Can dead fish swim upstream? Uncover the secret of the amazing vortex swimming

Produced by: Science Popularization China

Author: Wang Jiayin (Institute of Advanced Technology, Chinese Academy of Sciences)

Producer: China Science Expo

Everyone has seen fish swimming in water, but what about dead fish? It may sound a bit absurd, but the 2024 Ig Nobel Prize in Physics was awarded to a study on dead fish swimming.

Scientists from the United States were awarded the prize for "demonstrating and explaining the swimming ability of dead trout." The study not only reported a surprising and magical phenomenon, but also revealed the mysteries of fluid dynamics and provided new insights into how fish use eddies in the water to save energy.

Different fish have different swimming styles:

Fish swim in many different ways, and they are more complex than we think. The most common is oscillatory swimming, where the fish bends in an S-shape, creating a traveling wave from head to tail that propels the fish forward. But this is just the tip of the iceberg.

Some fish, such as swordfish and mackerel, swim in a cruising style. Their streamlined bodies and crescent-shaped tail fins allow them to maintain high speeds for long periods of time. In contrast, eels swim in a snake-like manner, with large, wave-like movements throughout their bodies, which is suitable for navigating complex environments.

Interestingly, some fish have developed special swimming methods, using their pectoral fins to "walk" on the seabed, while flying fish can jump out of the water and glide a distance using their pectoral fins. These diverse swimming methods reflect the adaptation of fish to different ecological environments.

flying fish

(Image source: Wikipedia)

A match made in heaven: fluid dynamics and fish swimming styles

Understanding how fish swim requires the principles of fluid dynamics. As fish swim, they are actually constantly manipulating the water flow around them. By moving their bodies and fins, fish are able to generate and control vortices, which provide propulsion.

Interestingly, the vortices created by fish when they swim are not random. Studies have found that fish that swim efficiently are able to create organized vortex systems. These vortices not only provide propulsion, but also reduce water resistance, allowing fish to swim faster and with less effort.

The energy efficiency of fish swimming has always been the focus of scientists. Studies have found that in addition to optimizing their body structure, fish also use a variety of strategies to save energy. Some fish, such as tuna, can migrate thousands of kilometers, which requires extremely high energy efficiency. Many fish also use "gliding" to save energy. After swinging a few times, they will stop moving briefly and use inertia to glide for a distance. In addition, schooling is also an energy-saving strategy. Following the vortex generated by the fish in front, the fish behind can save a lot of effort.

Next time you watch fish in an aquarium or by the river, pay attention to the way they swim. You may find that behind the seemingly simple movement lies the subtle mysteries of fluid dynamics. And these mysteries are inspiring us to create smarter and more efficient future technologies.

Vortex swimming: a unique way of swimming

Karman vortex street is a series of regularly alternating vortices formed behind a blunt object such as a cylinder when the fluid flows at a certain speed, presenting an orderly structure similar to a street. The research team was inspired by an interesting phenomenon: in rivers, fish often like to stay behind obstacles, and there are often vortices and even vortex streets behind the obstacles. Scientists are curious, can fish benefit from the special water flow in these vortex areas? To explore this question, they designed a clever experiment.

Karman vortex street animation

(Image source: Wikipedia)

In the experiment, the researchers placed a D-shaped cylinder in the tank to generate regular eddies. When live trout were placed in this environment, they exhibited a unique swimming style, known as the "Kamen gait." The fish's body sways in a large amplitude, low frequency manner, and its frequency is surprisingly consistent with the frequency of eddy formation. This swimming style seems to allow the fish to maintain its position while saving energy, and even swim upstream.

NASA image of the hurricane-induced Karman vortex street around the Juan Fernandez Islands off the coast of Chile

(Image source: Wikipedia)

But what was really surprising was that when the researchers conducted the experiment with dead trout, they found that even dead fish could exhibit similar "swimming" abilities!

Rainbow Trout

(Image source: Wikipedia)

The mystery of the dead fish's "resurrection"

So, how do dead fish "swim"? The answer lies in fluid dynamics and the soft nature of the fish body. When a dead fish is placed in a vortex, water flows from different directions act on the fish body, causing it to swing periodically. This passive swing can interact with the vortex in the water flow to generate forward thrust.

The researchers found that the frequency and amplitude of the swinging of dead fish were very similar to those of living fish. This means that when fish use vortices to swim, they are largely using a passive mechanism. The softness and shape of the fish body have evolved over a long period of time and are very suitable for this passive propulsion. Simply put, this study reveals an ingenious way of using energy in nature. In turbulent water, fish do not simply fight against the water flow, but learn to "go with the flow" and use the energy in the water flow to reduce their own energy consumption.

This study is not only interesting, but also has potential application value.

Understanding how fish use eddies efficiently could lead to new designs for underwater robots. These robots could be more agile and energy efficient in turbulent waters. Understanding how fish use eddies could also help us design more efficient ships and submersibles. For example, the design of a ship's hull could take into account how to better use the eddies it generates to reduce drag.

In addition, this study also provides a new perspective for us to understand the ecological behavior of fish. In rivers and oceans, the reason why fish choose to stay or migrate in specific locations may be related to their use of water flow characteristics. This is of great significance for fish conservation and fishery management.

This study, which won the 2024 Ig Nobel Prize in Physics, may seem absurd, but it is actually very inspiring. It reminds us that there are surprises everywhere in scientific exploration, and even a dead fish can reveal the mysteries of nature. This study not only deepens our understanding of fluid dynamics, but also shows how organisms cleverly adapt to and utilize the environment.

Next time you watch fish in an aquarium or by the river, pay attention to the way they swim. You may find that behind the seemingly simple movement lies the subtle mysteries of fluid dynamics. And these mysteries are inspiring us to create smarter and more efficient future technologies.

References:

1. James C. Liao, Neuromuscular Control of Trout Swimming in a Vortex Street: Implications for Energy Economy During the Kármán Gait

2. David N. Beal et. al., Passive Propulsion in Vortex Wakes