Gill ventilation and respiratory efficiency of Sacramento blackfish, Orthodon microlepidotus Ayres, in hypoxic environments

Respiratory and gill ventilatory responses of Sacramento blackfish to three environmental temperatures (12, 20 and 28°C) and four environmental dissolved oxygen concentrations (130, 90, 65 and 40 torr P_O2) were examined to determine physiological strategies of survival in eutrophic lakes and suitibility for culture conditions. Situated in van Dam-type respirometers, experimental blackfish showed increased gill ventilatory flows from increased ventilatory frequencies and stroke volumes to meet higher respiratory oxygen demands at increased temperatures. Ventilation volumes also increased at reduced environmental dissolved oxygen levels by increased ventilatory stroke volumes alone, except at 28°C where frequency increases were also measured. Oxygen consumption rates remained essentially constant with declining dissolved oxygen, except at 28°C where excitement elevated respiratory metabolism at 65 and 40 torr. Percentage utilization of oxygen increased with temperature from 12°C, but levels at 20 and 28°C were insignificantly different. Contrary to most studies on other species, there was no change in percentage utilization under hypoxic conditions even with 4.7-fold increases in ventilation volume in excited fish at 28°C. The ability of blackfish to survive in hypoxic waters is quantitatively compared with other species by calculation of a respiratory efficiency index (I), which includes the relationship between ventilation volume and percentage utilization of oxygen under normoxic and hypoxic conditions as well as the half-saturation value (P₅₀) of the species' blood with oxygen.