Cool Waters, Warm Bodies

Cool Waters, Warm Bodies
Marine Mammal Research Unit

image caption: Two northern fur seal pups amongst sleeping adult females on the Pribilof Islands.

Are young northern fur seals vulnerable to changes in ocean temperature?

Each fall, thousands of northern fur seals leave their summer island homes and take to the chilly waters of the Bering Sea to begin their annual winter migration. Most will follow a route that takes them into the North Pacific Ocean, with some traveling as far south as California, through a wide range of ocean temperatures.

For the most vulnerable members of this migration—the four-month-old pups beginning their life at sea—survival will depend on their ability to conserve energy, stay warm, and catch enough fish to keep them going until they return to their natal rookeries almost two years later. Two new Consortium studies, recently published in Marine Mammal Science, investigated the ability of young fur seals to regulate their internal temperature across a range of ocean temperatures.
“Northern fur seals are small, naïve animals that look like they should not be able to survive in the Bering Sea, except that they have these incredibly thick fur coats,” says Dr. David Rosen, a co-investigator in both studies. “Our main question was whether cooler water temperatures might cause young and vulnerable fur seals to require more energy to stay warm. We also wanted to know whether water temperatures might determine where they are found in the ocean.”
Thermal Neutral Zone
The two studies progressively followed six female northern fur seals, raised at the Vancouver Aquarium, through their first four years of life. Using a range of temperatures that northern fur seals might naturally encounter in the wild, the scientists sought to establish a “thermal neutral zone” (TNZ) for younger northern fur seals—the range of temperatures within which they do not need to expend any additional metabolic energy to stay warm or cool down. The TNZ varies across marine mammal species and has not been well studied in fur seals.

Michi, one of the young fur seals that participated in the study at the Vancouver Aquarium

Meechi, one of the young fur seals that participated in the study at the Vancouver Aquarium

“The northern fur seal pups seemed to have a broad TNZ,” says Alex Dalton, lead author of one of the studies, and who recently completed a Master’s of Zoology under Dr. Rosen and Dr. Andrew Trites. “For the most part, the older pups were  thermally neutral within water temperatures of 2-18 degrees Celsius throughout the year. This means that the range of water temperatures they would encounter throughout their migration is probably not placing any additional energetic demands on them.”
Dalton emphasizes that these studies are different from past TNZ research on northern fur seals, because they took place over four years, which allowed the scientists to gather data across all seasons and look for differences as the pups grew. “In the first two years of life, our study suggests that the fur seals would have to expend additional energy to keep warm in water below approximately 4-7 degrees Celsius”, notes Dr. Rosen. “However, our follow-up study of the fur seals when they were a few years older shows an increased ability to withstand much colder water temperatures.”
Driving Migration
Rosen believes that several factors seem to trigger the fur seals’ annual migration,  including storms and cooler air temperatures. He believes cooler sea surface temperatures may also play a role.
“Most younger fur seals probably could not survive winter in the Bering Sea,” Rosen explains. “The cooler surface temperatures may help to initiate the migration but they did not seem to be a driving factor in determining the route. In other words, the lower thermal limit did not match the migration pattern, which suggests they are migrating well within their thermal neutral zone and not just trying to stay ahead of the cold water.”

 Monthly average sea surface temperatures recorded in November between the Bering Sea (2 C at top of map) and Hawaii (22 C - bottom of map). Most fur seal pups are in the Bering Sea in November (dashed circle) and migrate into the warmer waters of the North Pacific Ocean as winter approaches.

Monthly average sea surface temperatures recorded in November between the Bering Sea (2 °C at top of map) and Hawaii (22 °C – bottom of map). Most fur seal pups are in the Bering Sea in November (dashed circle) and migrate into the warmer waters of the North Pacific Ocean as winter approaches.

If thermoregulation is not driving the migration route, except under extreme environments, Rosen believes the probable motivation is food sources.
“This relates back to a key question of whether a decrease in the availability of prey is adding to the decline in northern fur seal populations in the Bering Sea, as opposed to changes in ocean climate or more frequent winter storms,” Rosen says. He is also the lead author in a follow-up study recently published in Conservation Physiology, which found that nutritionally stressed young northern fur seals are better able to regulate their temperature in winter than in summer.
Dalton is interested in following up his study by investigating the reasons for the differences between thermoregulation abilities of northern fur seals versus sea otters, which are equally well known for their thick fur coats but significantly higher energetic requirements.


Broad thermal capacity facilitates the primarily pelagic existence of northern fur seals (Callorhinus ursinus).
Dalton, A.J.M., D.A.S. Rosen and A.W. Trites. 2014.
Marine Mammal Science 30:994-1013.
Thermoregulatory capacity may constrain the distribution of marine mammals despite having anatomical and physiological adaptations to compensate for the thermal challenges of an aquatic lifestyle. We tested whether subadult female northern fur seals (Callorhinus ursinus) experience increased thermoregulatory costs in water temperatures potentially encountered during their annual migration in the Bering Sea and North Pacific Ocean. Metabolic rates were measured seasonally in 6 captive female northern fur seals (2.75 to 3.5 yr old) in ambient air and controlled water temperatures of 2, 10, and 18 °C. Rates of oxygen consumption in ambient air (1 – 18 °C) were not related to environmental temperature except below 2.5 °C (winter only). However, metabolism was significantly higher during the fall seasonal trials (Sept – Oct) compared to other times of year, perhaps due to the costs of molting. The fur seals appeared thermally neutral in all seasons for all water temperat ures tested (2 – 18 °C) except during the summer when metabolic rates were higher in the 2 °C water. Comparing this broad thermal neutral zone to the average sea surface temperatures potentially encountered during annual migrations indicates wild fur seals can likely exploit a large geographic area without added thermal metabolic costs.
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Thermal limits in young northern fur seals, Callorhinus ursinus.
Rosen, D.A.S. and A.W. Trites. 2014.
Marine Mammal Science 30(3):1014-1028.
The thermoregulatory abilities of northern fur seals (Callorhinus ursinus) during their first two years in the frigid waters of the North Pacific Ocean may limit their geographic distribution and alter the costs for exploiting different species of prey. We determined the thermoneutral zone of 6 young northern fur seals by measuring their metabolism in ambient air and controlled water temperatures (0-12 °C) from ages 8 to 24 mo. We found that the ambient air temperatures within our study (overall 1.5-23.9 °C) did not affect resting metabolic rates. Calculated lower critical temperatures in water varied between 3.9 and 8.0 °C, while an upper critical temperature in water was only discernible during a single set of trials. These thermal responses provide insight into the possible physiological constraints on foraging ecology in young northern fur seals, as well as the potential energetic consequences of ocean climate change and altered prey distributions.

keywords     Northern fur seal, Callorhinus ursinus, thermoregulation, metabolism, bioenergetics
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Short-term episodes of imposed fasting have a greater effect on young northern fur seals (Callorhinus ursinus) in summer than in winter.
Rosen, D. A. S., B. L. Volpov and A. W. Trites. 2014.
Conservation Physiology 2:1-9.
Unexpected shortages of food may affect wildlife differently depending on the time of year it occurs. We imposed 48-hr fasts on six female northern fur seals (Callorhinus ursinus; ages 6 ? 24 months) to identify times of year when they might be particularly sensitive to interruptions in food supply. We monitored changes in their resting metabolic rates and their metabolic response to thermal challenges, and also examined potential bioenergetic causes for seasonal differences in body mass loss. Pre-fast metabolism of the fur seals while in ambient air or submerged in 4 ?C water was higher during summer (Jun-Sep) than winter (Nov-Mar), and submergence did not significantly increase metabolism indicating a lack of additional thermoregulatory costs. There was no evidence of metabolic depression following the fasting periods, nor did metabolism increase during the post-fast thermal challenge, suggesting that mass loss did not negatively impact thermoregulatory capacity. However, the fur seals lost mass at greater rates while fasting during the summer months when metabolism is normally high to facilitate faster growth rates (which would ordinarily have been supported by higher food intake levels). Our findings suggest that summer is a more critical time of year than winter for young northern fur seals to obtain adequate nutrition.
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