MARINE MAMMAL RESEARCH NEWSLETTER | February 2015 (Issue 12)
INTO THE LAB
How important is diet diversity to northern fur seals?
Eating a “balanced” meal composed of various food items is an essential message of proper human nutrition. In the wild, many pinnipeds (seals, sea lions, and walrus) also consume a variety of prey species. Whether this is because they are optimizing their nutritional intake or simply eating what they encounter is unknown.
Knowing the significance of diet diversity is important given the changes observed in fish populations in certain parts of the North Pacific Ocean where several pinniped species have declined. The diet of some of these pinnipeds — such as northern fur seals and Steller sea lions — are characterized by single fish species that dominate their local ecosystems. At the same time, many of these predator populations are decreasing. Many wonder whether these two observations are linked, and whether a lack of diet diversity might be having negative health impacts on individuals in the population.
The nutritional importance of a diversity of prey items in the diet of northern fur seals is being investigated by Mariana Diaz Gomez as part of her MSc work at the University of British Columbia. Mariana manipulated the diets of a group of northern fur seals held at UBC’s Marine Mammal Energetics and Nutrition Lab located at the Vancouver Aquarium to determine whether they can assimilate more energy from mixed diets than they can obtain from single species diets that contain the same gross energy. Marianna is also examining the physiological effects that different diets can have on the fur seals’ growth, metabolism, blood chemistry, and ability to thermoregulate.
Mariana notes, “with Steller sea lions, there is a statistical relationship between the uniformity of their summer diet and how well the local population is doing. There are also laboratory studies with true seals that indicate that mixed diets provide more energy than you’d expect. However, the potential nutritional benefit of a mixed diet has never been explicitly tested with sea lions and fur seals.”
Over the course of 6 months, Mariana alternated the diets of the fur seals between combinations of herring, pollock, capelin, and squid—either fed as a mixed meal or on their own.
In addition to collecting physiological data, Mariana had the “good fortune” of collecting scats from the fur seals. This allowed her to spend another 6 months in the laboratory, examining the energy, protein, and lipid content of the scat in order to calculate the amount of energy and nutrients that the fur seals retained.
“Usually, when we discuss the quality of a fish species as a diet item, we are talking about the gross energy contained in the fish.” Mariana explains. “But for a fur seal, what is important is the amount of energy they can retain after the digestive process has occurred, what we call net energy. No one has ever measured the digestive costs in fur seals, and whether this changes with diets of different quality and diversity.”
Mariana’s initial results, recently presented at the Alaska Marine Science Symposium in Anchorage, suggest that fur seals lose proportionally more energy from diets composed of lower quality prey items, and that mixed diets do not appear to provide any additional energetic benefit.
Mariana explains the importance of these results “This study illustrates how the cost of the digestive process can greatly exaggerate the differences in prey quality in the diet. The energy the fur seals derive from low quality prey is even lower than you’d predict from just looking at the fish.”
The cost of digestion of a lower quality diet was mainly affected by the amount of fish that the fur seals needed to eat in order to fulfill their required energetic needs. This means that fur seals in the wild would need to capture much more of the low quality prey items to meet their nutritional needs. Even though mixed diets did not alter the amount of energy the fur seals received, Mariana cautions that its importance should not be discounted.
“While diet diversity didn’t seem to affect the digestive process, it could still reflect ecosystem health, and be an indicator of how easily a fur seal might be able to find prey that meets its needs in the wild.”
Mariana’s study is the latest conducted with the group of fur seals kept at the Vancouver Aquarium that are helping scientists better understand their wild counterparts.