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)

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_tfБ%, 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.