|Abstract or Summary
- Juvenile coho salmon (Oncorhynchus kisutch (Walbaum)) and aquatic invertebrates were subjected to experimentally increased, but naturally fluctuating, temperatures in a model stream channel. Coho of the 1969, 1970, and 1971 year classes were reared in the heated model stream and in an unheated control stream, both located at the Oak Creek laboratory west of Corvallis, Oregon. The average increment of experimental over control temperature was 4.3 C over the entire experimental period of 22 months. Temperatures in the control stream were generally favorable for growth of coho if food organisms were scarce, while temperatures in the heated stream were favorable for growth if food organisms were abundant. Coho of the 1969 year class were reared for one winter as yearlings, and total production of this group was approximately the same in the two streams. Coho of the 1970 year class were reared from small fry to smolts over a one-year period. In this group, total production in the control stream was approximately twice as great as in the heated stream. While population size was nearly the same in the two streams, coho in the control stream grew much more rapidly than in the heated stream. Total production of coho of the 1971 year class, which were reared from the egg stage, was approximately five times as great in the control as in the heated stream when the experiment was terminated in August 1971. The difference in production resulted from larger population size and higher growth rate in the control stream. Production of aquatic stages of insects was approximately twice
as great in the control as in the heated stream over the period May 1970-May 1971. This difference between streams was especially marked in mayfly and stonefly nymphs, which were the most abundant insects in both streams, and which appeared to be the major sources of food for coho. The reduced growth rates of coho in the heated as compared to the control stream probably resulted from effects of temperature on the invertebrate food supply as well as from direct effects of temperature on the coho in raising their standard metabolic rates so that energy available for growth was reduced. Winter growth rates of coho in the heated stream were generally higher than in the control, probably as a result of an increase in appetite with increased temperature. Winter growth rates of the 1969 year class were high, probably because young coho fry were available as food. Winter growth rates of coho of the 1970 year class, which had a more natural food supply, were low in both streams. The insect fauna of the Oak Creek area is probably adapted to cool water temperatures, and this adaptation presumably contributed to the reduced insect production in the heated model stream. If heat tolerant species of insects were present in or able to migrate into a stream receiving a heated effluent, the simplication of the invertebrate fauna caused by the thermal pollution might increase the amount of food available to the top carnivore. Such heat-tolerant forms apparently were not present in the model streams or, if present, were not able to contribute significantly to total production.