Fish feeding behaviour can be complicated and time consuming to observe, analyse and quantify. The conventional single ballotini X-radiography method was modified to determine the timing of feeding within a meal in farmed fish reared at commercial densities. Feed pellets containing two bead sizes were fed at different times during a single meal, either for the first half or second half of the meal. The two different ballotini sizes were differentiated on X-radiographs by human eye and semi-automated bead counting software was able to count the different bead sizes accurately. This dual ballotini method was used to assess whether fish ate earlier, later or during both halves of the meal and showed that individual fish feeding patterns were similar across two repeated meals. The method was applied to Chinook salmon (Oncorhynchus tshawytscha), a premium farmed salmonid, to determine the relationship between feed conversion ratio (FCR) and the timing of feeding. Results showed that fish that ate smaller amounts of feed ate most of their meal at the end of the meal. There was no significant difference in when feed efficient or feed inefficient fish ate within the meal but, at the first dual ballotini assessment, there was a relationship between FCR and the timing of feeding where more efficient fish ate later in the meal. However, this relationship was absent at the second time point. Despite this, the dual ballotini method is a promising and versatile alternative method for the characterisation of fish feeding behaviours and can be applied under a range of conditions including higher rearing densities.

Improving the feed conversion ratio (FCR; the amount of feed consumed relative to the amount of weight gain) can reduce both production costs and environmental impacts of farmed fish. The aim of this study was to investigate what drives FCR to understand how nutrients are retained, as well as the amount of oxygen consumed for digestion, absorption and assimilation (a metabolic process known as specific dynamic action, SDA). Feed-efficient and inefficient Chinook salmon (Oncorhynchus tshawytscha) in fresh water were identified using ballotini beads and X-radiography that tracked individual feed intake across three assessment periods under satiated feeding. This allowed a comparison of physiological traits and body composition between the two FCR phenotypes over two time points as Chinook salmon grew from 305 to 620 g. Fish with higher daily feed intake (DFI) had higher daily weight gain (DWG) as expected. Nonetheless, the relationship between FCR and DFI as well as FCR and DWG was variable between time points. FCR and DWG were not correlated at the first time point and were negatively correlated at the second time point. In contrast, FCR and DFI were positively correlated at the first time point but not the second. Despite this, efficient fish ate smaller meals and retained more protein, lipid and energy in their body tissues. There was no detectable difference in metabolism between the two FCR phenotypes with respect to minimal resting metabolic rate, maximum metabolic rate, aerobic scope, or SDA parameters. In conclusion, FCR is not consistently associated with growth and metabolic differences in freshwater Chinook salmon, but FCR-efficient fish retain more nutrients and consume smaller meals.

Feed conversion ratio (FCR) is the ratio between feed intake and weight gain. By improving FCR within a species, feed intake can be reduced for the same amount of growth, thus reducing feed costs and environmental impacts. To enable selection for improved FCR, it is important to understand how FCR differs within a species and what factors might also be associated with FCR efficiency as potential drivers. This study identified high and low FCR phenotypes in farmed saltwater Chinook salmon (Oncorhynchus tshawytscha) and examined how the following physiological traits were correlated with FCR: daily feed intake (DFI), daily weight gain (DWG), protein, lipid and energy retention, and whole-animal metabolic oxygen consumption rates (including minimal and maximal metabolic rates and aerobic metabolic scope). High DWG and low DFI were strongly correlated with FCR efficiency and DWG and DFI were also correlated. FCR efficient fish also had higher retention of protein, lipid and energy and lower rates of minimal metabolism. Maximum metabolic rate and aerobic metabolic scope did not differ between FCR phenotypes. The results suggest that future breeding programmes selecting for feed efficient fish will likely also result in faster-growing fish that regulate their feed intake, retain a higher proportion of ingested nutrient, and have reduced maintenance costs, all of which are expected to reduce feed costs and environmental loading without impacting production.