Although most fishes are ectotherms, the efficiency of secondary production by fishes is substantially lower than the average of 50 percent measured for terrestrial ectotherms
What difference between aquatic and terrestrial environments might account for this difference?
Chapter 4 referred to the energy cost of osmoregulation, and that is a plausible hypothesis to explain the lower efficiency of secondary production by fishes. Evidence to support the hypothesis is equivocal because two different methods of measuring the cost of osmoregulation yield very different results:
• Measurements of oxygen consumption of fishes indicate that 20 to 50 percent of a fish's daily energy budget is devoted to osmoregulation. Furthermore, most (but not all) freshwater and marine fishes grow best in water that has a salinity between 5 and 18 parts per thousand.
• In contrast to these whole-animal studies, estimates of the cost of osmoregulation extrapolated from the cost of Na+ transport in isolated tissues indicate that the cost of osmoregulation is no greater than 10 percent of the energy budget.
(G. Boeuf and P. Payan. 2001 . How should salinity influence fish growth? Comparative Biochemistry and Physiology C 130:411-423)
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