Ecology of the Cutthroat Trout in Henrys Lake, Idaho

Abstract

Henrys Lake, a 6,500-acre reservoir at the elevation of 6,472 feet in eastern Idaho, has a maximum depth of 21 feet and an average depth of 12 feet. Seasonal variation of weather and shallow, fertile waters are the major environmental factors that influence the Henrys Lake biotic community. Phytoplankton is abundant and composed primarily of blue-green algae and cladocerans and copepods are the dominant zooplankters. Higher aquatic vegetation has developed over the entire bottom of the lake. The standing crop of bottom and plant-in-habiting fauna in Henrys Lake for 1951 is estimated to have been 606 pounds per acre. Fish species found in Henrys Lake are: Cutthroat trout, Salmo clarki; Rocky Mountain sculpin, Cottus bairdi semiscaber; and mountain whitefish, Prosopium williamsoni as native species. Introduced fish are: rainbow trout, Salmo gairdneri; eastern brook trout, Salvelinus fontinalts; and redside shiner, Richardsonius balteatus hydrophlox. Rainbow trout and rainbow X cutthroat hybrids were rare in the sport catch. Cutthroat trout and brook trout entered the creel in the ratio of 4 to 1. The body-scale relationship is described by a second-degree parabola. Cutthroat trout attain an average standard length of 21.6 millimeters prior to scale development. Analysis of scales provides evidence that the growing season of the cutthroat trout is limited to the open-water period of approximately 180 days, mid-May to mid-November. Male cutthroat trout of the 1951 sport catch matured at ages II and III while females matured at ages III and IV. Of 500 mature cutthroat trout marked and released from the Henrys Lake Spawning Station in the spring of 1950, only 12 were taken in the sport-catch. This low recovery indicates these marked trout suffered a severe natural mortality subsequent to spawning. The considerable variation of growth in length among the Henrys Lake cutthroat trout is attributed to individual differences in the time spent in tributary streams as compared to time spent in lake, seasonal variation of environmental conditions, and different age at maturity between male and female fish. The rate of growth in weight continues to increase during the troutˈs second and third years whereas the rate of growth in length declines after the first year. The sex ratio of female to male cutthroat trout in the 1951 sport catch declined not only with increase in age but also as the fishing season progressed. The midge-shrimp-damselfly food group comprised 90 percent by number and 74 percent by volume of the food utilized by the Henrys Lake cutthroat trout during the study period. The pronounced decline in feeding activity that occurred during August may have been due to the relatively high temperature and relatively low oxygen tension of the lake waters during this period. Pathogenic bacteria infected mature cutthroat trout of the 1950, 1951, and 1952 spawning migrations. The death of an estimated 3,600 cutthroat trout of all age-groups in small, open-water areas at the mouths of 3 tributary streams during January 1952, has been attributed to asphyxiation due to local oxygen depletion. The curves of growth in length and growth in weight of the Henrys Lake cutthroat trout are descriptive of a short-lived population. The mortality of the Henrys Lake cutthroat trout due to disease, asphyxiation, predation, and capture by anglers plus mortality of mature cutthroat trout subsequent to spawning (due possibly to causes both environmental and genetic in nature) combine to give a low average age and limit the number in the population.