All animals shift gear as they speed up. Horses switch from walking to trotting to galloping. They do this to conserve energy, changing to a higher gear when the cost of transport becomes too high. Fish also change gait as they speed up. Surf perches start out swimming with their pectoral fins at low speeds, before shifting to tail swimming and finally the burst and coast gait – where they intersperse swimming, using the tail, with coasting, using the pectoral fins – but no one knew the cost of transport of these gaits. According to Jon Svendsen, Eliot Drucker and Jeff Jensen put striped surf perches through their paces in the 1990s and suggested that the fish switch from aerobic respiration (fuelled by oxygen) to anaerobic respiration (not fuelled by oxygen) as they switch from one gait to another, but no one had tested the idea. Discussing the problem at a Friday Harbor Laboratories (University of Washington) Fish Swimming Class in 2005 with fellow students and mentors, Cristian Tudorache, Anders Jordan, John Steffensen and Paolo Domenici, the team decided to find out find out how striped surf perches manage their metabolism as they speed up through higher gaits (p. 2177).
Collecting surf perches from Puget Sound, the team placed individual animals in a swimming respirometer and measured their oxygen consumption rates. Setting the swimming speed at a leisurely 34 cm s–1, before ramping it up to 46 cm s–1 and finally a top speed of 56 cm s–1, the team filmed the fish and measured their oxygen consumption rates as they swam. After swimming the surf perches for half an hour at each speed, the team reduced the swimming speed to give the fish time to recover if they had switched to anaerobic metabolism. If the fish had been swimming anaerobically, their metabolic rate would be much higher during the recovery period than if they had been swimming aerobically, allowing the team to calculate how much energy the fish were supplying aerobically and anaerobically at each gait.
Back in Denmark, Svendsen's supervisor, Kim Aarestrup, joined the team to analyse the movies of the fish swimming and analysed their swimming gaits. At the slowest speeds, the fish generated lift and thrust by flapping their pectoral fins alone, but when they began moving at the second speed they recruited their tail fin in addition to the pectoral fins. Finally, at the top speed, the surf perches alternated bursting forward using the tail and coasting on the pectoral fins.
Correlating the respirometry readings with each gait, the team found that instead of supplementing their energy supply with anaerobic metabolism as they switched from the first to the second gait, the fish were able to keep going at the higher speed fuelled by aerobic metabolism alone. They only began supplementing aerobic metabolism with anaerobic metabolism at the highest speed.
The fact that the fish swim aerobically when using the tail in second gear is puzzling. Svendsen and his colleagues explain that only 0.3% of the muscle that powers the tail is aerobic red muscle so it seems unlikely that the red muscle alone can power the second gait tail beats. The team is curious to find out how the fish do this when 99.7% of the tail-beating axial muscles respire anaerobically and suspect that the fish switch gait to supply more power from the axial muscles for swimming at high speeds.
