Aquaculture Research Institute at the University of Idaho in collaboration with USDA has genetically selected several lines of rainbow trout (Oncorhynchus mykiss) that show higher growth rates when fed all plant protein diet (PPD) than non-selected lines of trout fed a fishmeal-based diet. So far, no commercial breeding programs have yet started to improve feed utilization efficiency in fish, mainly because of the difficulty in accurately measuring individual feed intake of fish reared in groups.
This study aimed at identifying trout families with better efficiency in using soybean-based diets through two experiments. In the first experiment, 15 families (CX-118, CX-125, CX-134, CX-135, CX-137, CX-138, CX-141, CX, 143, CX, 144, CX-145, CX-146, CX-147, CX-148, CX-149, and CX-152) of rainbow trout (80 fish/family) with an average initial weight of 32.7±9.4 g (±SD) were PIT tagged and randomly distributed in four tanks (450 L). Fish were reared in this environment for four months and went through two periods of feed deprivation (FD) and two periods of refeeding (RF). During the RF periods fish were fed at satiation with a soy-based extruded diet. Fish performance was classed as FD-, FD+, RF- and RF+ for fish exhibiting loss (FD) and gain (RF) of weight relatively lower (-) and higher (+) than the population mean (FD-/RF-, FD+/RF+, FD-/RF+ and FD+/RF-). These four groups were distributed in 800-L tanks (~45 fish/tank) with three replicates to measure the Feed Conversion Ratio (FCR). In the second experiment, another group of 1200 rainbow trout belonging to the same 15 families were individually PIT tagged and distributed into two groups of 40 fish/tank and fed two diets. Diet 1 was supplemented with lyophilized powder of spirulina whole cells and Diet 2 was supplemented with U-15N labeled spirulina. After 18 days of feeding, 0.4 mm diameter punches were used to take muscle samples from each fish to analyze for 15N isotopes.
Results for the first experiment showed families CX-148, CX-146, and CX-125 with significantly higher weight gain and families CX-138, CX-135, and CX-152 with lower weight loss during RF and FD periods, respectively (P<0.05). FCR and final weight were slightly improved in the best group (FD-/RF+) but results were not significant. Results for the second experiment showed a similar trend for weight gain (P>0.05). Also, families CX-148, CX-146, CX-147, CX-141, and CX-135 showed the highest 15N isotope in muscle.
Overall, the results of the present study, so far, have demonstrated that there are substantial genetic-based variations among different families of rainbow trout in utilizing soybean-based diets. The selected individual fish from both experiments will be used for the breeding program.