A pile perch swimming in a tank while its oxygen consumption was being measured. Photo credit: Melman Neill.
As fish move, there is a speed at which they use the least energy while moving over a given distance; wandering slowly consumes a lot of energy, as does moving at the fastest speeds. The optimal speed usually falls somewhere between the two, allowing creatures to move most efficiently for the distance that they cover. But there's more to life than just moving efficiently. You might need to slow down to spot a tasty morsel, and sometimes just hanging out with members of your own kind is worth a slower pace. So, do fish swim at the most efficient speed when going about their lives, or do they select a different speed as they mosey along? This was the question that John Steffensen (University of Copenhagen, Denmark) was curious to answer when he arrived to teach at the Fish Swimming course at the University of Washington Friday Harbor Laboratories in July 2023 with Paolo Domenici (Italian National Research Council) and Jacob Johansen (University of Hawai'i).
‘We discussed this idea with the students’, says Domenici, who recalls that Izzy Tiddy from the University of Glasgow, UK, was most enthusiastic about the project, adding that Melman Neill (University of Texas at Austin, USA), Alexander Rosén (Technical University of Denmark) and Yuha Hasegawa (Nagasaki University, Japan) were also fascinated. Then, the full class of 16 students went fishing off Jackson Beach, San Juan Island, USA. ‘We slowly drop the seine net from a boat in a U-shape near the shore, then two teams of about eight students each drag the net onshore. It's often like a competition for which teams pulls harder’, chuckles Domenici, remembering the students splashing in the water. They then selected pile perch (Phanerodon vacca) over 30 g body mass for transfer back to the lab. There, the workshop delegates measured the fish's oxygen consumption as the animals swam at speeds ranging from a stately 0.5 body lengths s−1 up to 4 body lengths s−1 – at which point the fish gave up after a few minutes – to calculate how much energy they were consuming at each speed. After analysing each individual's energy consumption as their speed increased, the researchers could see that the perches’ optimal swimming speeds fell around 2.25 body lengths s−1.
But how fast would the perch swim when they had the choice? The team transferred each fish individually to a 103 cm wide cylindrical tank filled with circulating water that was barely moving at the centre but flowing at 50–60 cm s−1 around the circumference, filming each fish as it manoeuvred around, sometimes remaining static at their most comfortable speed, which matched the water flowing toward them. When given this choice, the fish swam more slowly (0.78 body lengths s−1), even though they might have to use a little more energy over a given distance.
The teachers and students then wondered what would happen if they added a fish friend into the mix? Would the fish select a different pace again? After filming pairs of perch swimming convivially in the circulating tank, the team realised that the faster of the two fish slowed to the preferred speed of its slower companion. But why? The team suspects that moving more slowly might leave fish with spare gas in the tank, should they need to make a sudden break to avoid a predator or to snap up a passing morsel. Either way, gregarious perch take their colleagues into account when setting their pace, selecting a slower pace rather than zipping about at the speed that uses the least energy as they go along their way.
