Effects of body mass on physiological and anatomical parameters of mature salmon: evidence against a universal heart rate scaling exponent.

The influence of body mass (M(b)) on the physiology of large, adult fish is poorly understood, in part because of the logistical difficulties of studying large individuals. For the first time, this study quantified the influence of M(b) on the resting heart rate (f(H)), blood properties and organ masses of adults of a large-growing fish species, the Chinook salmon (Oncorhynchus tshawytscha). Surgically implanted biologgers measured f(H) and acceleration activity in sexually mature, male fish ranging in M(b) from 2.7 to 16.8 kg while they roamed freely in a controlled water body at ∼8°C. Blood parameters (at surgery and at death) and body organ masses (at death) were measured to investigate interrelationships with M(b). The scaling exponents for both f(H) and acceleration activity were not significantly different from zero. The lack of scaling of f(H) with M(b) contrasts with the situation for birds and mammals. All blood parameters were independent of M(b), while the masses of the compact myocardium, ventricle and spleen each scaled near-isometrically with M(b). These data raise the possibility that blood oxygen carrying capacity, mass-specific cardiac output and cardiac power output are maintained across M(b) in adult Chinook salmon. Biologging and biotelemetry should advance investigations into the effects of M(b) on the physiology and behaviour of large fish, where current knowledge lags far behind that of birds and mammals.