INFLUENCE OF LONG-TERM HYPOXIA EXPOSURE ON THE ENERGY-METABOLISM OF SOLEA-SOLEA .1. CRITICAL O-2 LEVELS FOR AEROBIC AND ANAEROBIC METABOLISM

Long-term hypoxia is a general phenomenon on the Italian Adriatic coas tline, and is mainly caused by continuous eutrophication. The sensitiv ity of sole Solea solea to long-term hypoxia was investigated. Healthy S. solea obtained from trawls were kept at 19 degrees C in aquaria fo r at least 2 mo. The fish were exposed to hypoxia after a preacclimati on period of 30 h at normoxia. Oxygen levels during normoxia were kept constant at 80% air saturation (16.6 kPa, 6.4 mg l(-1) O-2); during h ypoxia oxygen levels were set at 60, 40, 20, 12 or 6% air saturation ( 4.8, 3.2, 1.6, 1.0, 0.5 mg l(-1)). During the experiment oxygen consum ption was measured continuously. At the end of each experiment, blood samples were taken from anaesthetized specimens. Oxygen consumption pa tterns were statistically analyzed. A novel technique is described for the determination of the standard metabolic rate and the scope for ac tivity of free-swimming animals. The resting metabolic rate and the sc ope for activity showed significant changes at reduced oxygen levels. Activity levels declined progressively starting at 40% air saturation. Resting levels remained constant between 80 and 20% air saturation, b ut fell below the standard metabolic rate at 12 and 6%. Blood lactate levels were increased at 12 and 6%, indicating anaerobic metabolism. D ata show that 40% air saturation should be considered as a limiting le vel, while the incipient lethal level lies between 12 and 20%.