Animals rely on lipids as a major fuel for endurance exercise because they pack more joules per gram than any other fuel. However, in contrast to mammals, information on how the mobilization of lipids from endogenous stores is managed to meet the needs of energy metabolism in swimming fish is sparse. Information on in vivo rates of lipid mobilization in swimming fish has been limited to relatively low exercise intensities and has only been investigated in a single species. Therefore, the goal of my thesis was to address this paucity of information by quantifying lipolytic rate in rainbow trout during graded exercise and fatty acid mobilization in Atlantic salmon during prolonged endurance exercise. In the first part of my work, I hypothesized that like mammals, rainbow trout stimulate lipolysis above resting levels to a peak with increasing work intensity, but subsequently lower its rate at high intensities when ATP production from carbohydrates becomes dominant. To test this hypothesis, I measured the rate of appearance of glycerol (Ra glycerol) in the blood (resulting from the breakdown of triacylglycerol (TAG)) of trout at rest (control) and during graded exercise from rest to Ucrit. Results showed that Ra glycerol in trout averaged 1.24 ± 0.10 µmol kg -1 min-1 and that this rate was unaffected by exercise of any intensity. These experiments revealed that rainbow trout do not modulate lipolysis during exercise. Furthermore, I calculated that baseline lipolytic rate was much higher in trout than in mammals and that this rate is in constant excess of the requirements of energy metabolism. My second investigation focused on measuring fatty acid mobilization in Atlantic salmon. To date, the majority of studies on energy metabolism in salmonids have used rainbow trout as the ubiquitous model for salmonids. I postulated that domesticated rainbow trout may be far less impressive athletes than their wild anadromous form and other salmonids. In this regard, I proposed that studying energy metabolism in Atlantic salmon (even those from aquaculture) may help to deepen our understanding of the physiology of true long-distance migrant fish. To study the effects of prolonged endurance exercise on the mobilization of fatty acids from endogenous stores in these fish, I monitored the rate of appearance of fatty acids (Ra NEFA calculated from Ra Palmitate) in the blood during 72 hours of sustained swimming. I found that contrary to what has been previously described in rainbow trout, Ra Palmitate (and by proxy, Ra NEFA) is reduced by approximately 64% (from 0.75 ± 0.12 µmol kg-1min-1 to 0.27 ± 0.06 µmol kg-1min-1 and from 19.3 ± 7.8 µmol kg-1min-1 to 6.9 ± 2.0 µmol kg-1min-1 for Ra Palmitate and Ra NEFA, respectively) during prolonged endurance exercise in Atlantic salmon. However, like in trout, even this reduced rate of fatty acid mobilization exceeds the requirements of energy metabolism at rest and during swimming. While further experiments will be necessary, I speculated that this reduction in Ra NEFA may be caused by a partial inhibition of lipolysis to reduce the energetic cost of TAG:FA cycling and optimize fuel budgets during prolonged endurance exercise. This thesis provides the first in vivo measurements of lipolysis during graded exercise in salmonids and the first in vivo measurements of fatty acid mobilization in Atlantic salmon. From the results mentioned above, I concluded that salmonids mobilize lipids in constant excess of the requirements for energy metabolism, possibly to allow for rapid reorganization of membrane phospholipids in response to changing environmental conditions. However, more anadromous and migratory phenotypes may rely on a tighter control of lipolysis to minimize the costs of substrate cycling and conserve energy on limited fuel stores.