A Tale of Two Kelp Forests

The title above is the name of a science lecture I attended yesterday at the U. S. Geological Society. The lecture was being delivered as part of "Sea Otter Awareness Week". It addressed the difference in health between two sea otter populations, one in the Aleutian Islands in Alaska, the other in the adjacent Commander Islands off of Russia. The population in the Aleutian Islands is in serious decline (down 90% over the past 15 years), whereas the sea otter population in the Commander Islands is at steady state, at presumably the ecosystem's carrying capacity.

The lecturer described:
(a) How the marine ecology in those respective islands differed as a result of the numbers of sea otters.
(b) How the population density of otters affected the sea otters' behavior, diet and health

The lecturer also sought to explain why the sea otter population was in decline in the Aleutians, while the the population in the Commander Islands was not. Also, a few fascinating slides were spent detailing the difficulties of studying sea otters in these remote environments.

Impact to Marine Ecology

A solid case was made, in the form of box plots, bar graphs, and photographs that sea otters have a huge impact on the surrounding marine ecology. In very simplistic terms:

*An abundance of sea otters keep the sea urchin population in check, which keeps the kelp forest from being killed off.
*An absence of sea otters allows the sea urchin population to explode, which destroys the kelp forest.

Sea Urchins eating and killing Giant Kelp

The presence or absence of the kelp forest also has manifold effects:
*The kelp forest provides a nursery for healthy fish populations.
*A kelp forest also introduces increased nutrients in the water, which encourages the growth of filter feeders (mussels, clams etc...)

In summary, sea otters are what are called "keystone" predators. Their presence or absence has a large influence on the ecosystem. A subtle point, which I'm sure I don't fully understand, was the non-linear relationship between the number of sea urchins and the biomass of the kelp forest. As the number of sea urchins increases, their destructive effect upon the kelp forest accelerates.

My only challenge to the scientist's assertions are:
(1) Why are Monterey/Channel Island kelp forests relatively less impacted by the lack of sea otters? There are far fewer sea otters off California (2-3,000) vs. off Alaska, Canada, and Washington (~70,000), yet the kelp forests in California continue to exist.
(2) If there are other predators in California that keep the sea urchin population in check, why don't those predators exist off Alaska (wolf eels, for one)?

Impact to Sea Otter Diet/Health
Fascinatingly enough, the sea otters in the Aleutian Islands have grown in size in response to their population decine. The sea otters in the Aleutians (low populations) are the fattest, healthiest sea otters the scientists have ever seen. The scientist called them, "rotund", "jumbo" and happy. With so little competition, these few sea otters have plenty of sea urchins to eat. In comparison, the sea otters in the Commander Islands were smaller, equivalent to the size their Aleutian neighbors were before their population declined. Furthermore, the Aleutian sea otters also have a far less varied diet than their Commander Islands neighbors.

Funny Science Anecdotes

The lecturer got a few laughs describing the difficulties of studying sea otters, from their capture, to surgically implanting radio transmitters into their bodies. Here are some observations:
-Sea otters have a nasty bite
-Catching sea otters in a net is a nightmare, particularly when you are catching a dozen per hour, as was the case when they stretched a net across the sheltered lagoon.
-Rebreathers are required to encounter awake sea otters underwater, because they can smell your bubbles hundreds of feet away.
-A lot of hiking is required in the Russian islands, where there are no trails.
-Mina's aside: science is truly hard work, and often enormous amounts of effort is required to prove every logical step of one's scientific assertions (that's what doctoral students, and master's students are for).

Commander (Komandorski) Islands on the Left, Aleutian Islands in the Center and Right

Probable Cause for Sea Otter Decline in Aleutians
The answer might surprise you: the scientist from UC Santa Cruz hypothesized that the decline in sea otters in the Aleutian islands is attributed to increased attacks from orcas. The main evidence for his hypothesis are as follows:

(1) An increase in orca attacks witnessed over the past 15 years.
(2) An absence of sea otter carcasses found, or sick otters found.
(3) The sea otter population protected by a lagoon that was inaccessible to orcas declined much less rapidly than everywhere else in the Aleutians.
(4) A computer model accounting for the metabolic requirements of both orcas and sea otters predicted that it was a possible for a small group of 6-10 orcas to decimate the sea otter population over the years.
(5) Sea otters in the Aleutian islands stay closer to shore than they used to 15 years ago, and also stay closer to shore than the the sea otters of the Commander Islands.

One of the grey-haired scientists in the audience challenged this hypothesis and wondered if the population decline could not be attributed to pollution instead. The speaker did not have an excellent response to this challenge. He stated that the pollution was the same between Russia and Alaska, but he did not provide any numbers to back up his assertion.

Another scientist wondered why the orcas didn't attack the sea otters off of Russia. Apparently, different orca pods have very different geographical domains. Also, differing orca pods are extremely variant in their diets, behaviors, and even genetics. The "transient" (read: mammal-eating) orcas that supposedly ate the sea otters don't wander into the Russian islands. Supposedly, these orcas developed their sea-otter eating behavior due to the dwindling in their other food sources (harbor seals, sea-lions). The most fascinating thing I heard was that "resident" (read: fish-eating) orcas are as much as 1000 years genetically different from the "transient" orcas.

I went up to the mike and had to ask my bone-headed question. I was probably the only non-scientist in the 80-100 strong audience, and was perhaps the only person below the age of 50, but I braved it out with my unscientific question. I asked if: (1) the rumor that the California sea otter population decline was caused by "feline leukemia virus" was true, and (2) what could have been the cause of death for the 4 foot long dead sea otter I encountered off of Monastery Beach.

The scientist was quite nice in his response. First of all, it wasn't "feline leukemia virus" but another feline borne disease, Toxoplasmosis that was attributed to 17% of sea otter deaths a decade ago. He asserted that sea otters most likely caught toxoplasmosis through the prey that they ate, rather than kitty litter. Toxiplasmosis as a cause of death is in decline, however. He wouldn't elaborate on what caused the myriad of other causes of death in the pie-chart.

He couldn't elaborate on what caused the sea otter that I encountered to die. But he did mention that more than anywhere else, more sea otter carcasses were found in California than anywhere else. Also, a necropsy had to be performed on a fresh sea otter carcass in order to find the cause of death.

I left the lecture feeling good that there was so much science left to be done. There is still much to understand about why sea otters in California are not reaching carrying capacity. There is also so much yet unknown about orcas, and their behaviors.