Improving feed conversion ratio (FCR) can reduce both production costs and environmental impacts of farmed fish. To improve FCR, a better understanding of how it differs within a species and what factors might also be associated with driving FCR variation. Chinook salmon (Oncorhynchus tshawytscha) is a premium farmed salmonid in which very little is known about. This study looked to identify FCR variation within this species. Feed efficient and inefficient Chinook salmon, in both freshwater and saltwater, were identified using ballotini beads and X-radiography that tracked individual feed intake across three assessment periods under satiated feeding. This provided growth, feed intake and FCR values for individuals across two time points. Comparison of physiological traits and metabolism between the two FCR phenotypes and across timepoints was then carried out.
Daily feed intake (DFI), daily weight gain (DWG), and protein, lipid and energy retention were selected to compare to FCR. In saltwater salmon DWG negatively correlated with FCR efficiency while DFI was positively correlated. However, in freshwater salmon the relationship between FCR and DFI and FCR and DWG was variable between time points. FCR and DWG was not correlated at the first time point and was negatively correlated at the second time point. In contrast, FCR and DFI was positively correlated at the first time point but not the second. DWG and DFI was correlated in both freshwater and saltwater fish. Overall, FCR efficient fish had higher growth rates, ate smaller meals and had higher retention of protein, lipid and energy. Whole-animal metabolic oxygen consumption rates were measured to determine their minimal resting metabolic rate, maximum metabolic rate, aerobic scope as well as the amount of oxygen consumed for digestion, absorption and assimilation (a metabolic process known as specific dynamic action, SDA). There was no detectable difference in metabolism between the two FCR phenotypes with respect to maximum metabolic rate or aerobic scope for both freshwater and saltwater fish. However, the minimal resting metabolic rate was significantly lower in saltwater Chinook salmon but not in freshwater Chinook salmon. SDA parameters were measured only in the freshwater Chinook salmon and did not differ between phenotypes when fed a set ration.
In conclusion, metabolism did not differ between FCR phenotypes. However, feed efficient Chinook salmon are faster-growing fish that regulate their feed intake and retain a higher proportion of ingested nutrients, all of which are expected to reduce feed costs and environmental loading without impacting production.