Mountain regions are experiencing some of the highest air temperature increases and ice cover decreases. However, few studies have examined the effects of climate warming and earlier snowmelt on mountain lake thermal characteristics and energetic implications for fish. We assessed potential climate-induced thermal changes and energetic consequences for cutthroat trout (Oncorhynchus clarkii spp.) and brook trout (Salvelinus fontinalis) in the Southern Rocky Mountains, United States. We found that summer growing degree days increased by an average 21 percent with 2°C air warming and 43 percent with 5°C air warming. But earlier snowmelt increased growing degree days by an average 48 percent. The average maintenance ration with 2°C and 5°C warming increased respectively by 13.8 and 21.9 percent for cutthroat trout and 23.8 and 37.4 percent for brook trout. The average increase in food required with earlier snowmelt was 43.4 percent for cutthroat trout and 52.3 percent for brook trout. Thus, earlier snowmelt can have a greater effect on fish energy requirements than a 5°C rise in air temperatures. Snowmelt recession together with a 5°C air temperature rise could more than double food requirements for fish to maintain constant body weight. If lake productivity increases with these climatic changes, then trout growth could improve; otherwise, energetic demands may result in lower fish growth.