A knowledge of the basic biology of any fish is a primary requirement for the practical management of that stock of fish. This investigation was directed at a pure culture population of Yellowstone cutthroat trout, to describe the basic biology and provide a basis for management and further research. Kiakho Lake has a surface area of 67.42 acres, a maximum depth of 32 feet and a mean depth of 16.5 feet. Due to the rocky substrate, lack of littoral development and low total dissolved solids, the production of plankton and bottom fauna was small and characteristic of oligotrophic conditions. The food of cutthroat trout in Kiakho Lake in May was comprised of 83.9 percent by volume and 81.3 percent by occurrence of chironomid pupae. In June the food was 46.7 percent by volume and 45.8 and 35.5 percent by occurrence of chironomid larvae and Gammarus respectively. In July the Gammarus were 57.8 percent by volume and 60.3 percent by occurrence. In Lumberton Reservoir and Monroe Lake the Gammarus comprised 51.0 and 55.6 percent by volume and 34.4 and 78.2 percent by occurrence respectively of the food. In Garcia Lake, Chaoborus was 32.9 percent by volume and 36.0 percent by occurrence and the redside shiner, Richardsonius balteatus, was 27.8 percent by volume and 31.8 percent by occurrence. The fish appeared to be second in preference to Chaoborus. The body-scale relationship is described by a straight line having a slope of 1. A graph of instantaneous growth rate plotted against length, revealed that faster growing fish have a faster decrease in growth rate. Due to the absence of certain characteristics e.g. a concavity in the upper limit of the graph, the growth of Kiakho Lake cutthroat appeared to support the view that faster growing fish are selected by the fishery, and that it can be demonstrated in this type of graph. The data, fitted to a Parker and Larkin (1959) growth equation gave a z value of 0.71. The absence of "Lee's Phenomenon" gave support to the premise that the phenomenon can result from selection by a fishery, and invalidated the other ideas concerning the causes as far as this population was concerned. The spawning run in Kiakho Lake was estimated at 3,000 fish. A tagging program revealed that the fish spent on the average of 13 days to spawn, and that there was approximately a 54 percent mortality. The male fish appeared on the spawning grounds first. The female fish showed a decrease in size, later in the run, which was not shown by the males. The eggs hatched sometime in mid June and the young fish apparently spend one year in the outlet stream. The female fish mature between the ages of 2—4 and the males between 1—3. The mean number of eggs per female, plus or minus two standard deviations was 944± 393.29. A multiple regression analysis revealed that body length affected the number of eggs produced, 2.5 times as much as egg diameter. Recommendations were made, due to the probable effects of competition, that cutthroat trout be kept in pure culture populations. It was further suggested that cutthroat trout numbers be maintained in view of the severe reduction and almost extinction of the species in other areas. / Science, Faculty of / Zoology, Department of / Graduate

In May, adult Yellowstone cutthroat trout (Salmo clarkii lewisii) moved from Kiakho Lake into its outlet to spawn. Upon emergence from the gravel, most young fry descended at night from the fast flowing, gravel bottom areas into slow flowing, mud and sand bottom regions where they spent the summer. The majority of fry over-wintered in the creek before ascending to the lake. The research was conducted in the summers of 1959 and 1960. It had two objectives: (1) to describe fry distribution and (2) to examine some characteristic fry "behavior" including movement. Traps and an experimental trough were operated. Behavior was observed, movements of marked fish were studied, and fry distribution was determined. Temperature, barometric pressure, water level, and light were recorded. Darkness apparently influenced the downstream movement of recently emerged fry for 90% of it occurred at night. A slow current apparently greatly slowed the rate of descent. The gradual descent of these fry in the "sluggish" Beaver Meadow during July evidently produced the late July uniform Beaver Meadow fry density. Long distance upstream and downstream movement of "healthy" fry in August was quite limited though short temporary movements of fry from their "home", may have been common. At least 60% of the fry held the same "home" for two weeks. Fry defended territories. The August 1960 Beaver Meadow fry population decrease was due to natural mortality. Month-old Beaver Meadow descending fry were characterized by small size and "physical weakness". In a week, the number of day versus night descenders was about equal, but within a 24 hour period, there was no correlation between the two. Bright moonlight appeared to depress night downstream movement, a fluctuating water level may have increased it, and a week long drop in creek temperature apparently depressed all downstream movement. Fry moving upstream were generally larger than average size fry. There was a moderate association between rising daily maximum water temperature over a period of several days and increased upstream movement. Fry in a trough ascended more at 20°C. than at 9°C. A week long drop in water temperature likely depressed movement. Because more than 80% of the movement occurred in the day, a certain minimum light intensity may be required before fry ascend much. The Kiakho outlet observations were compared to the findings of other researchers. Generally, stream dwelling Salmoninae appear to exhibit territorialism, homing, and limited movement,, Several other generalizations about stream dwelling trout, were suggested,, / Science, Faculty of / Zoology, Department of / Graduate

The study of the cutthroat trout in the Logan River drainage was initiated in cooperation with the Utah Cooperative Wildlife Research Unit and the Utah Fish and Game Department during May of 1948. The first field work began that month. The taking of fish was terminated in November, 1949, although several trips were made into the study area as late as January, 1950.