| Abstract | During sea-cage rearing, Atlantic cod and haddock are fed commercial diets that have high protein content (∼ 50–58%) and are designed for optimum growth at temperatures of 10–12 °C. However, these fish spend several months at temperatures below 5 °C in Atlantic Canada, and there are data to suggest that growth could be enhanced at low water temperatures by feeding a diet with lower protein (LP) content and higher lipid. Thus, we explored how dietary protein and lipid (P/L) levels of 42/16 (low protein, LP diet) and 55/11 (high protein, HP diet) and water temperature (11, 6 and 2 °C) influenced the specific growth rate (SGR), hepatosomatic index (HSI), adjusted specific growth rate (SRGA), food consumption (FC), gross conversion efficiency (GCE) and gastric evacuation (GE) of juvenile Atlantic cod and haddock. Several temperature and species-specific effects were observed for the measured variables. Significant decreases in SGR, SGRA and FC (range 60–96%) were evident for both species when reared at 2 vs. 11 °C. The GCE of haddock was 46 and 31% (for HP and LP diets, respectively) higher than that measured for Atlantic cod at 11 °C, but 35 and 40% lower at 2 °C. With regards to the effects of dietary protein level, it was found that while feed composition did not affect growth rate (with the exception of haddock at 11 °C) or GE, GCE was significantly lower in fish fed the LP diet. These results indicate that 1) low temperatures present a particular challenge to haddock aquaculture; and 2) based on the similar growth rate, but lower GCE values, there is no advantage to feeding an LP diet even at cold temperatures. |
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