How can we test the exercise capacity of a fish?

It's difficult to measure the speed of a fish through the water, since both the fish and the water is moving, sometimes in the same direction and sometimes in opposite directions. Think about yourself swimming or canoeing up or downstream to visualize this problem.

In a laboratory it is easier to measure how fast a fish can swim. Today this is done in a stationary experimental setting where the water is moving through a tube, either by a pump or a propeller, and the fish is swimming against the generated current. At the same, time the water speed (which, since the fish remains in the same position, is also the fish swimming speed) can be accurately measured and controlled.

The figure above shows one example of a tread-mill for fish, where the fish swim inside a tube and the waterflow is generated by a motor connected to a propellor. In this setting it is also possible to measure things like heart rate, cardiac output and respiration of the fish at rest and at various swim speeds.

The swimming capacity in fish depends on a number of factors such as body form (the more streamlined you are the faster you can swim), body size, swimming mode, the amount of red versus white muscle and the effectiveness of the cardiovascular and respiratory systems.

It is not easy to measure the sustained swimming speed (see previous page), and most studies instead look at the prolonged swimming speed. This is tested in the lab by increasing the water speed stepwise until the fish fatigues. Then a value called critical swimming speed (Ucrit) is calculated. In most cases the unit of swim speed is body lengths per second instead of cm per second. It is easier to compare fish of different sizes if the value is given as body lengths per second. Still it is important to be careful when comparing fish that are very different in length.