CLUES FROM SEA OTTERS
Scientists sometimes use instruments or robotic probes to collect samples and data on environmental quality. In a sense, each individual sea otter is a living, breathing probe constantly accumulating data about the coastal waters it lives in.
Sea otters are an
apex predator of the nearshore ecosystem
. This means that they're at the center of the local
food web, and every other link in this web connects back to otters. Sunlight and nutrients are absorbed by tiny algae and giant kelp, and these plants are eaten by shellfish, which in turn are eaten by otters.
It's good to be an apex predator, but it also means that any contamination or damage to even the most distant links of the food web could have an impact on your health. So if an apex predator's health is suffering, that's a clue to problems in the ecosystem.
To gather these clues, project scientists sail to study locations from California to Alaska to capture wild sea otters. With careful, expert handling by project divers and veterinarians, wild otters are safely captured, anesthetized and given a full health physical. And just as with human patients, project scientists take biopsies and record observations as clues:
Blood, Skin and Genes
Police detectives often collect DNA during investigations, and so are our project scientists. Skin and blood samples from otters yield DNA for an exciting tool called
gene transcription analysis -- which can show whether an otter has been exposed to
stressors such as oil spills, toxins, tumors and viruses. Here's how.
Remember that the
genes of an organism (be it human or otter) is its body's instruction manual -- the genes in our DNA tell our body how to react and what molecules to make as it encounters stress and disease. Specific genes are activated byspecific stressors, and each time a gene is "transcribed" to react to a stressor, it leaves a unique signal.
That means by analyzing gene transcription clues, we can figure out a list of stressors ailing each otter. Scientists can use this as a starting point to explore what the sources of those stressors are in the nearshore waters.
Scientists will also analyze blood samples for serums, antibodies and other molecules, which also hint at the health condition of each otter.
Feeding and Diet
As apex predators, sea otters eat
a lot -- a 75-pound otter can eat about 1,500 sea urchins a day, or about 25 pounds of seafood. They need to eat a lot to sustain their unusually high energy levels as they survive the cold, 45 to 60 degree Fahrenheit waters and hunt in depths up to 300 feet for prey like crabs,
sea urchins and octopus.
So if an otter is not getting enough nutrition for its energy needs, that can provide clues about the productivity of the nearshore ecosystem, and the types of stresses faced by sea otters. For example, when preferred prey becomes scarce, sea otters broaden their diet to include lower-quality prey, which in some cases may expose them to new disease-causing parasites.
Fortunately, sea otters make diet observations easy.
Otters only eat their prey at the surface and typically hunt within sight of the shoreline. This means that scientists can watch individual otters through a high-power telescope on shore, measure their dive times and record the species and size of prey being eaten. From maps, scientists can estimate otter diving depths and energy used, and from lab studies they can estimate the nutritional and caloric gains from each prey species. Combine all this information in a powerful computer model, and you can calculate each otter's daily energy budget.
Whiskers and Isotopes
Of course, scientists can't observe otters every single day and record their long-term feeding habits. That's where stable isotopes come in.
In nature, chemical elements like carbon and nitrogen can exist in different masses -- think of them as "heavy" and "lite" versions. We call these different versions isotopes. Your body can contain both isotopes of the same element, but the cool thing is that the ratio of regular to light isotopes in each organism's body depends on its position in the food web, also called its trophic level. For example, if you eat a lot of sea urchins -- which only eat plants -- you feed at a lower trophic level and your body will a have a certain stable isotope ratio. If you eat a lot of octopus -- which eat crabs and other animals that eat other animals -- you feed at a high trophic level and your body will have a different stable isotope ratio."
We take whiskers (also called vibrissae) to study isotope ratio because whiskers grow slowly over an otter's lifetime. Analyze a segment near the tip of the whisker and you can guess at an otter's trophic level and diet when it was younger; analyze a segment near the base to guess at trends in more recent months.
Body Condition
Just as doctors measure Body Mass Index (BMI) in humans, project scientists measure the mass and length of sea otters to calculate their body condition.
Body condition data help us understand whether otters are malnutritioned or healthy in each study location. If the animals are well-fed, healthy and robust, they will have a high weight relative to their bodylength. If food is scarce, they will weigh less.
Combined with stable isotope and diet analysis, body condition data paint the long-term picture of an otter's place in the local food web. If we know a certain prey species has poor nutritional and caloric value, yet we discover that otters continue to feed on that prey species, that could be a clue to ecological problems in these waters. It's like finding out what's on the shelf at the otters' local supermarket -- are the shelves full or empty, and if they're full, it is full of nutritious food options, or not?
Strandings
Not all clues come from living otters. When sea otters die, they are often found stranded on a beach or floating in the waves. And as in police investigations, otter bodies are given a complete necropsy to collect clues.
Instead of a coroner, otter bodies are sent to wildlife laboratories. There, veterinarians try to identify a cause of death -- perhaps parasites, shark bite or algal toxins -- and assess the otter's health condition before death. The number of stranded bodies counted each year also given scientists an estimate of how many otters die from non-predator causes. Altogether, necropsy results give scientists more clues to track down abnormalities in the nearshore ecosystem.
Explore more clues below, or meet the research team:
Updated 2011.06.13
U.S. Department of the Interior |
U.S. Geological Survey