metabolism. More fish are needed to fuel the sea lions on their frequent journeys.
The pinnipeds had the best fishing success below the second powerhouse -- an estimated 0.84 fish per hour as compared to 0.46 at the first powerhouse and 0.22 percent at the spillway. "Mature" adult sea lions had the best catch rate (0.58 fish per hour) but only slightly better than adults (0.51) and sub-adults (0.48).
The researchers spent little time watching for pinniped predation at night -- only 3 percent of the possible nighttime hours during the study period.
"Sea lions were never observed at night in the tailrace of Bonneville down to Beacon Rock," according to the abstract. "Most or our observations during the hour after sunset were of sea lions leaving the tailrace area.
"In mid-May, we did see some sleeping/resting behavior in the tailrace of PH1 during the day and evening, but nighttime observations using night-vision binoculars and spotlights indicate that sea lions are not actively feeding at night in the Bonneville Dam tailrace."
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