BRILL: STANDARD METABOLIC RATES OF TROPICAL TUNAS 



into the red muscle immediately adjacent to the 

 spinal column. Thermistor probes were also 

 mounted in the incoming seawater line and in the 

 respirometer box itself. Red muscle and water tem- 

 peratures were determined by measuring the resis- 

 tance of the various thermistors with an HP 3456A 

 digital multimeter. 



Oxygen concentration (milligrams per liter) of the 

 water upstream and downstream of the fish was 

 determined with a dissolved oxygen meter (Yellow- 

 springs Instrument, model 51A) equipped with a 

 Clark-type polarographic electrode oxygen-temper- 

 ature probe. The probe was normally in the outflow 

 seawater line, but was moved to the inflow seawater 

 line to determine inflow seawater oxygen levels 

 every 30-60 min. The analog output of the oxygen 

 meter was also measured with the HP digital multi- 

 meter. An HP 9825A computer was used to control 

 an HP 5930A six-channel relay actuator which per- 

 mitted the digital multimeter to determine sequen- 

 tially the resistances of various thermistors and the 

 analog output of the oxygen meter. Seawater oxy- 

 gen level, red muscle and water temperatures, meta- 

 bolic rate, and heart rate were calculated and 

 printed by the computer at 5-min intervals. 



After being sealed, the respirometer box was 

 covered with black plastic to minimize disturbance 

 to the fish. Temperature of the seawater supplied 

 to the respirometer was maintained at 21° -22 °C for 

 the first 1-2 h because reduced water temperature 

 has been shown to help tuna survive after handling 

 (Barrett and Connor 1964). Seawater temperature 

 was then changed to either 20° or 25°C, and the 

 fish maintained at the test temperature until its 

 metabolic rate remained relatively stable for at least 

 1 h. The SMR was estimated by averaging the last 

 5-12 metabolic rate measurements. The standard 

 deviations of the metabolic rate measurements used 

 to estimate SMR were <11% of the mean (i.e., SMR) 

 in all cases, and in 70% of the cases, the standard 

 deviations were <5% of the mean. 



To determine the SMR at a second temperature, 

 the water temperature was changed to either 20°, 

 25°, or 30° C, and metabolic rate measurements con- 

 tinued again until the fish's metabolic rate remained 

 stable for 1 h. 



SMR Measurements-Aholehole 

 and Rainbow Trout 



Aholehole were obtained from Sea Life Park (Wai- 

 manalo, HI) and rainbow trout from a commercial 

 fish farmer (through the University of Hawaii, Hilo). 

 The former were maintained in an outdoor tank with 



running seawater at 25°C ( ± 2) and the latter, in an 

 indoor tank with running freshwater at 15°C (± 2). 

 Both species were fed daily, but individuals were not 

 fed for at least 20 h prior to use in an experiment. 



The respirometer used for aholehole was essen- 

 tially identical to that used by Davis and Cameron 

 (1971) and Jones and Schwarzfeld (1974) to measure 

 water flow and gas exchange across the gills of rain- 

 bow trout. The aholehole were anesthetized in 1: 

 10,000-1:30,000 MS222 (Tricaine methanosulfo- 

 nate). A thin, rubber membrane was sutured around 

 the fish's mouth and sealed with a small amount of 

 tissue glue (Histoacryl, B. Braun Melsungen AG, 

 West Germany). The fish was then placed in a black 

 Plexiglas box that was open at both ends. This box 

 was then placed in a larger tank that was divided 

 into two chambers by a partition with a hole through 

 it. The membrane sealed around the fish's jaws was 

 attached to the edge of the hole and sealed in place 

 with a Plexiglas plate held with stainless steel wing 

 nuts. This system allowed separation of the inspired 

 and expired water, yet allowed the fish to make nor- 

 mal respiratory movements. Water level in the two 

 chambers was maintained by standpipes (constant 

 level drains). Ventilation volume was determined by 

 measuring the water flow rate from the standpipe 

 in the chamber containing the fish. By lowering this 

 standpipe, the fish could be force-ventilated. 



Water samples were drawn from the anterior 

 chamber, and from the black Plexiglas box contain- 

 ing the fish, approximately every 15 to 20 min. 

 Water oxygen level was determined with a water- 

 jacketed oxygen electrode (Radiometer, Copen- 

 hagen) maintained at 25 °C. Metabolic rate was 

 calculated using the oxygen content difference be- 

 tween inspired and expired water and the ventila- 

 tion volume. 



Aholehole were given 2 h to recover from the 

 anesthesia before metabolic rate measurements 

 were begun. A series of metabolic rate measure- 

 ments were made with the water level in the two 

 chambers even and the fish actively pumping water 

 over its gills, until the its metabolic rate remained 

 relatively stable for at least 1 h. The water level in 

 the chamber containing the fish was then lowered 

 and measurements taken while the animal was being 

 force- ventilated, continuing again until the meta- 

 bolic rate stablized. Finally, the fish was given 0.1- 

 0.3 mL Flaxedil (intramuscularly) and metabolic rate 

 measurements continued while the animal was para- 

 lyzed and force-ventilated. In two cases, the fish was 

 left in the respirometer overnight on forced venti- 

 lation to allow the effects of Flaxedil to wear off. 

 Metabolic rate measurements were made again 



27 



