Alternative model structures for bioenergetics budgets of a cruising predatory gadoid: incorporating estimates of food conversion and costs of locomotion

Abstract

Swimming costs of North Sea saithe (pollock) (Pollachius virens) were estimated from a balanced energy budget equation using field estimates of food ration and growth together with two alternative food conversion functions. Food ration estimates were obtained by application of a gastric evacuation model to field data on amount and composition of stomach contents. Laboratory-based net conversion efficiency, κlab, produced values of 1.61–6.72 of the activity multiplier, whereas estimates of 1.44–3.27 were obtained from application of an activity-modulated net conversion efficiency, κwild. The activity multiplier ranged from 2.0 to 2.5 at optimum cruising speed. The high activity levels obtained by application of κlab probably reflect low-cost accumulation of lipid in laboratory saithe. All together, the results indicated that net conversion efficiency in North Sea saithe was better described by κwild (ranging from 0.43 to 0.50) as opposed to κlab (0.51–0.76). Model estimates of food ration and body growth using activity costs at optimum cruising speed further demonstrated the sensitivity of estimates from the balanced energy budget equations to applied parameter values and field estimates of variables.