Over twenty years later, Brode was the first scientist to study the food habits of Crater Lake fish. Hasler carried the research a step further by determining the growth rates of fish in the lake. In July and August 1937, 1,387 anglers spent a total of 1,714 hours at Crater Lake catching 1,302 fish, an average catch of 0.75 fish per hour. Hasler randomly collected scales from 175 fish (124 trout and 51 salmon) to determine their age and then separated them into age groups. He computed the average length of fish in each age group and determined growth by estimating the length of fish after each year of life. The largest fish caught among the trout examined was twenty-six and a half inches long, weighed seven pounds, and was beginning its seventh year of life. Rainbow trout and coho salmon attained average lengths of 11.7 and 14.1 inches, respectively, by their third year. Based on fishery data collected between 1937 and 1940, Hasler and Farner reported that both rainbows and coho were reproducing naturally in Crater Lake. They also concluded that stocking the lake was ineffective and that "natural reproduction plays a dominant role in the maintenance of the population and stocking, by present methods, only a minor one." (20)

In 1939, four unusual fish were collected by F. F. Fish of the U.S. Fish and Wildlife Service, who was investigating a fungus infestation in the lake's salmon population. The fish were delivered to Orthello Wallis and Carl Bond at Oregon State College. Wallis and Bond identified the fish as kokanee salmon (Oncorhynchus nerka), an identification confirmed by Carl Hubbs of Scripps Institution of Oceanography The kokanee were apparently few in number, since none were found again at the lake until 1947. Although there was no record of kokanee ever being stocked in the lake, Wallis and Bond speculated that this species was reproducing and had become established there. Currently, the lake's fish populations consist entirely of rainbows and kokanee, although a brown trout was reportedly caught in 1966. The coho salmon have apparently disappeared. (21)

University of Washington oceanographers pose together at Crater Lake in July 1940. From left are Thomas Thompson, Lyman Phifer, Rex Robinson, Clinton Utterback, and Donald Farner. Photo courtesy of the National Park Service