| Summary: | Bioenergetic studies can quantify the conversion of chemical energy contained in food to biologically useful energy to understand how changes in diet quality and quantity affect overall energy budgets and nutritional status. However, chemical energy is intrinsically linked to the macronutrients contained in food (i.e., lipid and protein) in terms of energetic density and digestive efficiency. For northern fur seals (Callorhinus ursinus) it is unknown how efficiently they transform dietary gross energy to net energy. I fed six trained adult female fur seals eight experimental diets composed of four prey species (capelin, walleye pollock, Pacific herring, and Magister squid), alone or combined. I measured the fur seals’ digestive efficiency for energy and macronutrients across diets. I also investigated the effect of dietary intake on digestive efficiency, and tested the hypothesis that mixed-species diets provide a greater nutritional return than equivalent single-species diets. I quantified net energy uptake by measuring excreta energy loss and measuring heat increment of feeding. My results revealed significant differences between digestive parameters across diets. I found that digestible energy (95.9–96.7%) was negatively affected by both ingested mass and dietary crude protein. Furthermore, urinary energy loss (9.3–26.7%) increased significantly with increases in dietary crude protein. I also found that the heat increment of feeding (4.3–12.4%) increased with decreasing dietary lipid content. Overall, net energy gain (57.9–83.0%) was positively correlated with lipid content. I found that macronutrient digestibility differed across diets and that, overall, lipids were more digestible (96.0–98.4%) than crude proteins (95.7–96.7%). Also, dietary protein influenced the ability of fur seals to digest lipids and proteins. Overall, my results demonstrate that low lipid prey not only contain less gross energy, but result in proportionally lower net energy gain following digestion, partly due to decreasing digestibility of lipids in high protein diets. I also found that, counter to predictions, mixed-species diets do not provide fur seals with greater energetic or macronutrient gains than single-species diets. These findings contribute to understanding the nutritional ecology of northern fur seals and the impact that changes in diet can have on the fur seals’ nutritional state. === Science, Faculty of === Zoology, Department of === Graduate
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