Temperature and dietary effects on the physiology of two gadids, the Atlantic cod (Gadus morhua) and haddock (Melanogrammus aeglefinus)

Perez-Casanova, J. C. (2009) Temperature and dietary effects on the physiology of two gadids, the Atlantic cod (Gadus morhua) and haddock (Melanogrammus aeglefinus). Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

Atlantic cod and haddock grown in sea cages are exposed to changes in water temperatures that can have a significant impact on their physiology and/or biology. During winter months these species are exposed to low temperatures that have a negative impact on their food consumption and growth rates. On the other hand, during summer months, these fish can be exposed to temperatures above their upper critical temperature that can affect their stress physiology, immunology and ultimately lead to high mortality. To investigate the effects of low temperature on several aspects of the biology of these two species, I used tank respirometry to explore whether feeding low protein diets at cold temperatures (2°C) would decrease the metabolic cost of digestion and the time required for digestion/gastric evacuation, thus leaving more available energy for growth. In addition, using biochemical and molecular biological tools, I studied the effects of high water temperatures (acute and chronic challenges) on the stress response and immune function of juvenile Atlantic cod and determined their upper critical temperature. My results showed that: 1) low water temperature (2°C) significantly reduces food consumption, growth rate, gross conversion efficiency and absolute specific dynamic action (SDA; mg O₂‚) and slows gastric evacuation; 2) reducing dietary protein levels during the winter is unlikely to be of benefit to Atlantic cod and haddock culture operations. With regards to studies at elevated water temperature, I identified oxygen consumption and changes in plasma total Cortisol levels as reliable indicators of high temperature stress in Atlantic cod, and showed that juveniles of this species can tolerate short-term exposure to temperatures above 20°C, although their stress response differs when exposed to acute vs. chronic thermal challenges. Moreover, my results indicated that their immune function during chronic temperature stress is influenced by interactions between thermal effects and temperature-induced stress, and that not all immune-related parameters are affected by exposure to high temperatures.

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