Understanding the Mechanisms that Determine Feed Conversion Ratio in Chinook Salmon – Physiological and Behavioural Aspects

Abstract

Finfish aquaculture production costs can be reduced by improving feed conversion ratio (FCR), the ratio of feed intake to weight gain. The aim of this thesis was to identify key factors that influence individual FCR variation in Chinook salmon (Oncorhynchus tshawytscha). Feed efficient and inefficient Chinook salmon, in freshwater and saltwater, were identified using ballotini beads and X-radiography to determine individual feed intake. Comparisons of physiological traits and metabolism between the two FCR phenotypes found that freshwater and saltwater feed efficient fish consumed smaller meals, had higher growth rates, and retained a larger proportion of ingested protein and lipid. No detectable difference was found between FCR phenotypes with respect to maximum metabolic rate (MMR) or aerobic scope (AS). The minimal resting metabolic rate (RMRmin) varied between FCR phenotypes in Chinook salmon reared in saltwater but not in freshwater. Specific dynamic action was measured only in freshwater fish and did not differ between phenotypes when fed a set ration. To assess feeding behaviour a novel application of the ballotini method was developed to determine the timing of feeding in fish. Two bead sizes (dual ballotini) were fed in different halves of the meal to assess when fish ate. No difference was found when the FCR phenotypes ate within the meal. Video imaging was used to analyse feeding behaviours and FCR more in-depth. Feed inefficient fish carried out more turns and swam further to obtain food and spat out more pellets, further increasing energy expenditure. In conclusion, feed efficient Chinook salmon were faster growing, ate less and retained a higher proportion of ingested nutrients while exhibiting more efficient feeding behaviours. RMRmin was shown to be lower in feed efficient fish reared in saltwater, however, MMR and AS did not differ between FCR phenotypes regardless of salinity. The new knowledge that feed intake and behaviour are key drivers of FCR will enable industry to improve FCR. Feeding Chinook salmon to satiation increases overeating and spitting, reducing feed efficiency. Controlling feed intake, whether through selection for individuals that consume smaller meals or feeding a ration slightly below satiation, could improve farm FCR.