Reproductive Biology of Rainbow and Brown Trout in a Geothermally Heated Stream: The Firehole River of Yellowstone National Park

Abstract

Geothermally heated waters discharged into the Firehole River increase downstream temperatures by about 10.5/sup 0/C, and affect the reproduction of the resident brown and rainbow trout. Gross and histological comparisons of gonadal development and annual maturation cycles between trout from an upstream, unheated station and a station downstream in the warmest part of the river indicate that the two species respond differently to the heating of the stream. Brown trout from the unheated station reproduced well, with all adults examined having normally maturing gonads and young of the year abundant in the area by early summer. However, those from the warm station had poor reproductive success, with a low percentage of adults having normally maturing gonads, prespawning degeneration of ova in some females, and very few young of the year evident in the area. Rainbow trout at the warm station reproduced successfully and had changed their annual spawning time from spring to fall. In the warm, fertile waters of the Firehole River, this early spawning not only avoided the high temperatures that would prevail during the spring and early summer, but also enabled the young to grow through the winter.