Leapin’ Lipids! In Search of the Ultimate Steller Diet

One of the key challenges for scientists is identifying the critical nutrients in the sea lion diet. Even a non-scientist can appreciate the difficulties associated with prompting a one-ton wild animal to change its feeding habits to allow scientists to test which foods are more nutritious. This, combined with the complexity and unpredictability of the North Pacific Ocean, makes dietary studies on wild sea lions very difficult. To address this problem, scientists have been conducting dietary studies in a controlled environment with the cooperation of a few Steller sea lions at the Vancouver Aquarium.

A new study by Drs. David Rosen and Andrew Trites of the University of British Columbia's Marine Mammal Research Unit, sought to better understand how changes in the chemical composition of prey might impact the health and physiology of sea lions. The results of the study, which included three different feeding experiments, were published in the May 2005 issue of the Journal of Comparative Physiology.

Looking at Lipids
Rosen and Trites studied the effect of high-lipid (fatty) versus low-lipid (lean) diets on the physiology of young sea lions. The study mimicked two very real scenarios faced by wild Steller sea lions: periods in which there is enough food to meet their energy requirements (a 'maintenance' diet), and periods in which food is limited (a 'submaintenance' diet).

In the first experiment, sea lions showed no significant physiological effects after being fed a maintenance diet of both high-lipid (herring) and low-lipid (walleye pollock) fishes. In the second experiment, however, in which the sea lions were fed a short submaintenance diet of high lipid (herring) and low lipid (Atka mackerel) prey, the results were more telling. While the sea lions lost the same amount of weight on both diets, they actually lost more of their critical lipid reserves while consuming the low-lipid prey (Atka mackerel; see accompanying Figure).

In the third experiment, the sea lions were fed a maintenance diet of high-lipid herring, as well as naturally lean pollock whose lipid content had been artificially raised, to determine whether the physical composition of the lipid was a key dietary factor. Again, the sea lions lost more of their own fat reserves while on the artificially enhanced pollock but gained fat on the naturally fatty herring, suggesting that some lipids are more readily absorbed than others.

Unexpected surprises
There were two big surprises during the second experiment when animals were forced to lose body weight, that hint at additional, unexpected consequences of inadequate diets. First, Rosen and Trites expected a drop in metabolism during the submaintenance diet. This so-called 'metabolic depression' is common in wild animals during both predictable (i.e. seasonal) and unexpected food shortages, and serves to limit weight loss during periods of low energy intake. During the trials, however, metabolic rates actually increased despite substantial weight loss. This suggests a natural 'foraging response' to hunger kicked in rather than a 'fasting response.' In a wild scenario, a foraging response triggered by a lack of available food may actually speed up starvation. However, Rosen and Trites caution that the exhibited 'foraging response' could have been triggered by training or other human-induced factors.

Scientist had also routinely believed that seals and sea lions lost almost all of their weight from their internal fat (blubber) reserves. Therefore, changes in relative body condition (that is, lipid mass as a percentage of total body mass) were assumed to be a good measure of general health. However, Rosen and Trites found that changes in blubber accounted for only about half of the sea lions’ weight loss. It seems that the sea lions broke down a "surprising amount" of body tissue to meet their energy requirements. This meant that, although the animals were obviously nutritionally stressed, their body condition changed very little. This led Rosen and Trites to question whether relative body condition is the best indicator of nutritional stress in wild Steller sea lions. Current research is directed at fine-tuning a more accurate physiological index for both captive and wild studies.

Simple questions not so simple
The study's combined results suggest that sea lions can function on a low-lipid diet when food is abundant but when food is limited, a low-lipid diet promotes rapid blubber loss. However, scientists also realise that the response of sea lions to changes in their prey are extremely complex. Several studies, including this one, have highlighted that the effect of prey quality on sea lion physiology differs according to the season and the age and sex of the animal. Also, the health effects of different diets can be measured by a variety of different means, from changes in body and fat mass down to changes in cellular biochemistry.
14 June 2005

Publication:
Examining the potential for nutritional stress in young Steller sea lions: physiological effects of prey composition. Rosen, D.A.S. and A.W. Trites. 2005. Journal of Comparative Physiology 175:265-273. abstract The effects of high- and low-lipid prey on the body mass, body condition, and metabolic rates of young captive Steller sea lions (Eumetopias jubatus) were examined to better understand how changes in prey composition might impact the physiology and health of wild sea lions and contribute to their population decline. Results of three feeding experiments suggest that prey lipid content did not significantly affect body mass or relative body condition (lipid mass as a percent of total mass) when sea lions could consume sufficient prey to meet their energy needs. However, when energy intake was insufficient to meet daily requirements, sea lions lost more lipid mass (9.16±1.80 kg±SE) consuming low-lipid prey compared with eating high-lipid prey (6.52±1.65 kg). Similarly, the sea lions lost 2.7±0.9 kg of lipid mass while consuming oil-supplemented pollock at maintenance energy levels but gained 5.2±2.7 kg lipid mass while consuming identical energetic levels of herring. Contrary to expectations, there was a 9.7±1.8% increase in metabolism during mass loss on submaintenance diets. Relative body condition decreased only 3.7±3.8% during periods of imposed nutritional stress, despite a 10.4±4.8% decrease in body mass. These findings raise questions regarding the efficacy of measures of relative body condition to detect such changes in nutritional status among wild animals. The results of these three experiments suggest that prey composition can have additional effects on sea lion energy stores beyond the direct effects of insufficient energy intake.