Mortality of cutthroat trout fry, Salmo clarki, in Yellowstone Park, Wyoming

Abstract

A field study to determine mortality of cut throat trout fry was conducted in Yellowstone Park, Wyoming. The total adult run upstream was counted in order to predict numbers of downstream migrating fry. A total of 36,098 trout were counted migrating upstream. Of these, 11,997 were males and 24,101 were females. Average fecundity was 1000 eggs per female. Allowing 21.0 percent for death in redds, 19,039,790 fry were expected in the downstream migration. A trap was set across Clear Creek to determine numbers of downstream migrating fry. Total fry count was 116,895. Excessively high water runoff hampered further trapping. One hundred adults were collected from Clear and Cub Creeks at their outlets into Yellowstone Lake to determine if predation was a major factor in fry mortality. Adults were collected during daylight and dark. Results showed that adult predation was insignificant in both creeks and outlets. A total of 50 moribund fry kidney smears were made on blood agar plates. Twenty-five healthy fry kidneys were used as controls. All plates were incubated from 45° to 60°F. None of the plates had growth, therefore fry mortality due to bacterial invasion was considered to be insignificant. A sampling device used for counting downstream migrating fry was tested for accuracy. Five lots of 4000 fry each were poured from the trap through the sampling device. The range of inaccuracy was from 57. 5 percent below the actual count to 42.0 percent over the actual count. A library study analyzing various fry trapping operations and methods was included.