Fish must spend energy to swim when they forage, reproduce and avoid danger. While it's well known that fish metabolize energy quicker at fast speeds to propel themselves, slow swimming should, theoretically, also demand high energy costs as the fish works to maintain its posture in the water. But this predicted U-shaped relationship between energy and swimming speed has never been found in fish. Intrigued by this paradox, Valentina Di Santo and her colleagues from Harvard University and Boston College, USA, wondered whether the fabled U-shaped curve had evaded other researchers because they only analysed a narrow range of swimming speeds, so they set about measuring how a fish's energy demand changes over a much larger range of speeds.
To test their hypotheses, the authors had clearnose skate swim upstream in a controlled water flow so that the fish maintained a stationary position in the tunnel by swimming against the water at the same speed. By measuring the amount of oxygen in the water as the 9 cm-long fish swam over a wide range of speeds, ranging from about 7 to 20 cm s−1, the authors inferred the energy that the skate used at each speed. However, after each swimming bout, the team allowed the fish to rest to avoid fatigue, while they continued measuring the skates’ oxygen consumption, in order to determine whether the fish were supplementing the oxygen consumed during their swim with anaerobic respiration: elevated oxygen consumption during the rest period would indicate that anaerobic metabolism had contributed to the skates' energy demand while they were swimming. In addition, the team filmed the fish swimming in the tunnel, to find out how they adapted their movements at different speeds.
When the team plotted the oxygen used by the fish during the swimming bouts, they confirmed that the fish's energy demands were high at slow swimming speeds, low at intermediate speeds and high again at the highest swimming speeds, to produce the U-shaped graph that was predicted by theory. After analysing the fish's movements, the team also realised that their high energy demand at slow speeds was explained by the need to create upward lift to prevent themselves from falling to the bottom. To increase lift, the skates beat their fins faster and tilted their head higher than their tail, much like an airplane would at take-off. Following each swimming bout, the team also found that the skates’ oxygen levels remained high. This suggests that in addition to using aerobic respiration while swimming, the fish were also using anaerobic metabolism. And, when the anaerobic contribution was included in the total energy demand, the team found that the rate of energy use increased by ∼50%.
Di Santo and colleagues then decided to test how different swimming protocols affect the maximum sustainable speed that a fish can achieve. Comparing the ramp-up protocol – where the speed is gradually increased, without breaks, until the fish fatigues – against their new protocol with rest periods between swimming bouts, the team found that the fish fatigued at a slower speed when they were using the ramp-up protocol than when they were allowed to rest between periods. This suggests that the widely used ramp-up protocol underestimates the maximal sustainable speed, possibly because of fatigue caused by anaerobic metabolism when swimming at slower speeds.
Di Santo and colleagues have confirmed that fish that swim slowly spend much energy to sustain their posture and show that anaerobic metabolism contributes to the energy demand even at slow speeds. These results give a more complete description of fish energetics from which we can begin to appreciate why fish exhaust themselves when swimming slowly.