CARDIAC-OUTPUT AS A PREDICTOR OF METABOLIC-RATE IN COD GADUS-MORHUA

Adult Atlantic cod (2 kg Gadus morhua) were fitted with Doppler ultras onic flow-probes to measure ventral aortic outflow (i.e. cardiac outpu t). The probes remained patent for upwards of 3 months, during which t ime detailed relationships between cardiac output ((Q) over dot), hear t rate (fH) and rate of oxygen consumption ((M) over dot O-2,) were de termined as a function of swimming speed and temperature (5 degrees C and 10 degrees C), The rate of oxygen consumption increased linearly w ith (Q) over dot and exponentially with swimming speed. A very good co rrelation was observed between (M) over dot O-2 and (Q) over dot (r(2) =0.86) compared with the correlation between (M) over dot O-2, and fH (r(2)=0.50 for all 10 degrees C data and r(2)=0.86 for all 5 degrees C data). However, the (M) over dot O-2, versus fH correlation gradually improved over approximately 1 week after surgery (r(2)=0.86). The rel ationship between (M) over dot O-2, and (Q) over dot was independent o f temperature, while the relationship between (M) over dot O-2, and fH changed with temperature. Hence, calculating (M) over dot O-2, from ( Q) over dot is simpler and does not require that temperature be record ed simultaneously. Variations in cardiac output were determined more b y changes in stroke volume (Vs) than by fH; therefore, fH was a less r eliable predictor of metabolic rate than was (Q) over dot. Given that (Q) over dot can be used to estimate (M) over dot O-2, so faithfully, the advent of a cardiac output telemeter would enable robust estimates to be made of the activity metabolism of free-ranging fish in nature, thereby strengthening one of the weakest links in the bioenergetic mo dels of fisheries biology.