The migration and production of indigenous populations of brown, brook, and cutthroat trouts in Spawn Creek, Cache County, Utah, were studied in 1973 and 1974 to elucidate the role of this stream vis- a - vis the rest of the watershed . Spawn Creek and its watershed were described climatically, hydrologically , geologically , vegetatively, recreationally, and chemically. The prevalent, aquatic taxa of fauna and flora were also listed. The equipment and techniques used in the study t o collec t data, including a two-way fish trap and electro- fishing gear, were delineated . The collected data consisted of periodic estimates of mean weight and density stratified by year classes . These year classes were separated by length-frequency histograms, the length of individuals of known age , and the determination of age via counts of annuli on scales. This last method of aging was adjusted for the failure of cutthroat trout to form an annulus in their first year of life . The estimates of density were obtained through a modified form of the two-catch removal estimator. This estimator was designed to correct the estimates for emigration from the sampled area between the two sampling efforts . The biases in several unmodified multi-catch removal e , · imators vis- a-vis the populations of trout in Spawn Creek were discussed relative to the different , individual sizes of several year classes. Mathematical models to describe t he time-dependent growth and density were constructed from the von Bertalanffy equation and the equation of depensatory mortality, respectively, by adding trigonometric components to each. The added constraints imposed by these components were discussed for models of both growth and dens ity . These models were fitted to the appropriate , periodic estimates of growth and density to provide functions of these states vis-a-vis time. The calculation of production for each year class consisted of differentiating the appropriate model of growth, multiplying the resultant differential equation by the appropriate model of density , and integrating the product for various interval s of time. A numerical routine for integration was used when the solution of the integral of the aforementioned product was unattainable in closed form . The basic alogarithm of this routine was discussed. The periodic estimates f rom the data showed that several year classes such as the 1971 year class of cutthroat trout and the 1970 and 1971 year classes of brook trout, were numerically dominant in 1973 but not in 1974. When most of t he individuals in the last two year classes died via senescence in 1974 , the density and productive capacity of the brook trout population declined. This decline was a result of the failure of this taxa to produce new dominant year classes in 1973 and 1974. The periodic estimates from the data also showed that growth was similar in 1973 and 1974 except for the 0+ age group. The brown and cutthroat trout s of this age grew less in 1974 while the brook trout grew more. Immigration occurred seasonally with the adult cutthroat trout migrating in the spring, and the juvenile cutthroat and brown trout ~ ~oving in the fall. No precedent for the fall emmigration of cutthroat juveniles was found in the literature. Emigration was a random affair save for the 0+ age group of cutthroat trout. Migration of brook trout was ~ot significant. Production was greater in the growing season of 1973 than in 1974 for all species . During the year of trap operation only the 1974 year class of cutthroat trout and the 1970 year class of brown trout showed an increase in produced biomass. The failure of the brook trout to produce new dominant year classes was discussed and was attributed to the evacuation of beaver in Spawn Creek and the subsequent dilapidation of their ponds. The underestimation of the production of the 1974 year class of cutthroat trout and the probable causes for said error were discussed. The net emigration of juvenile cutthroat trout and the loss of produced biomass were tied together as a partial cause and effect. The failure of the net immigration of cutthroat trout adults to enhance the productive capacity of these year classes in this stream was attribut ed to reproductive activity and subsequent mortality of these individuals. This relationship along with the predominant emigration of cut throat juveniles showed Spawn Creek vis - a-vis this species to be primarily a vehicle to enhance the reproduction of migrants exclusive to any other activity by this species. The relationship among migration , production, and reproduction of brown trout appeared as described above for t he cutthroat trout , however, the small density of the population of this t rout prevented any firm illation.