Fig. 2. Patterns of changes in rates of oxygen consumption (O₂, mg h^-1 kg^-1, red lines) with increasing exercise levels in Atlantic cod (Gadus morhua) and Greenland cod (Gadus ogac) from various latitudes based on data for North Sea cod (Schurmann and Steffensen, 1997) and for the Greenland populations of both species (Bushnell et al., 1994). Solid lines depict the actual performance range up to the critical swimming velocity (U_crit). Dashed lines depict extrapolated costs at higher swimming speeds (not reached) for a comparison between acclimation temperatures. For comparison, results at 0°C for Antarctic stenotherms (Notothenia neglecta and Pagothenia borchgrevinki) are included (Johnston et al., 1991; Forster et al., 1987). (A) In accordance with a compensatory increase in mitochondrial densities and overall aerobic metabolic capacities (see text), the intersection of extrapolated lines suggests that cold-acclimation at 10 or 5°C compared with 15°C causes a larger metabolic increment for the same exercise level in cod (as well as a rise in temperature-specific standard metabolic rate, SMR; not shown, see text). Metabolic cold adaptation observed in northern cod populations (at 4°C) also elevates SMR. However, the metabolic increment with rising swimming speed reflects enhanced energy efficiency and a somewhat higher U_crit. (B) A model of the transitions between acute warming and long-term warm acclimation (from 5 to 15°C) based on data for cod by Claireaux et al. (1995) and Schurmann and Steffensen (1997). The reduction in baseline oxygen demand associated with a warm-induced decrease in mitochondrial densities should contribute to the increase in U_crit observed during long-term warm acclimation. For further explanation, see the text.

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