Long-term studies quantify the prey requirements of pinnipeds, and help predict the effects of nutritional stress.

Long-term studies quantify the prey requirements of pinnipeds, and help predict the effects of nutritional stress.
Marine Mammal Research Unit

Two new studies answer the question: “How much fish does a seal need?”

One of the most common threats to marine mammal populations is reduced food supply, either due to the effects of climate change or human fishing pressure. The inability to acquire adequate prey has been a leading hypothesis to explain the decades-long decline of the endangered Steller sea lion.

Rapid environmental changes are predicted to negatively affect Arctic seals through a combination of sea ice loss, changes in their prey base, and increases in the costs for foraging and resting.

Bearded seal Long Marine Lab. NMFS Permit #18902. Photo Colleen Reichmuth

Central to understanding how environmental changes will affect individual animals and their populations, is the need to determine how much food marine mammals require, how prey demands change during the year, whether there are critical times of year when securing adequate food is more important, and what the physiological consequences are of not catching enough fish. Unfortunately, it is difficult to answer these questions for wild pinnipeds.

Fortunately, invaluable data can be obtained from seals and sea lions maintained under human care. Dr. David Rosen, an Assistant Professor at UBC’s MMRU/IOF, recently published two papers that directly addressed  questions related to the food needs of seals and sea lions and the consequences of undernutrition.

Spotted seal Alaska SeaLife Center. NMFS Permit #18902. Photo Madeline Meranda

In one of the papers published in Aquatic Mammals, Dr. Rosen and colleagues Dr. Collen Reichmuth (UCSC) and Dr. Nicole Thometz (USF) continuously measured the food intake and body mass for three ice-dependent Arctic species of seals — ringed seals, bearded seals, and spotted seals — over a 9-year period. They assessed how body mass and food intake changed as the seals matured from pups to adults, and also identified predictable seasonal cycles where both mass and food intake varied during the year.

Although maintained in research facilities, the ringed seals, bearded seals,  and spotted seals mimicked the same hormonally-driven changes in physiology believed to occur in wild seals. This allowed the research team to identify potential critical periods when the seals relied on ingesting large amounts of fish needed to restore vital fat stores lost during earlier periods (when they ate less and were more active), as well as prepare for future seasons when on-board fat reserves are essential to survival and reproduction.

Their research raises the question of “what happens when an animal doesn’t get enough food”? In some ways the answer is obvious — as with any other animal, generally eating less food means losing body mass. But Dr. Rosen wondered whether there was a way of predicting exactly how much mass the animal would lose — that is, could the effect of a certain level of nutritional stress on the growth of a seal or sea lion be quantified?

Ringed seal Alaska SeaLife Center. NMFS Permit #18902. Photo Colleen Reichmuth

The ability to predict the effect of a certain level of nutritional stress on an individual animal’s physiology is central to modern species conservation models. Dr. Rosen realized that the answer might lay buried in data collected during numerous feeding studies conducted with groups of Steller sea lions maintained at the Vancouver Aquarium over the past 2 decades.

By combing through the records of dozens of previous trials, Dr. Rosen uncovered a mathematical relationship predicting how a certain level of underfeeding would decrease the normal growth rates of sea lions. Even better, the equation, recently published in Marine Mammal Science, was robust enough to be applied to animals across a broad range of ages and during different times of the year.

Dr. Rosen feels that the findings from both of these studies are exactly the type of hard data needed to formulate science-based management plans to aid in the recovery and conservation of pinniped species. They also demonstrate the tremendous value that animals maintained under human care contribute to conservation efforts of their wild counterparts.

The work was also recently presented at the Alaska Marine Science Symposium (watch the video below).


Dr David Rosen is an Assistant Professor at University of British Columbia’s MMRU/IOF


Molting strategies of Arctic seals drive annual patterns in metabolism.
Thometz, N.M., H. Hermann-Sorensen, B. Russell, D.A.S. Rosen and C. Reichmuth. 2021.
Conservation Physiology, 9(1), coaa112.
Arctic seals, including spotted (Phoca largha), ringed (Pusa hispida), and bearded (Erignathus barbatus) seals, are directly affected by sea ice loss. These species use sea ice as a haul-out substrate for various critical functions, including their annual molt. Continued environmental warming will inevitably alter the routine behavior and overall energy budgets of Arctic seals, but it is difficult to quantify these impacts as their metabolic requirements are not well known—due in part to the difficulty of studying wild individuals. Thus, data pertaining to species-specific energy demands is urgently needed to better understand the physiological consequences of rapid environmental change. We used open-flow respirometry over a four-year period to track fine-scale, longitudinal changes in the resting metabolic rate (RMR) of four spotted, three ringed, and one bearded seal trained to participate in research. Simultaneously, we collected complementary physiological and environmental data. Species-specific metabolic demands followed expected patterns based on body size, with the largest species, the bearded seal, exhibiting the highest absolute RMR (0.48±0.04 L O2 min-1) and the lowest mass-specific RMR (4.10±0.47 ml O2 min-1 kg-1), followed by spotted (absolute: 0.33±0.07 L O2 min-1; mass-specific: 6.13±0.73 ml O2 min-1 kg-1) and ringed (absolute: 0.20±0.04 L O2 min-1; mass-specific: 7.01±1.38 ml O2 min-1 kg-1) seals. Further, we observed clear and consistent annual patterns in RMR that related to the distinct molting strategies of each species. For species that molted over relatively short intervals—spotted (33±4 days) and ringed (28±6 days) seals—metabolic demands increased markedly in association with molt. In contrast, the bearded seal exhibited a prolonged molting strategy (119±2 days), which appeared to limit the overall cost of molting as indicated by a relatively stable annual RMR. These findings highlight energetic trade-offs associated with different molting strategies and provide quantitative data that can be used to assess species-specific vulnerabilities to changing conditions.

keywords     Arctic seals, spotted seal, ringed seal, bearded seal, sea ice, molt, climate change, respirometry, resting metabolic rate, energetic trade-offs
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Seasonal and developmental patterns of energy intake and growth in Alaskan ice seals.
Rosen, D.A.S., N.M. Thometz and C. Reichmuth. 2021.
Aquatic Mammals 47(6):559-573.
Quantifying the energy needs of individual animals and understanding the relationship between food intake and physical growth are necessary to determine species-level food requirements and model potential responses to changing environmental conditions. To provide fine-scale information about developmental and seasonal patterns in the energetic requirements of Arctic phocid seals, we documented longitudinal changes in food intake, body mass, and standard length in four spotted seals (Phoca largha), three ringed seals (Pusa hispida), and two bearded seals (Erignathus barbatus). The seals were studied for up to 9 years in sub-Arctic and/or temperate climates while living under human care. Seals were fed using behavioral criteria that allowed their food intake and body mass to vary naturally. Gross energy intake (GEI) increased with age in all species, reaching a plateau as seals matured. GEI was greatest for the largest species (bearded seals) and lowest for the smallest (ringed seals). Mass-specific GEI declined with age, and was similar between spotted and ringed seals, with bearded seals consuming about half that of the smaller species. Overlaid upon long-term developmental changes were predictable seasonal cycles in food intake and body mass which became more pronounced as seals matured. Seasonal cycles in food intake and body mass did not always reflect simple cause-and-effect relationships. For example, seasonal peaks in food intake were regularly associated with simultaneous declines in body mass. The consistency of energy intake patterns, despite seals being maintained in semi-artificial conditions in different local climates, supports the hypothesis that seasonal oscillations are guided by underlying hormonal changes linked to key life history events and mediated by the physical environment. The described physiological patterns serve to highlight times of year when free-ranging Arctic seals may be more sensitive to environmental perturbations.

keywords     Phoca largha, Pusa hispida, Erignathus barbatus, food intake, caloric intake, body mass, growth, development, environmental conditions, nutrition
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