| Summary: | In this research, the effects of temperature, body size, and/or feeding level on a variety of bioenergetic parameters in early (age-0) juveniles of both Atlantic cod (Gadus morhua L.) and haddock (Melanogrammus aeglefinus L.) were quantified. Bioenergetic parameters were measured at temperatures (5, 8, 12, and 15°C) experienced by field fish, using a range in fish sizes (3 to 13 cm TL) encompassing the first year of juvenile growth. In the first study, the relationship between cod and haddock growth rate (GI) and feeding-level (C) was quantified at different temperatures. The shape of the GI - C relationship and growth efficiency changed with temperature. RNA:DNA of white muscle and temperature explained a significant portion of the variability observed in GI of juvenile cod (R 2 = 0.716) and haddock (R2 = 0.637). In the second and third studies, the effects of temperature, body size and feeding on rates of energy loss via respiration (R) were measured. An equation was generated using data from the present research and other studies, Log10R = −0.8822 + 0.7745Log10DW + 0.8469Log10T (R 2 = 0.9871, n = 274), describing the relationship among R (mg O 2 fish−1 hr−1), temperature and body size over the 8 orders of magnitude difference in body size from larvae to adults. In the fourth study, physiological mechanisms contributing to growth rate differences were evaluated by comparing metabolic energy loss and metabolic substrates among relatively fast- and slow-growing cod. Fish reared within the same tank and experiencing the same feeding conditions, exhibited 2-fold differences in growth rate. The fastest growing fish had significantly lower condition factors (weight/length3) and tended to have lower metabolic expenditures compared to slower growing individuals. In the fifth study the effects of temperature and body size on the volitional swimming of juvenile cod were measured. Fish of 3, 9, and 30 cm TL were used, corresponding to sizes of field fish before, during, and well after the juvenile pelagic to benthic habitat transition. Relative swimming velocity (body lengths s−1) was unaffected by fish size, but increased exponentially with temperature.
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