These furry marine mammals seem to have bottomless stomachs — what does that mean for the ecosystem around them?
Eating, sleeping, playing, and giving birth in the sea; sea otters can spend their whole lives in the water without ever venturing to shore. Well known for their skillful ability to crack open shellfish with rocks, these furry creatures are ecosystem engineers whose voracious appetites can significantly impact the kelp forests and rocky intertidal zones they frequent.
Exactly how large are those appetites? A recent study published in Aquatic Mammals quantified the food and energy intake data of ten sea otters, located at the Vancouver Aquarium, from husbandry records spanning three decades. Wild sea otters are difficult to monitor over long periods of time, and quantifying managed sea otters’ food biomass and energy intake can be a valuable way to understand their wild relatives’ potential ecological and population impacts on ecosystems, and how they may be affected by future environmental changes.
Sea otters eating urchins at the Vancouver Aquarium. Photo credit: Juliana Kirkelie-Kim
“Sea otters are considered a “keystone species,” which essentially means that they have a disproportionate impact on their local ecosystems compared to other species within that same ecosystem,” said Julia Adelsheim, co-author and M.Sc. student in the Marine Mammal Research Unit at the Institute for the Oceans and Fisheries. “This study demonstrated that one of the reasons they have such a large impact is because they require a large amount of food on a daily basis.”
Sea otters live along the rocky, tidal zones lining the Pacific coast of North America and dive for food within one to two kilometres offshore. They can dive to anywhere from 10 to 100 metres deep, using the kelp forests and reefs for hunting prey and for rest and shelter. They have great impacts on species living below the ocean surface, often helping to enhance local biodiversity and provide positive effects to nearby coastal communities and fisheries.
“One of the ways they affect their environment is by preying on sea urchins hiding in kelp forests, which provide valuable habitat for other aquatic species,” said Shannia Iskandar, lead author and graduate of the Applied Animal Biology program at the University of British Columbia. “Urchins are a gluttonous predator that consumes vast amounts of small kelp plants. They also happen to be one of the sea otters’ favourite foods, and are regularly preyed upon, which helps keep them in check and facilitate the growth of young kelp.”
Sea otter eating at the Vancouver Aquarium. Photo credit: Juliana Kirkelie-Kim
Young sea otters eat the most food relative to their size, with their food mass equivalent to around 26 per cent of their total body weight. Food-to-body mass ratios decline as the otters grew older, leading to only a 20 per cent ratio in adult otters. Male aquarium sea otters were also 25 to 42 per cent larger than females, and their energy intakes were 23 to 58 per cent higher.
“We also showed that, while sea otters do consume more food compared to other marine mammals in aquariums, it is largely because their prey contains less energy,” said David Rosen, senior author and assistant professor at the Marine Mammal Research Unit at the Institute for the Oceans and Fisheries. “The amount of energy required to ‘power’ an aquarium otter is only five to fifteen per cent more than for other marine mammals such as seals and sea lions. Despite having different lifestyles, the energy intake of the aquarium otters was only slightly lower to that estimated in wild otter populations.”
The sea otters under study showed the general mammalian model — seen in dolphins, killer whales, and seals — where relative food intake is highest while the pups are young and growing very rapidly. This also presents challenges to their mother. The mothers of pups often spend most of their energy finding food for their offspring, often at the expense of their own health. Higher food intake needs may also be due to thermoregulation, as pups need extra energy to stay warm. As with most young animals, sea otter pups also spend the majority of their time playing, swimming, and learning; all energy-intensive activities. As the otters get older, their food requirements slowly level to more sustainable levels as they become more adept at diving and foraging, with male otters needing slightly more energy intake than female otters.
Sea otter eating at the Vancouver Aquarium. Photo credit: Juliana Kirkelie-Kim
“Understanding the food intake and energy requirements of managed sea otters, and how sex and age influence such requirements, can help us more accurately estimate the prey needs of wild sea otters,” said Iskandar. “This type of data is beneficial for the conservation of this species, especially when certain local populations are declining or are stagnant, and will allow us to assess whether resource availability is limiting populations, and help us predict the effect of future ocean environments and human activities.”
With worsening climate change events, the future of sea otters in the Pacific Northwest is uncertain. With a narrow range of habitats, sea otter populations may be put at risk, also causing negative impacts on other marine species around them. “We saw the devastating impacts of climate events during the 2021 summer heat wave on coastal and rocky intertidal ecosystems here in BC,” said Adelsheim. “There was widespread mortality of many of the sea otters’ preferred prey. Like many species globally, climate change is sure to have an effect on populations of sea otters.”
The effects of age and sex on the energy intake of captive sea otters (Enhydra lutris): implications for captive management and species conservation was published in Aquatic Mammals
Tags: David Rosen, IOF students, Kelp forests, Marine Mammal Research Unit, Marine mammals, MMRU, Pacific Ocean, Sea otters, Sea urchins, Tidal Zones, Vancouver Aquarium