Metabolism, Critical Oxygen Tension, and Habitat Selection in Darters (Etheostoma)

Six species of darters (Etheostoma) were studied in the field and laboratory to relate respiration to habitat selection. The species ranged in habitat preference from a fast—water form (Etheostoma rufilineatum) to a slow—water form (Etheostoma fusiforme). Field measurements showed that winter O_2 tensions were not limiting the distribution of the darters when these tensions were compared with the critical O_2 determinations made in the laboratory at a low temperature (10 degrees C). However, summertime ambient O_2 levels were as low as 3.3 ppm [= 7.33 kilopascals or 55 mm Hg], a level that was 1.0—1.7 ppm below the wintertime critical O_2 tension (P_c) of several of these species. The fishes compensated for this by lowering P_c in the summer. This finding is contrary to previously reported results on the relationship between P_c and temperature. While not all darters selected habitats such that the downward shift of P_c in the summer was of ecological consequence, this was the case in 1 of the slow—water stream forms (E. boschungi) which inhabits waters in the summer that would be uninhabitable without such a shift in P_c. Additionally, one of the darters that typifies fast—water habitats (E. rufilineatum) had a P_c of 6.1 ppm [= 14.0 kilopascals or 105 mm Hg), which prevented it from occupying slow waters in the summertime. However, such respiratory exclusion from available habitats is not the case in the other stream forms studied (Etheostoma flabellare, Etheostoma squamiceps, and Etheostoma duryi), nor do these other stream forms occupy waters as slow as they are capable of on the basis of respiratory abilities. Therefore, a limited tolerance to low O_2 levels at summertime temperatures is not the general cause of the preference of Etheostoma for streams, although this is the case in some specific instances. In addition, the stillwater from (E. fusiforme) does not have any respiratory advantage over many of the stream forms in terms of a low metabolic rate or low P_c. However, darters as a group do show a rather low metabolic rate when compared to freshwater fishes in general.