Shooting, apparently, did not bring the end for six sea lions found dead late Sunday morning in two closed, floating traps stationed below the lower Columbia River's Bonneville Dam.

Preliminary results of a necropsy, conducted beginning late Sunday night and completed late Monday, found no evidence of recent gunshot wounds, according to NOAA's Fisheries Service. The cause of the sea lion deaths remains unknown. The necropsies did identify numerous shallow puncture wounds in one animal that are consistent with sea lion bite marks.

The results "eliminate what everyone though was the cause," said NOAA spokesman Brian Gorman.

"The two big mysteries are how did the animals die and how did the doors get closed," according to Gorman. The traps were on-site as part of an ongoing effort by the states of Oregon and Washington to trap California sea lions for transfer to zoos and aquariums around the country. The solitary goal of that program is to reduce the predatory sea lions' impact on spawning salmon and steelhead runs.

"We're trying to figure out the two mysteries, and if they are related," said Gorman, who acknowledged that the deaths may not have been the result of foul play.

"I do know there have been cases in the past where doors have come down spontaneously," i.e. slammed shut without being triggered by humans, he said. It's possible the animals could have been inadvertently trapped and prevented from returning to their cool, watery habitat.

That theory is puzzling, Gorman said, because two traps are involved and it "suggests two doors have to come down simultaneously." Had one fallen, it seems logical the animals in the trap alongside would have fled, he said.

The states' departments of fish and wildlife had scheduled trapping each Monday and Thursday. Before the trapping began April 24, and between trapping days, the traps are left open so the animals become comfortable with the traps as part of their environment. The sea lions would "haul out" on the platforms to take a rest from their hunts and to digest.

An x-ray examination conducted Tuesday identified metal fragments in soft tissue around the neck area of two of the dead animals. In addition, a metal slug was found in the blubber of one animal. However, neither the fragments nor the slug appear to have caused death, and may have been associated with old wounds, according to NOAA.

The agency said it would continue its investigation to determine how the animals died and how the doors to the traps on which they were found could have been closed. The traps are closed manually with ropes from behind a nearby blind. They traps are in a "boat restricted zone" below the federally owned dam where public access is prohibited.

Each trap held two dead California sea lions and one dead Stellar sea lion, the smallest a young animal weighing about 300 pounds and the largest 1,300 pounds, Gorman said.

The possibility that the animals may have killed each other in some sort of a fight "is something people are talking about," Gorman said. It is unlikely, however, since only one animal showed evidence of wounds, the shallow punctures.

"We are not discarding anything," Gorman said of potential causes of death. Anyone with information that may assist in the investigation is asked to call the NOAA Enforcement Hotline at 1-800-853-1964.

Sharon Young of the Humane Society of the United States called on the federal government to conduct an around-the-clock investigation to find perpetrators she feels exist.

"They didn't just die on their own," she said. Young said that a fight between the animals is a highly unlikely cause of death.

"If they fight, it's pretty messy" so it would have been immediately obvious, she said.

NOAA-Fisheries, the federal agency that manages migratory marine mammals, is investigating the deaths, with assistance from the Washington Department of Fish and Wildlife, the Oregon Department of Fish and Wildlife, state police and the Corps, which operates Bonneville Dam.

Tissue samples from the animals have been sent for analysis at the National Fish and Wildlife Forensics Laboratory in Ashland, Ore. The heads of the animals were sent to the lab too, primarily for the x-rays. The tissue samples could potentially reveal signs of poisons or other causes of death. Gorman said he did not know when NOAA would receive results from the analysis.

The deaths occurred between 7 p.m. Saturday night when U.S. Army Corps of Engineers observers saw the traps, doors open, and when the dead animal were found just before noon Sunday.

Washington and Oregon wildlife managers immediately called off efforts to capture and relocate California sea lions, canceling a planned Monday effort.

"In light of yesterday's incident, the states are focusing their efforts on assisting the investigation and have suspended capture operations at Bonneville Dam until further notice," said Guy Norman, regional director of the WDFW's southwest Washington region.

Both California and Steller sea lions are protected under the federal Marine Mammal Protection Act. Steller sea lions, which are not subject to removal, are also federally protected under the Endangered Species Act. Penalties of up to $20,000 and one year in federal prison can be imposed for each animal protected under the MMPA, and up to $50,000 and one year in prison for each animal protected by the ESA.

The HSUS is offering a reward of $5,000 for information leading to the identification, arrest and conviction of the person or people responsible for killing the six federally protected sea lions.

Sea lion trapping and removals from below the dam will be stalled further, until at least next March, according to the terms of an agreement reached Tuesday by the states, HSUS and federal government.

In March, NOAA-Fisheries granted Washington, Oregon and Idaho the authority to remove -- lethally or otherwise -- up to 85 California sea lions annually from the area below the dam to stem salmon predation.

That approval was challenged in U.S. District Court by HSUS, which also sought a preliminary injunction to prevent the states from implementing its sea lion approval plan. The injunction was denied April 16 but HSUS appealed the decision and sought an emergency stay in the in the U.S. Court of Appeals for the Ninth Circuit. The appellate court granted the stay in part, prohibiting the lethal removal of California sea lions but allowing their capture and transfer of California sea lions to zoos.

The federal agency said the agreement it signed with the HSUS to suspend trapping and any lethal removal of sea lions from the Columbia River until next spring would allow it to make a full and complete presentation of its case to the district court and complete its investigation.

The stipulated agreement from HSUS, the states and federal government says the permanent removal of sea lions from the Columbia is not allowed before March 1 "(unless this Court enters a final decision in this matter that is not timely appealed or the Ninth Circuit enters a decision on an appeal of a final decision before that date, in which case this restriction shall be lifted upon entry of such a decision); provided however that nothing in this agreement prevents NMFS and the States from engaging in the non-lethal temporary trapping, branding, and return of such sea lions to their natural habitat."

The agreements submitted to the district and appeals courts ask for expedited hearings of the lawsuit in both courts. Regardless of the district court decision an appeal is anticipated, Gorman said.

The parties to the lawsuit are asking that the district court proceedings be concluded by Sept. 15. The proposed schedule asks the Ninth Circuit to conclude its deliberations by March 1.

Over the past two weeks, Washington and Oregon wildlife managers have captured seven California sea lions for relocation to zoological facilities. Six sea lions have been flown to SeaWorld facilities in Orlando, Fla., and San Antonio, Tex.

One of those animals died last week when it failed to resume breathing after being sedated for a health examination. A completed necropsy -- the non-human version of an autopsy -- "concluded that the death of the animal resulted from a combination of the very large size of the individual and the duration of the anesthesia. It was decided that limiting the duration of anesthesia depending upon the size of the animal should decrease the probability of mortality of adult male California sea lions in the future.

"The ACC concluded that it is highly probably that B198 was in a diseased state, having a urogenital carcinoma which has been reported as a cause of mortality in many wild California sea lions. The diseased condition of the animal probably did not contribute to its mortality during anesthesia," according to a report released this week by the animal care committee formed to monitor the sea lion removal program.

The sea lion dubbed B198 weighed 1,452 pounds, a record among California sea lions catalogued by researchers and nearly double the average weight of an adult male.

"The necropsy results indicated the animal was exceptionally fat, having 120 mm [nearly 5 inches] of ventral blubber over the sternum, extensive deposits of fat in the peritoneal and thoracic cavity and among the organ systems," according to the report.

California sea lions are part of a robust West Coast population that since 2001 has taken an increasing toll on federally protected salmon and steelhead below the Bonneville Dam.

Despite three years of efforts to deter them through hazing, sea lions last year consumed more than 4 percent of the returning spring chinook salmon run in just the area visible to observers on the dam.

Researchers at the dam say the numbers of sea lions at the dam seems to be past its peak for this year, holding at 40 animals in recent days. A count of 63 on April 16 set a record high for any one day over the course of the research, which began in 2002. The research was launched to evaluate the impact sea lion predation was having on salmon and steelhead, which includes stocks listed under the ESA.

Preliminary data this year indicates 73 different California sea lions, and at least 17 Steller sea lions and two harbor seals, have visited the dam, located 146 river miles from the river's mouth at the Pacific Ocean. At least 38 of the California sea lions seen this year have been also been observed at the dam in previous years.

The California sea lions, which rarely swam so far upstream historically, typically begin leaving the Columbia in May, bound for breeding grounds off the Southern California coast. It's theorized that big spring salmon runs to the Columbia early in the decade caught the marine mammals attention. Male sea lions forage up the coast between breeding seasons.

Corps researchers estimate that the sea lion take actually observed below the dam from Jan. 11 through May 5 includes 3,160 chinook, 283 steelhead, 607 sturgeon (21 larger than 5 feet), 9 lamprey and 648 unidentified fish. They stress that the data is preliminary.

The sea lions are on track to match or surpass last year's record salmon catch, 3,589.