DEMONSTRATION OF TANK EFFECT ON GROWTH INDEXES OF JUVENILE RAINBOW-TROUT (ONCORHYNCHUS-MYKISS) DURING AN AD-LIBITUM FEEDING TRIAL

Growth indices were examined in 24 identically managed tanks, each con taining 120 diploid juvenile rainbow trout (initial mean body weight, 9.3 g), during a 12-week study to examine tank effects associated with tank location in a multi-user research facility. Growth indices inclu ded mean body weight, feed intake, feed conversion index, and specific growth rate. The null hypothesis that tank effect had no effect on gr owth over the 12-week period was rejected (P = 0.038), and mean weight in individual tanks differed by as much as 18.7%. During the study, i t was determined that the proximity of tanks to common-use walkways in the facility could affect growth indices. This was indicated by signi ficant differences in the mean fish weights among blocks of tanks sewe d by different header tanks after 4 (P = 0.001) and 8 (P = 0.024) week s. The block containing tanks of fish with the highest mean weight was nearest to the 2 common-use walkways in the facility. Fish in this bl ock of tanks, compared with those in other blocks, had significantly g reater feed intake but no significant differences in conversion effici ency. Compensatory growth, a well known growth attribute in fishes, di minished the difference in mean weight between these blocks of tanks b y the end of the study. Comparison of paired tanks within header tank blocks indicated that fish in those located nearest to walkways had hi gher feeding rates over the 12-week period (P = 0.048), but less effic ient feed conversion (P = 0.040) than did fish in matched tanks locate d farthest from walkways. However, there were no differences in moan w eight of fish. Results of this trial document the risks involved in id entifying fish in a tank as the experimental unit when treatments are administered to the tank of fish, the latter being the true experiment al unit.