Physiological and behavioral responses to hypoxia in the bonnethead shark, Sphyrna tiburo: routine swimming and respiratory regulation

We examined the effect of hypoxia on the swimming speed, respiration rate (oxygen uptake), gape and ventilation volume of the bonnethead shark, Sphyrna tiburo. We used a sonic flowmeter developed for this study to examine swimming speed changes of sharks held in artificial lagoons during diurnal dissolved oxygen changes. Sharks were observed to swim at about 34 cm s(-1) during the day but increased to about 40 cm s(-1) at night when dissolved oxygen levels fell to < 3 mg l(-1). Using a closed system respirometer we examined changes in swimming speed, respiration rate and gape at four dissolved oxygen levels. Swimming speeds averaged 24 to 25 cm s(-1) under normoxic conditions but increased to 38 to 40 cm s(-1) during hypoxia. Similarly, respiration rate increased with increasing speed and decreasing dissolved oxygen. Gape averaged about 1.0 cm under normoxic conditions and increased to a maximum of about 3.5 cm during hypoxia. Using assumed oxygen extraction efficiencies of 25, 50 and 75% and observed respiration rates, we estimated that ventilation volumes of about 25 to 470 l h(-1), depending upon oxygen concentration, would be necessary for gill ventilation. These experiments suggest that changes in swimming speed and mouth gape are important for respiratory regulation in ram ventilating sharks.