Research Analysis

Growth

One of the most visible potential effects of undernutrition is on the size of an individual. The Steller sea lions at the Vancouver Aquarium are constantly measured to see how much they weigh and whether they are getting longer or stouter, leaner or fatter. Information on growth rates, drawn from twice-weekly measurements of body length and girth, as well as estimates of blubber depth, give an indication of how the costs of growth change, both during the year and with sea lion age. The sea lions at the Aquarium are providing insight into the effect of different diets on size and condition, and allowing researchers to develop measurements of sea lion health that can be applied to animals in the wild.

Metabolism

To determine whether Steller sea lions in the wild are obtaining sufficient energy from their food, it is necessary to quantify the amount of energy required for different activities, beginning with basic energy use, or resting metabolic rate. Researchers can then convert a sea lion’s behaviour into an estimate of the amount of food it needs. While the sea lions rest quietly inside a special “metabolic chamber,” scientists measure the amount of oxygen that the animal consumes and the amount of carbon dioxide produced, then convert the data into energy use estimates.

Results so far indicate that sea lions undergo developmental and seasonal changes in metabolism. They exhibit an initial decrease in metabolism within the first 18 months of life. Metabolism then decreases at a much lower rate over the next several years. Overlaid on these developmental trends is an emerging seasonal pattern of metabolic changes that increase in magnitude as the sea lion matures. There appears to be a peak in metabolism associated with the spring “fattening” period, and a period of low metabolism in the early winter. In addition to providing information on changes in the basic energy use of sea lions, data have also been incorporated into concurrent bioenergetic studies, serving as the backbone of a Steller sea lion computer model.

Keeping Warm

Previous studies with the captive Steller sea lions have measured the additional energy expended by to keep warm at different temperatures (thermoregulation). However, understanding the interactions between bioenergetic parameters is important for a realistic interpretation of the consequences of changes in an animal’s energy budget.

Using a “swim flume,” the marine counterpart of treadmill, Dr. David Rosen of the University of British Columbia and a team of Consortium scientists have been measuring the heat generated from digesting a meal, known as the heat increment of feeding (HIF), and the cost of thermoregulation. In this study, the apparent HIF was measured in the flume while water temperature was regulated between 2 and 10 degrees C. The metabolism was measured by having the sea lion hold steady in the swim flume before and after meals. In addition, the experiments measured the oxygen consumption of the sea lions while they were active within the swim flume. The results from these tests suggest that, contrary to predictions, juvenile Steller sea lions do not use heat generated through digestion to offset the increased thermoregulatory costs of decreasing water temperature.

Heart Rate

Until now, energy expended by Steller sea lions in the wild could only be calculated as a gross average calculated over an extended time period. Measuring heart rate of captive animals, however, can be used to calculate the energy expenditure of wild sea lions performing specific behaviours over short time periods. A project led by UBC researcher Jan McPhee has established a linear relationship between heart rate and oxygen consumption across various levels of activity in four captive Steller sea lions.

To find the relationship, the sea lions were outfitted with two subcutaneous electrodes and a harness housing a computerized data-logger for monitoring and recording heart rate. Oxygen consumption and heart rate data were collected simultaneously while an animal was at rest in the metabolic chamber or swimming against different water current speeds. From this relationship, recorded heart rate may be used to estimate metabolism and, thus, energy expenditure in the wild. However, further research is needed into how the relationship may change in a more natural environment before this monitoring method is used in the field.

Hydrodynamic Forces

A team of researchers has been studying the hydrodynamic forces encountered by Steller sea lions as part of the effort to estimate their energy requirements. Such information is vital if we are to understand the dietary choices of the species, and the possible effects of changes in diet associated with changing habitats and climate. A study carried out by Lei Lani Stelle involved videotaping six sea lions at the Aquarium as they glided past an observation window. Measurements of the change in velocity as the animals decelerated, along with each animal’s size, shape and mass, produced estimates of the drag coefficient for Steller sea lions.

The results of the experiments showed that drag coefficients for Steller sea lions lie within the typical range expected for marine animals – greater than that of some penguins, but less than values associated with bottlenose dolphins. In general, it appears that Steller sea lions experience relatively low levels of drag, resulting in a swimming performance similar to that of other otariids, including the California sea lion. They also appear to swim at close to an optimum speed, based on calculations for a minimum cost of transport. The next step is the development of models that can estimate the energetic costs of swimming and the contribution of swimming costs

Effects of Diet

The nutritional stress hypothesis is primarily concerned with what happens when sea lions are forced to change diets, from one dominated by fatty, high-energy prey, such as herring, to one with a larger proportion of leaner, lower-energy fish, such as pollock. Such a shift appears to be occurring in part of the North Pacific, and lies at the heart of the nutrition stress hypothesis. Consortium researchers recorded the metabolic and growth-rate effects of switching the diets of captive sea lions. Changes in body mass, morphology and condition, and overall activity were monitored. Blood samples were also taken to measure the clinical effects of the diets on iron and vitamin levels in blood. Results are expected in 2001.

Fasting Mechanisms

Given that changes in nutrition may be contributing to the decline of Steller sea lions in the wild, it is important to understand the physiological mechanisms they use to cope with periods of poor nutrition and to develop methods of detecting this condition in the wild. To that end, the Consortium undertook a joint research project with Dr. Lorrie Rea of the University of Central Florida to examine the effects of undernutrition on blood chemistry.

The study investigated whether sea lions are able to cope with food limitation equally well during all times of the year. Data from previous fasting trials suggested that sea lions experience faster rates of mass loss during the non-breeding seasons. Differences in blood chemistry also suggested that sea lions, which voluntarily fast during the breeding season, do not readily enter a fasting-adapted metabolic state when forced to fast outside of the breeding season. Paired fasting trials (breeding season and non-breeding season) conducted on four sea lions held at the Vancouver Aquarium supported the findings of this study.

Energetic Models

A complete explanation of the impact of predation on fish stocks requires a detailed understanding of the food requirements of the predators. Traditionally, researchers have relied on analysis of stomach contents from wild sea lions, but such research suffers from considerable logistical and economic hurdles. Captive studies, meanwhile, are often limited by small sample sizes and the difficulty in approximating conditions in the wild. A third option is the development of a bioenergetics model. Such a model for Steller sea lions in Alaska constituted a major portion of a study conducted by UBC’s Arliss Winship. Using existing physiological, diet and population data from a variety of sources, including captive sea lion studies, Winship constructed a model of how much food the sea lions needed at each stage of their lives and what those requirements imply for populations of prey.

Winship was able to provide estimates of the amount and type of fish necessary to support each population, in effect evaluating the nutritional stress hypothesis. The model showed that changes in the energy density of a sea lion’s diet leads to significant changes in the amount of fish the animal needs to eat. Sea lion populations from Southeast Alaska consumed fewer fish per individual during the breeding season than sea lions in the Aleutians. The trend is associated with the relatively low energy density of the main prey species in the western region compared with the high-energy prey in the east. On average, sea lions in the central Aleutian Islands had to eat 12 per cent more fish than did sea lions in Southeast Alaska.

The model predicts that a 10-year-old male needs between 20 and 32 kilograms of herring or other small schooling fish, which are high in energy content, each day. To get a comparable amount of energy from a diet comprised of pollock, cod or other gadids typically low in energy content, the same animal would have to consume between 32 and 48 kg of fish. Those differences may be even greater when digestive efficiency of each prey type is taken into account. A comparison of the amount of energy required to digest different types of fish was undertaken by Dr. Rosen, who concluded sea lions use more energy digesting pollock compared with herring and squid due to the high proportion of bony material in pollock.