The Steelhead Rainbow Trout, Salmo gairdneri gairdneri Richardson, and Silver Salmon, Oncorhynchus kisutch (Walbaum), are two of the most important fishes found along the Pacific Coast of North America. A considerable amount of published material regarding their biology, distribution, systematic status, propagation, and management already exists. However, up to the present time, and especially to the start of the experiments described in the present paper, there has been a notable lack of quantitative data regarding both species, particularly with regard to their life histories.

Because of this lack of quantitative data, so necessary for sound regulatory, stocking, and other management programs, the California Trout Investigations, a cooperative unit of the California Division of Fish and Game (now the California Department of Fish and Game) and the U. S. Bureau of Fisheries (now a part of the U. S. Fish and Wildlife Service) in 1932 initiated a program of study at Waddell Creek, a typical coastal stream in Santa Cruz County, California. Upon the termination of the formal cooperative agreement in 1937, these studies were conducted independently by the California Division of Fish and Game.

The plan of the experiment was to study the steelhead and the silver salmon in their natural habitat. Since both fishes are anadromous, the logical approach was to construct a dam or weir at which both the upstream and downstream migrants could be counted. In the process of counting, observations could be made on the migrants (measurements, scale samples, sexual maturity, parasites, etc.), fluctuations of populations determined from the counts, and the counts complemented by observations made on the fishes in the stream (spawning activities, feeding habits, etc.).

Waddell Creek was chosen for the following reasons: It was a stream under as nearly natural conditions as could be found in California at the present time and was still reasonably accessible; it was large enough to possess a full biota and small enough to be dammed at reasonable cost and to permit complete counts of at least all upstream migrants, and thus avoid errors that might result from sampling; it was so situated that it could be kept under observational and legal control as a unit, with the general public excluded.

Waddell Creek in its general characteristics is typical of the great majority of California coastal streams of like size. Moreover, in miniature it is almost a replica of the larger stream systems, such as the Klamath and the Eel. This fact is of great importance in that the habits and ecology of the trout and salmon in the small streams and the large ones are similar. Consequently, the conclusions regarding the proper management of these fishes derived from the present study are applicable, at least in the broader aspects, to the coastal streams in general.

Obviously, certain limitations are imposed by a program that consists of studying the natural fluctuations of a population in a limited area. Large-scale sampling involving the killing of specimens cannot be carried on without danger of disturbing the natural balance. Thus, it is not possible to make various measurements such as egg counts and pyloric caeca counts, stomach analyses, etc. The very great advantage of Waddell Creek in this respect was that its drainage basin is adjacent to that of Scott Creek, a stream of comparable size, with comparable environmental conditions and a similar fauna, in which the lacking data could be gathered. Scott Creek had the advantage of being the location of a State egg collecting station and a State hatchery (the latter situated on a tributary, Big Creek) and of being set aside as a State Fish Refuge. Consequently, it was possible not only to gather data on egg production and to secure measurements but also, through marking of the naturally-spawned fish in Waddell Creek and the artificially-spawned and hatchery-reared fish in Scott Creek, to carry out a comparative study of two adjacent streams, one under natural conditions and the other under artificial management, and to study the amount of "homing" and "straying" between the two streams.

As will be discussed further in this paper, certain conditions already existed or were created by the experiment which altered natural conditions to varying degrees, especially in the direction of making difficult a true evaluation of population fluctuations under natural conditions, but the essential quantitative picture of the life histories of the species concerned has remained a correct one.