Spying on sea otter activities lets biologists measure populations of other species in the otters’ habitat. “It’s just a lot of work to get densities of clams and marine species like that,” described wildlife biologist Daniel Monson. To get a better idea of ecosystem health, scientists can use Unmanned Aerial Vehicle (UAV) technology to take a look at what sea otters are eating.
Otters explore the sea floor for available food before surfacing to eat their prey. “If the otters are basically sampling it for you, and telling you what the availability is, it’s a much more efficient way to get at that type of habitat information,” Monson said.
Daniel Monson, research wildlife biologist with the U.S. Geological Survey’s Alaska Science Center in Anchorage, Alaska, has studied sea otters since 1987.
USGS Alaska researchers collaborated with Brenda Konar, professor of marine biology at the University of Alaska Fairbanks School of Fisheries and Ocean Sciences, to test using Unmanned Aircraft Systems (UASs) to fly over sea otters in Kachemak Bay, Alaska, and record video of what the otters were eating. During the pilot test they flew a rotary wing UAV and took advantage of the technology’s stare-and-hover capabilities to record otters in their natural environment.
The scientists found that with careful flying it is possible to hover over otters without disturbing them: an important requirement when they’re collecting information about natural foraging habits. They were able to fly the remote-controlled small aircraft as close as 10-15 meters [33-50 feet] over otters without disturbing them. Monson: “At 10-15 often the animals didn’t seem to notice the UAS, especially if there was any tiny bit of breeze, or if they were busy.”
Otters are a keystone species with a large impact on their environment. Their diets and the prey they eat guide conditions in kelp forests. When sea otters aren’t present to keep sea urchin populations in check, urchins can munch plants so aggressively that annual kelp forests can’t establish themselves– instead of kelp forests they enforce ‘urchin barrens’. When otters are present they dive and forage on the sea floor to catch food: sea urchins, clams, crabs, fish, octopus and squid, snails and more benthic (sea floor) invertebrates. Otters can hold their breath over 5 minutes during these dives, but bring prey back to the surface and float on their backs while eating. Staying warm in Alaskan waters requires lots of energy gained by eating, and otters eat a lot: sea otters in Alaska eat food weighing an average of 25% of their body weight daily. Have you seen otters cracking hard-shelled prey open using rocks while floating on their backs?
Recording otters eating
Sea otter observations are usually either based on shore where scopes can be set up on solid ground, or are based on spotting sea otters from the passenger seat of small aircraft. Monson described “When you fly a survey, a manned survey,” and “Similarly in foraging observations, you do that in real time.” A researcher has to very quickly write down what they see and estimate the number of items otters have caught. What species is the prey? What size? Monson described having to do all of that in real time while otters eat and swim. “You are not able to ever recheck it. You do your best when you are up there, but you can’t go back.”
With new techniques, scientists hope to record footage of sea otters foraging further from shore. “It would be a big advantage to have all the data recorded,” Monson said. Video cameras mounted on flying UAVs can help study otter diet. “Now we’ll be able to do that on a video screen and you can even use image analysis techniques,” Monson explained. “You could go back and check it again, you could play it slow motion, you could do frame-by-frame analysis.” With the UAVs’ cameras “Recording images, the images themselves become the data and you can do all of your analysis in a post-processing way.”
With the high mobility of UAVs and their cameras’ views, Monson hopes to “Watch them feed and get that same foraging information and same energy recovery information from offshore locations,” instead of from scopes’ limited sights. Monson: “That’s a hole in our foraging data that we’ve always known we had and we didn’t have a way to get around it in the past. And so now we might have a tool here in a rotor wing UAS to get unbiased foraging information.” Indeed, otters might be eating a different diet when they hunt in deep waters. Monson stated “They might feed on a lot smaller things near shore where it is shallow,” whereas offshore “They don’t really dive deep unless they are getting kind of a big pay off. So if you are offshore and diving deep, you are more likely to be getting bigger things.”
I’m amazed to see an otter in the video ‘UAVs over Otters’ eating a whole octopus arm.
Energy recovery rates
Watching adorable tummy feeding is very useful for understanding a habitat’s limits. Monson told Frontier Scientists that for the scientists investigating sea otters in Kachemak Bay, Alaska, learning “Energy recovery rates is our primary goal and that tells us a lot about the status of the habitat and the status of food resources.” Researchers can get a grasp on the local population health of prey species based on how often otters surface with i.e. a specific species of crab. And for otter population health, spotting sea otters foraging allows scientists like Monson to calculate energy recovery rates.
Monson described energy recovery rate as a “Useful metric in terms of thinking about where the population is relative to the carrying capacity of the habitat.” When a habitat has many resources– plenty of healthy plants and prey species– it can support more otters. “The animals can get all of the energy they need in a relatively short time, so their energy recovery rate is really high.” On the other hand, when otters can’t find food easily their energy recovery rate is low.
A sea otter would ideally forage and eat during a short amount of time during the day, leaving the rest of its time for socializing or taking care of pups. Monson said “Their energy budget and thermal budget depends on quite a bit of resting time. They’ll feed, then the energy that’s produced while digesting keeps them warm while they rest.”
Video footage from UAV cameras can allow Monson and colleagues to very precisely estimate the size of food items “And convert that to a caloric value,” filling them in on energy recovery rates. “One thing about sea otters and wildlife in general is: they don’t really eat to excess,” Monson said. While they do want some energy reserves ‘on board,’ they don’t want excess reserves. “It can become detrimental if you have too much energy on board.”
As otter population numbers increase, their habitat eventually reaches carrying capacity– the maximum population size of the species that the environment can sustain. Only a certain number of otters as can realistically survive on the resources of a habitat. “As they have hit carrying capacity, their energy recovery rate drops down,” Monson said. “Prey tends to be harder to find, obviously: smaller and fewer in number. So they cannot bring up as many things and as big of things.” It takes more time, effort and energy to find food.
An otter cannot spend more than half a day seeking food. “If they can’t get their energy requirements in 12 hours, they are really in trouble,” Monson said. So knowing where an otter population sits in the continuum of energy recover rates, and how many hours it takes for a healthy otter to consume enough food every day, “Gives you a scale of where they are at relative to the carrying capacity of the habitat.”
Laura Nielsen 2016
Frontier Scientists: presenting scientific discovery in the Arctic and beyond
(Sea otters energy recovery rates UAVs habitat health)