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.