Sunday, 9 April 2023

Decline of the Sea Otters

Sea otters (Enhydra lutris) are an endangered species, native to the coasts of the northern Pacific from Japan to California. I've discussed them in detail before, but suffice it to say here that they are remarkable creatures, able to survive without setting foot on land, digging no dens as other otters do, and even giving birth at sea. They also have the densest fur of any mammal, making it especially valuable.

Which, of course, explains how they became an endangered species in the first place.

Humans have been hunting sea otters for their fur for many centuries, but for much of history, this was small scale, with the local tribes being unable to make any lasting dent in their numbers even if they had wished to. Estimates for the worldwide population of sea otters prior to the 18th century are naturally difficult to come by, but it may have been as high as 300,000. The herald of coming doom came in 1741.

A few years previously, Emperor Peter the Great of Russia had employed Danish explorer Vitus Bering to map the Arctic coasts of eastern Siberia, in what was one of the largest organised expeditions of discovery in history - employing over 3,000 explorers and lasting ten years. 1741 was the year that Bering felt able to push on ever further, crossing the straits that bear his name and, in July of that year, he discovered Alaska. This did not end well for him; he was already ill at the time (possibly from scurvy, although there is apparently some dispute about this) and by the time he returned to eastern Russia in the winter, he was essentially incapacitated. 

A storm forced his ship to seek shelter on one of the Commander Islands, where he died on the 19th December. However, the expedition had included several scientists, there to document the plants, wildlife, and geography of any newly discovered lands. So, with nothing much else to do until the weather cleared, the ship's physician, George Steller, explored the islands - despite also being one the expedition's leading scientists, he may still have been miffed that he had been given only ten hours to explore the whole of Alaska. 

Steller's name is well-known to modern taxonomists because of the number of things that he discovered over the course of his career. At least three species of birds, two mammals, a fish, a flower, and even a mineral are all named after him, and even that is not a full list of his discoveries. For our purposes, the most important thing is that, while on the Commander Islands, he made the first scientific description of the sea otter.

This was very unfortunate from the perspective of the sea otters, because the expedition then brought back several pelts of the animals to Russia (along with some from seals) and that prompted Russian fur trappers to head east and start exploiting the animals. To being with, this largely affected the Asian and Alaskan populations, but other countries couldn't help but notice what the Russians were up to, and sent their own expeditions over the next few decades to check out the area. James Cook's expedition to Vancouver Island in 1778 also brought back sea otter pelts and from then on, it was basically open season.

So it continued, from the east coast of Russia all the way around to California, for over a hundred years. Then, in 1911, the United States, Russia, Japan, and the United Kingdom (representing Canada) signed the North Pacific Fur Seal Convention. Despite the name, this covered all fur-bearing sea mammals, not just seals, banning open-water hunting and giving the US jurisdiction over on-shore hunting. Congress then used that jurisdiction to immediately ban commercial hunting altogether for the next five years to give stocks a chance to recover. It was the first international treaty to deal with such conservation efforts.

By this point, it is thought that the worldwide population of sea otters was around 2,000, and restricted to just thirteen specific locations. Surveys in the 1930s revealed that protection measures, at least in the US and Canada, were working, and the animals had started to rebound. When the IUCN published its first list of endangered species worldwide in 1996, sea otters were not on it, instead being described as "low risk".

It would be nice to say that things have continued to improve ever since... but it wouldn't be true.

Indeed, even the 1996 assessment was, in retrospect, overly optimistic; populations have been dropping again in some areas since around the late '80s. A 2005 study estimated that the population in the Aleutian Islands had dropped by a staggering 90% in the previous 15 years or so, and similar, if less dramatic declines have occurred elsewhere; it's thought that the Russian population may have halved over the last thirty years. The current worldwide estimate is 130,000 individuals, with over three-quarters of those living in Alaska - indeed, two of the thirteen populations thought to have been around in 1911 had vanished by the mid-'60s.

The question, obviously, is what's happening. We do, after all, have more thorough conservation policies in place now than we did even in 1911, with the US Endangered Species Act of 1973 being a case in point. Obviously, that doesn't cover Russia, where many of the declines have been happening, and the truth is that we aren't really sure what's going on there. 

Perhaps the single greatest threat to sea otters, now that we've stopped hunting them, is pollution, especially from oil spills. Unlike seals, sea otters do not have a thick layer of blubber, so they rely entirely on that super-thick coat of fur for insulation; if it gets matted with oil, they are likely to die of hypothermia. That's ignoring the toxic effects of oil and other pollutants; at least 2,600 sea otters are thought to have died in Prince William Sound following the Exxon Valdez disaster in 1989.

Global warming and accidental drowning in fishing nets are also significant threats. While it's likely not a major cause of the initial decline, recovery of populations has been hampered in some areas by infection with Toxoplasma, which gets into the sewage system from cat faeces and is, eventually, often flushed out to sea to infect the local wildlife.

To help understand this, various surveys of sea otter mortality have been conducted over the years and, in 2016, one threw up an anomaly. This was conducted in the Katmai National Park and Reserve in Alaska, a 16,600 km² (6,400 square mile) wilderness area lying on the eastern side of the northern end of the Alaskan Peninsula. (For reference, that's a little larger than either Connecticut or Yorkshire). 

When the area was first designated as a National Monument, back in 1918, there were no sea otters there, but they had just started to return when the National Park was created in 1980, probably from a surviving population around Augustine Island, just to the north. By the end of that decade, it's thought that around 900 lived in the area, reaching around 7,000 by 2008. It's stayed stable since then, probably because that's around the maximum number of otters the local food supply can support and thus, about as good as it's ever going to get.

The 2016 study collected carcasses of sea otters from along the coast and had been running for ten years at the time of publication. What the study showed, among other things, was that around half of the carcasses belonged to animals in the prime of life - between two and eight years old, in the case of sea otters. That's worrying, because, if the population were stable, you'd expect most of the dying individuals to be old, with a second peak consisting of juveniles. Those in the prime of life, in other words, should be the least likely to die.

A recent re-evaluation of the study, however, appears to show the reason, and it isn't as alarming as it first appeared. It turns out that, in the particular study area used, brown bears are regularly sneaking down to the coast to kill and eat sea otters that have hauled themselves up onto land for a bit of a rest - not something they need to do, but that they certainly can. Camera traps placed at locations along the coast managed to catch some of the bears in the act, and the bears, being much larger than the otters, aren't picking on elderly or underage individuals, just whatever is around.

That the overall population in Katmai isn't declining suggests that this is an unusual feature of this one particular area. Indeed, we wouldn't really expect bears to be hunting an animal that spends most of its time at sea but, here at least they do. Indeed, the camera trap caught some attacking seals, albeit with notably less success.

While this may have been a false alarm, it does point to what's likely the real reason for the decline in sea otter numbers outside of the occasional oil spill: predation. It's just not predation by bears or wolves. Nor can it be great white sharks, because, while they will indeed, eat sea otters, those aren't common around Alaska. Instead, the finger of blame is thought to rest with killer whales

While killer whale populations worldwide are in an unclear position, those in the North Pacific seem to be increasing, perhaps partly because of improved protection efforts but more likely because the animals that they used to eat were themselves declining, forcing them to find alternative food sources. In Katmai, that's likely not a serious issue, since the sea otter population is stable and probably as high as it could be but elsewhere, especially in the Aleutian Islands to the southwest, it's a serious issue.

It's all a complex web. The cetaceans that killer whales eat decline in numbers, so the killer whales start eating sea otters (and seals) and then those decline in numbers as well. Which means you get more of the sort of animals that sea otters eat, such as clams, crabs, and sea urchins. Which then eat the kelp that forms the "forests" that dominate the local habitat. Which affects the entire ecosystem.

And so it goes...

[Photo by Mike Baird, from Wikimedia Commons.]

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