by Cameron Douglas
Nearly driven to extinction in the early twentieth century, sea otters are not only enjoying healthier numbers but are also on a sort of instinctive crusade to help save the planet. A recent study has shown otters to be a crucial link in the chain of balance that occurs in estuaries.
Citing research by the U.S. National Academy of Sciences, the BBC reported last month that re-introduction of sea otters to the estuary at Elkhorn Slough (near Moss Landing) apparently has had a positive effect on marine vegetation, specifically sea grass. A group of scientists studied the increase and decline of sea grass at Elkhorn over the past 50 years. The only reason they could find for the recent recovery of sea grass at the slough is the presence of sea otters, which were recently re-introduced to the area.
Sea grass suffers from agricultural runoff that gets into the seabed, mostly nitrogen-rich fertilizers. Algae that goes unchecked grows to excess in this environment. The sea grass that does grow is deprived of sunlight when the algae starts growing on the leaves of the sea grass. Nature has a balance with tiny invertebrates that eat the algae. Just one problem: crabs feed on those invertebrates. If there are no invertebrates to consume the algae then the sea grass dies back.
The researchers suggested this is where sea otters come to the rescue. Otters eat the crabs, freeing the invertebrates to do their job on the algae, and so the sea grass can thrive.
It’s a complex chain of events; so to make sure, the researchers did comparison studies at similar estuaries with and without sea otters, and backed that up with lab and field experiments, which included blocking otters’ access to areas where sea grass was growing. The results confirmed the researchers’ hypothesis.
Sea grass is regarded as the “canary in the coal mine” in terms of predicting levels of adverse nutrients in the water. Sea grass serves other purposes. It acts as a nursery habitat for many species of fish, while it also uses CO2 from the air and water, potentially helping with climate change. In addition, sea grass helps protect the stability of the shoreline much the same as coral and kelp.
Since otters were re-introduced to Elkhorn a year ago, the sea grass there has recovered considerably. Brent Hughes, lead author of the study, states that Elkhorn has a heavy load of pollution coming into it — more than most systems in world — “yet you can still get this healthy, thriving habitat, and it’s all because of the sea otters…it’s almost like these sea otters are fighting the effects of poor water quality.”
Hughes hopes that coastal managers can benefit from this research and better understand the broader effects of sea otters.
Things you otter know…
Sea otters (Enhydra lutris) are native marine mammals of the northern and eastern Pacific Ocean. Unlike other marine mammals, the otter’s primary insulation comes from its exceptionally thick coat of fur (up to 150,000 strands of hair per cubic centimeter), the densest in the entire animal kingdom. They are the heaviest members of the weasel family, but among the smallest marine mammals. Otters can walk on land, but it’s a rare sight and they generally stick to the water.
Sea otters like to cruise near the shoreline, diving to the sea floor for food. A sea otter uses rocks to dislodge prey and open shells, making it one of few mammal species that use tools. It can be difficult to think of these furry, pixie-faced critters as predators, but technically they are. Otters prey mostly on sea urchins, mollusks, crustaceans and some species of fish. An otter must consume an estimated 25 to 38 percent of its own body weight each day to burn enough calories to offset body heat loss from the cold Pacific water. Otters have large kidneys that can process fresh water from salt water, allowing the otter to drink directly from the ocean.
Their educated pallet sometimes leads them into occasional conflict with humans when the otter steals a tasty catch. It gets worse when groups of otters are attracted to fisheries and fishing boats. However, gunshots and conflicts with fisheries make up only a tiny percentage of sea otter deaths, most of which are caused by shark bites, starvation, diseases, and the leading cause of otter deaths, worms.
The sea otter population is estimated to have been between 150,000-300,000 before they were hunted for their fur from 1741 to 1911. At that point there were only 1,000-2,000 individuals left, living in a fraction of the wide range they once occupied. An international ban on hunting, along with conservation efforts and reintroduction helped the population to recover and occupy about two thirds of their original range. This is considered an important success in marine conservation; however, otter numbers have declined again in the Aleutian Islands and California. Sea otters are still classified an endangered species.
