Experiments were conducted to determine the preferred temperatures and the final preferenda of Pimephales promelas and Lepomis cyanellus. Specimens of P. promelas and L. cyanellus were acclimated to five and four different temperatures for 30 days. A gradient tank was built permitting a temperature gradient of 2° C. per chamber. Openings in the partitions between the chambers let the fish move freely throughout the tank. One fish was used to a test and ten tests for each acclimation level. Recordings of the position of each fish were made every 15 seconds for 40 minutes. The temperatures corresponding to the modes were averaged to obtain the preferred temperature for the acclimation level. Pimephales promelas and L. cyanellus acclimated rapidly to high temperatures but required about two weeks to acclimate to low temperatures. Preferred temperatures increased as the acclimation temperatures increased for both species until the final preferendum was reached. The final preferendum was significantly higher for L. cyanellus than for P. promelas. The final preferenda of fishes may have important effects upon distribution in environments. The importance of temperature in field work is emphasized and problems for future investigations are discussed. The study reported herein was to determine the preferred temperatures and the final preferenda of two Oklahoma fishes, Pimephales promelas (Rafinesque), the fathead minnow, and Lepomis cyanellus Rafinesque, the green sunfish. The study was made possible through Research Grant No. WP-67 (C2) from the National Institutes of Health. Interest was centered primarily with behavioral reactions of fish to temperature changes rather than physiological effects of temperature. Fry (1952) and Doudoroff (1957) have made available excellent articles about the physiological effects of temperature on fish. Chidester (1924) reviewed the literature on fish migration and concluded that temperature was the primary physical factor in the migration of fish. Sprugel (1951), while investigating the effect of temperature on the distribution of fish, found that bluegills would leave the higher oxygenated levels in lakes and penetrate lower strata where the dissolved oxygen was much reduced. The length of time spent at the lower levels was limited. Dendy (1946) found that dissolved oxygen levels of 1.5 ppm did not necessarily constitute a barrier to fish. He also found that pressure did not influence distribution as strongly as did temperature.
