Using Cs-137 to measure and compare bioenergetic budgets of juvenile Atlantic salmon (Salmo salar) and brook trout (Salvelinus fontinalis) in the field

Through the Cs-137 mass balance method, annual consumption rates were estim ated for juvenile Atlantic salmon (Salmo salar) parr and precocious males a s well as for brook trout (Salvelinus fontinalis) at four sites in the Ste- Marguerite River system, Quebec. With explicit age analysis, consumption ra tes and growth rates were derived on an individual fish and age-class basis . These represent the first consumption estimates for Atlantic salmon in th e wild. Precocious males had consumption rates 1.5 times greater than nonma turing parr, while Atlantic salmon parr consumption rates were 2.7 times gr eater than brook trout. There was a strong positive relationship between in dividual annual consumption and growth rates for Atlantic salmon and brook trout at all sites. Subsequently the concept of field maintenance ration wa s introduced as the intercept of consumption over growth. Maintenance ratio ns for Atlantic salmon parr ranged from 0.010 to 0.016 g.g(-1).day(-1) betw een sites. Brook trout had maintenance rations approximately half those of Atlantic salmon at 0.0059 g.g(-1).day(-1). Precocious male growth efficienc ies were half those of nonmaturing parr despite higher feeding and growth r ates. Brook trout growth efficiencies were significantly greater than those of Atlantic salmon parr. The lower growth efficiencies observed for Atlant ic salmon are likely due to increased metabolic costs associated with highe r activity. On average, Atlantic salmon parr spent 2.4-fold more energy in activity than brook trout. Atlantic salmon precocious males spent 1.7 times more energy in activity than parr.