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FISH

The maximum recommended temperature for trout is 68°F or 20°C, yet rainbow and brown trout live in sections of the Firehole River where the water temperature exceeds 75°F (24°C). The fish in the heated portions of the river have changed important life functions such as diet, size, growth patterns, and reproduction.

Life Function	Unheated	Heated	Brown Trout	Rainbow Trout
Diet	Bottom-dwelling insect larvae	Mollusks, winged insects
Size	Smaller body size compared to counterparts in heated parts of the river	Larger body size compared to counterparts in unheated parts of the river
Growth	One annual growth period	Two annual growth periods
Reproduction		Successful

In contrast to brown trout, rainbow trout not only reproduce successfully in the warmer parts of the river, but have changed their annual spawning time from spring to fall.

Because of its hydrothermal additions, the water in much of the Firehole River stays relatively warm year-round. The coldest temperatures usually occur in mid-June when runoff from melting snow is greatest, while the warmest temperatures occur in August when the snowmelt has ended and the summer sun is heating the water. The shift in rainbow trout spawning time serves two purposes-it avoids the highest water temperatures of late summer and it allows the young fish to grow throughout the winter before coping with the stress of high water temperatures.

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