SOLVING THE SEA LION PUZZLE
(previously published in the Kodiak Daily Mirror, 18 March 2004. Written by Deanna Cooper)

Some of the hypotheses that have been put forward to explain the decline of Steller sea lions in western Alaska.

Dr. Andrew W. Trites from the North Pacific Universities Marine Mammal Research Consortium presented a seminar at ComFish on the cause of the Steller sea lion decline. For many years it was thought the decline was due to commercial fisheries, but other hypotheses were put forward and researched, such as environmental toxins, storms, regime shifts, changes in food, emigration, entanglement, predation by sharks and killer whales, parasites and disease.

"The approach we have been taking is to go through all these possible hypotheses and look at them. As researchers, we are reductionists — we start crossing the possibilities off one at a time, and hopefully we are left with one thing at the end," Trites said. He said that many of the possible factors in the sea lion decline are interrelated.

"I see it as a hierarchy of changes and at the bottom of the pyramid is ocean climate — the regime shifts. I think this is the major driving force that establishes which species are going to be dominant in the Gulf of Alaska or the Bering Sea, which species are going to thrive and which species are going to do poorly," he said. Sea lions prefer about 10 different species and, like humans, what they eat determines their overall health. Pollock, herring and sand lance are an integral part of their diet. While herring and sand lance are considered “energy rich" foods for the sea lions, pollock is not. A diet high in pollock may supply insufficient energy and inadequate levels of vitamins and minerals to sea lions. They naturally eat whatever is most abundant and accessible.

Conceptual framework for how ocean climate affects sea lion numbers by determining the relative abundances of fish available to eat, which in turn affects the pregnancy rates, birth rates, and death rates (through disease and predation) of sea lions.  Also shown are the effects of human activities that can directly or indirectly affect sea lion numbers.

The overall health of sea lions translates into how many offspring they leave and whether the populations go up or down.

"A knife has cut through the entire ecosystem and affected all levels of it. Marine mammals are not separate from the ecosystem — they are a part of that system."

"The amount of body fat an animal puts down affects how much they are going to grow, their fertility and at a cellular level, whether or not their cells are going to be more susceptible to viruses. It is no different than people," Trites said. Biologists have determined that over the last century there have been two periods where conditions have been good or bad for sea lions. One of these good periods was 1947 to 1976 and another may have started in 1999.

The decline of Steller sea lions, as well as harbor seals and fur seal pups occurred about the same time starting in 1977.

Trites said the animals were physically smaller in the 1980s and showed signs of stunted growth. They had lower pregnancy rates and high mortality of juveniles from 6 to 23 months of age.

"When things were good for sea lions, their diets appeared to contain a healthier mix of energy-rich fish. They ate pollock, but overall were getting more fat in their diet. Energy rich (in terms of health,) is very positive and should result in an increase in population size.

On the other side of the coin, we have had two periods of bad conditions — ‘bad’ in terms of sea lions. If you were a pollock fisherman, you would say these were great times. But it appears a high amount of pollock is associated with a poor period for sea lions," Trites said.

Though diet is an important factor, Trites firmly believes the root of the problem is ocean conditions. He said this is still considered a hypothesis and is difficult to prove, but there is a predominance of evidence that is consistent with the hypothesis.

Trites said that while human factors, such as fishing, can have an effect by removing some of the key species, there is evidence that there were times when sea lions were not very abundant long before commercial fisheries.

"People have thought the change in food could be caused by fishing, but I have not seen any evidence outside of the conviction that some people have that this is a factor," Trites said.

He pointed out that sea lion populations declined in the Western Aleutians, which did not have a pollock fishery and did not have the same extended fishery development that has gone on in the Kodiak area.

"A lot of people miss that point. They say joint-venture fisheries caused the sea lion decline. That pertains to our backyard, but let’s look at the other backyards in Alaska. The theory is that if it happened here it should have happened there and it didn’t," Trites said. There is strong evidence that killer whales eating sea lions is a significant part of the problem. Though it may be preventing populations from recovering, it is not the key to the sea lion decline, he said.

There is no evidence that sea lions are starving to death; their disappearance has been subtle, and more than one factor plays a role, but people have been looking for a single cause, a smoking gun solely responsible for the decline.


"The decline has occurred because of the synergistic effects of multiple things but what seems to be driving it is nutritionally-based. That is what is going to make an animal more susceptible to being caught by a predator or catching a disease."

“However, when there is any kind of doubt, human activity is restricted. We cannot say with 100 percent certainty that people have not played any role whatsoever in the decline," Trites said. Because commercial fisheries were thought to be a factor in the decline, the National Marine Fisheries Service placed restrictions on commercial fishing in the Bering Sea and the Gulf of Alaska. One outcome of the restrictions requires a vessel-monitoring system be installed on certain boats.

Dr Trites on far right with other scientists

The Vessel-Monitoring-System (VMS) requirement came about to implement Steller sea lion protection measures and is used when endorsed fisheries are open.

"What I am hoping is people will sit back and re-look at some of the basic assumptions they have had and rethink the reductionist approach they have been taking to solve this mystery."

"The fact is that nobody has found any of the (sea lion) bodies. It has been a very subtle change that has gone on — yet dramatic too. It is happening right underneath our noses. We have the best technology today that money can buy. It is being applied to the sea lions, yet we cannot figure it out for sure," Trites said.

Killer whales, disease and failed pregnancies play a significant role, but people need to recognize that ocean climate — regime shifts — affect not just sea lions but the whole ecosystem, Trites said.

"A knife has cut through the entire ecosystem and affected all levels of it. Marine mammals are not separate from the ecosystem — they are a part of that system."

(previously published in the Kodiak Daily Mirror, 18 March 2004. Written by Deanna Cooper)

14 May 2004