Understanding the role of sea otters as a keystone species in marine nearshore environments provides an excellent example of species interconnections. In this post we’ll wade on into the coastal waters of Oregon and take a look at how a healthy kelp forest works to maintain a balance of the marine nearshore ecosystem, the role of the southern sea otter (Enhydra lutris nereis), the rise in what is called “urchin barrens”, and current efforts to reintroduce sea otters to the Oregon coast.
It’s estimated that between 16,000 and 17,000 southern sea otters once lived in the waters along the Pacific coast. By the early 1900’s, sea otters were hunted to near extinction by fur traders and commercial fishermen who thought the otters were competing with them for shellfish (see previous post Beaver: The Ultimate Keystone Species). The number of sea otters off the central coast of California was at a low of about 50 in 1938. The last sea otter in Oregon was killed in 1906, and in Washington in 1910. In 1969-1970, the U.S. government was planning to perform nuclear testing at Amchitka Island in Alaska. Thankfully, marine biologists working there at the time intervened and facilitated the moving of 59 sea otters from Amchitka Island to the northern coast of Washington prior to the blast taking place. Thousands of sea birds and about one thousand sea otters died in that nuclear blast. The relocated population of 59 otters has grown to 2,962 (in 2019) and is now mixing with another sea otter population in Vancouver, B.C. as well as expanding their range south. The southern sea otter is currently federally listed as a threatened species. Oregon is the only state where sea otters once lived that remains unpopulated.
Key Players in an Ideal Sea Otter Habitat
The sea otter is both the heaviest member of the weasel family and one of the smallest marine mammals. They do not have an insulating blubber layer like other marine mammals yet they live in an environment where the ambient temperature is about half their body temperature. They maintain their body temperature with the aid of their very thick fur, and by eating a lot. Their fur is the thickest of all animals in the animal kingdom with one billion hairs per square inch! They groom their fur constantly which allows a layer of air next to their skin, effectively creating a dry suit for themselves. They weigh up to 100 pounds and, in the wild, generally live to be 14 – 17 years old.
Sea Otters are highly social animals and live together in groups called “rafts”. A raft will consist of either all females with one or two males that mate with these females or a group of bachelor males. Otter pups are dependent on their mother for 5-6 months after birth. Pregnancy and caring for pups are a huge energy burden on the female and their health can suffer as a result. Their diet consists of marine invertebrates (sea urchins, mollusks, crustaceans) and some species of fish. They use rocks to pry invertebrates off the sea floor and pound them open them making it one of the few mammal species to use tools.
A favorite meal of the sea otters is sea urchins. The sea urchin’s favorite meal is kelp. In some areas such as the Alaskan coast, the relationship between sea otters present and kelp forest abundance is starkly apparent; when sea otters are present in an ecosystem the urchin population remains under control which, in turn, allows kelp forests to grow. When sea otters are absent we see urchin populations multiply and kelp forests severely degraded or missing altogether. This environment is called an “urchin barren” and reflects an ecosystem out of balance, but keep in mind that the variables that lead to an urchin barren state are numerous, making this a complex issue.
Giant kelp forests in the nearshore marine environment are considered highly productive and highly diverse ecosystems. Kelp are large brown algae that form in shallow, cold, nutrient-rich waters off the coast. Kelp forests are found around the world with the Pacific Northwest coast being particularly well-suited for them. The largest species off the Oregon coast is bull kelp (Nereoctstis leutkeana). Both annual and perennial species of kelp exist; perennial species can live up to 20 years. Seaweeds are anchored on the sea floor and have long blades that can grow up to 18 inches per day. These blades grow straight to the waters surface, staying aloft by a gas-filled bladder at the base of each blade. Kelp are photosynthesizers that play a role at the base of the food chain. On decomposing, they enrich the ocean water with nutrients which are then utilized by filter feeders (muscles, barnacles, etc) and other invertebrates. They modify light levels and help control sedimentation, reduce erosion by blunting the force of incoming waves, and provide a significant amount of carbon sequestration. They are biodiversity hotspots and serve as a nursery for many species including many types of invertebrates, juvenile rockfish, seals, sea lions, whales, sea otters, gulls, terns, snowy egrets, great blue herons, and other shore birds. To thrive, kelp need only light for photosynthesis and abundant nutrients, typically coming from the upwelling of deep ocean cold water.
Historically, kelp forests have occupied about 25% of the world’s coastlines. Unfortunately, kelp forests in the Pacific Northwest are now in decline due to climate change, sea star wasting disease, urchin population explosion, and the absence of sea otters. The unhealthy corollary to a healthy kelp forest is a referred to as a sea urchin barren. There are some areas off the Southern California coast where kelp forests thrive without sea otters being present. However there urchin populations are kept in control by the presence of other predators, including lobsters and California sheepshead. If these predators are removed from the ecosystem (through over-fishing or by other factors) it will again allow urchins to thrive and the kelp forest to degrade. Other species that feed on kelp such as sea stars, isopods, kelp crabs, and herbivorous fishes tend to feed on drift kelp — kelp that has been dislodged from its substrate. When sufficient drift kelp is available, these species do not impact the attached kelp plants. Currently, 50% of the kelp beds in Oregon are found at the Port Orford reef in southern Oregon.
There are two types of sea urchins: red (Mesocentrotus franciscanus) and purple (Strongylocentrotus purpuratus). The larger red urchins are the ones being fished for marketing of the uni (gonads). Red urchins commonly live over 30 years and can live up to 200 years. They are not always successful at reproducing as conditions have to be just right; some years will result in a boom of red urchin larvae, others not. Red urchins mainly eat drift kelp. The red urchin population was fished out along the west coast in the 1990’s but with protections are making a comeback. They are managed as a fishery product.
Purple urchins live in shallower sub-tidal zones but as their populations explode they move into the zone where red urchins are found. Purple urchins will feed on the base of the living kelp plant causing the plant to die. When urchins have eaten all the food available in their area and created an “urchin barren”, they starve but they don’t die; they go into a state of low metabolic activity and can survive for years in this state. Starved out sea urchins do not produce uni, which is what sea otters feed on. Sea stars also feed on purple sea urchins. Interestingly, the urchins will part their spines and allow an approaching sea start to get close to them before launching a surprise attack; the urchin uses its pinchers to gnaw on the sea star’s tube feet causing the sea star to back away. However, sunflower sea stars are unaffected by this attack and will continue advancing to eat the urchin whole — spines and all!
Sea stars are a large and diverse class of Echinoderrms with over 1,900 living species. There are 11 species that are typically found on Oregon’s coast including the Pacific blood star (Henricia leviuscula), the Sunflower sea star (Pycnopodia helianthoides), and the purple or ochre star (Pisaster ochraceus) typically seen in tidal pools. Sea stars are highly adapted, mobile creatures that have few natural enemies.
Together, sunflower sea stars and sea otters can control sea urchin populations by preying on them. In the ecology world, this is known as functional redundancy — the idea that when two or more species in an ecosystem community perform similar functions, there should be a buffering effect that protects against the loss of either species.
In 2013 – 2014, multiple species of sea stars began dying off in huge numbers — up to 90% of species were lost on the Pacific Northwest coast. Scientists have identified the virus responsible for the disease as a single virus — sea star associated densovirus (SSaDV) — yet it remains unclear what environmental factors caused this outbreak to be so severe. Basically, the sea stars outer layer (the “skin”) melts away allowing their bodies to deteriorate. This event, dubbed “Sea Star Wasting Disease” (SSWD), began in Howe Sound just north of Vancouver, British Columbia and killed all of the sunflower sea stars along with 20 other species of sea stars. The scale and speed of this event was shocking. Howe Sound appears to have been ground zero for this event, but SSWD quickly spread along the entire west coast of the Pacific. Recovery of the sea stars since then has been uneven. Some species seem to have bounced back, but there was 100% loss of sunflower sea stars in the PNW (except for a small population in the Puget Sound area), and they are not coming back. At many locations, urchin populations have exploded and created urchin barrens. Note that echinoderms, in general, experience boom-bust cycles and may take decades to return to an area previously inhabited (if at all).
Some of the Challenges to Marine Nearshore Environments
The kelp forests of the Pacific Northwest have been in moderate decline for many years and the problem is growing worse. Even more severe declines have been documented elsewhere in the world. Ocean kelp forests can be seen from space using the Landsat satellites. Scientists can use this data to document the changes to kelp forests over time. This method is being used in Oregon.
In 2013 – 2015, a heat wave formed in the Pacific Ocean off the coast of North America. It was officially termed “the blob”. The warm waters of the blob were nutrient-poor and extremely hard on kelp forests. In northern California, there was a 90% loss of kelp forests and sea stars attributed to this event. To date, the kelp has not come back and there has been a huge increase in the number of sea urchins as well as an increase in the mussel population. Efforts are underway to reintroduce sea otters and sea stars to help restore the kelp forests. The kelp forests off the Oregon coast were not as severely affected by the blob. The dynamics of these ecosystems are very complex and not well understood. Scientists are monitoring many different areas to try to better understand what is driving kelp decline. Recent studies have shown that kelp forests can thrive in deeper oceanic zones of the warmer, tropical waters where light can penetrate to greater depths. The growth of kelp is not affected as much by water temperature as it is by nutrient availability, particularly nitrogen. Under conditions of adequate light and nutrients, kelp can thrive in water temperatures of up to 23ºC.
One of the ecosystem threats that has been associated with climate change is an increase in domoic acid (DA) intoxication. DA is a potent neurotoxin produced by a diatom species (genus Pseudo-nitzschia) that can accumulate in the food web. This diatom is found worldwide and has the greatest impact on oceanic eastern boundary upwelling systems, which includes the west coast of the U.S. DA toxicity is an important cause of marine wildlife mortality as well as a threat to human food safety. DA enters secondary trophic levels of a food web when suspension feeds such as shellfish and anchovies ingest the toxic diatom cells. High DA levels have been shown to be associated with warmer ocean temperatures and are associated with both the Pacific Decadal Oscillation (PDO) and El Niño events, particularly when these two conditions coincide. If these warm ocean events become more persistant due to global warming, West Coast DA events may also increase. Due to their small body size, high metabolism, and diverse prey preferences sea otters are particularly susceptible to DA exposure. In one study that reviewed findings for southern sea otter necropsies of 560 animals over a 15 year period, probable DA intoxication was a primary or contributing cause of death for 20% of the sea otters.
Ocean acidification is a whole topic unto itself but I want to give it a quick mention in this section. Ocean acidification refers to a reduction of the ocean pH over an extended period of time, caused primarily by the uptake of carbon dioxide (CO2) from the atmosphere. When CO2 is absorbed by seawater, a series of chemical reactions occur resulting in an increased concentration of hydrogen ions. This increase causes seawater to become acidic and reduces the availability of carbonate ions necessary for the building of sea shells and coral skeletons. Sea urchins, corals, oysters, diatoms, and other organisms with shells or skeletons end up with very thin shells and skeletons and ultimately the entire oceanic food web is at risk of collapsing.
So far, ocean pH has dropped from 8.2 to 8.1 since the industrial revolution, and is expected by fall another 0.3 to 0.4 pH units by the end of the century. A drop in pH of 0.1 might not seem like a lot, but the pH scale, like the Richter scale for measuring earthquakes, is logarithmic. For example, pH 4 is ten times more acidic than pH 5 and 100 times (10 times 10) more acidic than pH 6. If we continue to add carbon dioxide at current rates, seawater pH may drop another 120 percent by the end of this century, to 7.8 or 7.7, creating an ocean more acidic than any seen for the past 20 million years or more.
How well marine organisms will adapt to a rapidly changing environment due to climate change is not well understood. An evolutionary perspective is necessary to better understand climate change effects on our seas and to examine approaches that may be useful for addressing this challenge.
Feasibility of Sea Otter Reintroduction in Oregon
Efforts are underway to return sea otters to the southern Oregon coast. This section of the coast has a rocky surface that can support the players’ necessary for a successful return of sea otters to the environment. The state of the current environment here is degraded and will need some restoration work upfront to help ensure the success of sea otter reintroduction. The Elakaha Alliance is working with a number of cross-disciplinary groups and researchers to first target restoration work in certain areas (culling purple urchins, nurturing kelp oasis’).
The Elakaha Alliance recently completed an in-depth scientific study to assess the feasibility of reintroducing sea otters to the Oregon Coast. Here are their five main takeaways:
- Reintroductions (through translocation) are a successful conservation tool. Previous reintroductions into southeast Alaska, British Columbia, and Washington have increased species viability, helped recover genetic diversity, and improved gene flow in sea otter populations.
- Reintroducing sea otters to Oregon is likely to succeed, with appropriate considerations. A model developed specifically for evaluating population success of reintroductions in Oregon suggest that several areas, mostly along the southern coast, would likely support a successful reintroduction of sufficient numbers of otters. The model also indicates that multiple release locations may be more effective than a single release site.
- Estuaries may be an important reintroduction environment, especially when close to a suitable nearshore ocean habitat. These environments support sea otter populations in some areas of California. Further research is recommended to review potential sea otter – human interactions in estuaries, however otters could potentially move into estuaries and sloughs as populations recover.
- Return of sea otters will have many direct and indirect effects. As a keystone species, sea otters have inordinately strong effects on the nearshore ecosystems they inhabit. Indirect ecosystem enhancements include: increases in kelp forest and eelgrass beds which, in turn, increase fin fish and invertebrate species, increase in overall biodiversity and productivity, increase in carbon capture and fixation. Sea otter reintroduction can also have a negative social and economic impact on the shellfish industry.
- Social, economic factors and regulatory issues must be considered. Reintroductions can only occur if these issues are fully addressed. Outreach and engagement with a broad array of affected stakeholders are essential.
Elakha Alliance is currently working on its second of three phases — achieving consensus of key partners including tribes, shellfish harvesters, fisherman, ports, businesses, conservation organizations, and local, state, and federal governments. This phase is expected to conclude no earlier than 2024. The final phase will be to restore a viable, sustainable population of sea otters to a few select places along the Oregon coast. This is expected to take from 2-4 years before actual restoration begins, followed by monitoring, research, and continued stakeholder engagement. Experience and the models show that, following reintroduction of sea otters to a new environment is typically followed by a significant loss of the animals immediately, followed by a slow rebound of several years, then a more rapid increase in population. So, it will be several more years before we will be able to watch sea otters off our Oregon coast, but what a delight that will be!
Making the Connection
My position on restoration work has shifted over the past several years since I was first studying to become an Oregon Master Naturalist. I used to believe that Mother Nature was best left alone to recover from human disturbance in the way that only she knows how to do best. This conviction was borne out of the belief that, in many cases, when humans try to “restore” an area to a more “natural” state they quite often fail or make things worse simply because they don’t have a full understanding of all the relationships and interconnections between species and the environment. I have much more respect for someone who is considered an expert in their field if they freely acknowledge how much they still have to learn about their field. Amazing discoveries continue to be made daily in the scientific world — sometimes blowing long-held beliefs out of the water — and will continue to be made into the future. The complexity of Earth’s ecosystems is simply mind-blowing, when you have even a small glimpse into the finely-tuned workings of Mother Nature you are humbled and awe-struck.
I have come to understand that, in many cases, restoration work is not only necessary but vital in assisting Mother Nature in the recovery of human-disturbed complex ecosystems. The work of The Elakha Alliance is an excellent example of how this work can be done successfully; with intensive study and engagement with known experts in all the identified fields related to the restoration work, with the understanding that the work will take many years to complete, with the humbleness to acknowledge that, while we know a lot about this environment, there are things we do not know that may affect the outcome but we are going to give this our best shot.
I encourage you to watch this excellent 7-minute video made by KQED and Quest titled Sea Otters v. Climate Change — https://www.kqed.org/quest/65409/sea-otters-urchins-kelp-climate-change
- Goodell, J. (host), March 202 – April 2021, Northwest Nature Matters podcast, season 2, episodes #28, 29, 30, 32, 33, 37, retrieved from www.myowf.org
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- “Bringing Back Oregon’s Sea Otters”, The Elakha Alliance; elakhaalliance.org
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- Kone, Dominique, “Oregon sea otter reintroduction: opinions, perspectives, theories”, 28 May 2018, blogs.oregonstate.edu
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- Kone, D.V., et. al., 2021, “Informing sea otter reintroduction through habitat and human interaction assessment”, Retrieved from https://www.elakhaalliance.org/wp-content/uploads/2021/02/Kone-et-al-2021.pdf
- Miller, M. A., et. al. (2020) “Predators, Disease, and Environmental Change in the Nearshore Ecosystem: Mortality in Souther Sea Otters (Enhydra lutris nereis) from 1998 – 2012″, retrieved from https://www.frontiersin.org/articles/10.3389/fmars.2020.00582/full
- McKibben, S.M., et.al., 2017, “Climatic regulation of the neurotoxin domoic acid”, www.pnas.org
- “Sea Stars of the Pacific Northwest”, 2018, https://www.seastarsofthepacificnorthwest.info
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- “What is ocean acidification?”, retrieved from www.oceanservice.noaa.gov
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