An experimental and theoretical analysis of the effect of added weight on the energetics and hydrostatic function of the swimbladder of European sea bass ( Dicentrarchus labrax )

Abstract

We conducted experiments to determine the effect of the increasing ultrasonic/radio transmitter weight on the routine metabolic rate of sea bass. We measured the oxygen consumption (MO2) of fish tagged externally with a dummy transmitter made of a hollow pipe, the weight of which was adjusted with lead to represent in water 0, 1 and 4% (R tf) of the animal weight. We then developed a theoretical model to estimate, for a given fish size, the range of added weight that fish can compensate for through swimbladder regulation. When R tf≤1%, MO2 of untagged and tagged fish did not differ significantly. However, when R tf reached 4%, fish that carried a tag incurred a significant elevation of oxygen consumption, which represented 28% of their total useable power (or metabolic scope). This result strongly supports the view that a high R tf ratio contributes to a decrease in available metabolic energy by diverting energy from, e.g., growth or swimming performance. A comparison between the tagged fish and the theoretical model reinforced the hypothesis that, when R tf attained 4%, the increase in metabolic rate reflected a supplementary and costly swimming effort necessary to maintain vertical position. In this condition, the swimbladder cannot regulate the buoyancy of tagged fish.