Home Fish Science Do Fish Swim with the Current? (Detailed Explanation)

Do Fish Swim with the Current? (Detailed Explanation)

by gvald

The destiny of a fish is simple: to eat and be eaten. It reproduces when the season comes, and makes us pretty little tadpoles, which will have exactly the same destiny as him. Eternal repetition, until the end of time. Or the drying up of the oceans.


Most fish do not get tired and swim in the direction of the current, there are however some exceptions:


Which fish swim against the current?

Salmon is the fish that swims against the current the most, in fact, to complete its entire life cycle, salmon needs to migrate between the sea and fresh water to reproduce.


The ability to cross an obstacle is closely linked to swimming and jumping abilities, which in turn depend on the morphology of the species, its size and the water temperature.


The silhouette and morphology of the salmon make it a very good swimmer able to jump. With the sea trout, it has, at the adult stage, the greatest capacities of crossing obstacles by swimming and/or jumping:

  • It can swim at a sprint speed of about 800 to 1100 ft/mn
  • It can jump over heights of 3 to 7.5 ft


However, this capacity is weighted by the physical conditions of the fish, the configuration of the obstacle to be crossed and the conditions that the fish finds at the foot of the obstacle:

Salmon must be in good health. The larger an individual, the higher it jumps and the deeper the drop-off should be.


The drop height should not exceed a height proportional to the length of the fish. 

The obstacle should not be composed of several steps too close together, otherwise the fish does not have enough space to take its momentum and reach the next step.

A sufficiently deep take-off pit must precede the obstacle so that the fish can gain momentum. The water level must be sufficiently high at the weir and the flow rate not too high for the fish to swim upon landing. Otherwise, the fish will be swept downstream.


Which fish swim with the current?

99% of all fish swim naturally with the current, whether they are freshwater or saltwater. Salmon are virtually the only fish that are forced to swim upstream to fresh water to ensure the safety of their offspring.


Why and how do salmon swim upstream?

Swimming in salt water most of its life, a salmon must eventually swim upstream to the fresh water where it was born. The salmon then spawns (reproduces) and lays its eggs before dying, thus completing its life cycle. It is thus by making an immeasurable physical effort that it allows the existence of a new generation. But how does a salmon’s heart hold up in the middle of an upstream current to make such an effort? The salmon obtains a “boost” of oxygen by the action of an enzyme normally absent from blood plasma.


Unlike birds and mammals, the circulatory system of fish is unidirectional. The heart, simple and only venous, only serves as a pump to send the blood back to the gills, which will allow its oxygenation. This oxygenated blood gradually returns to the heart, providing a maximum of oxygen to the other organs on the way and leaving only the remains to the heart organ. During an effort as intense as the ascent, the myocardium has an increased need for oxygen to function at the required rate.


To keep the blood pumping properly, the heart then takes advantage of the effect of carbonic anhydrase (CA), an enzyme responsible for converting CO2 into bicarbonate, H2CO3. Through this reaction, it lowers the pH, acidifying the blood and forcing the release of O2 from hemoglobin as a result. A chemical mechanism observed directly on the heart of a salmon, by measuring the pH with probes as thin as a human hair.


How do fish swim against the current?

Water is a very dense material offering a strong resistance to anything that moves. The more streamlined the object, the easier it slides: the water is first separated at its head and then closes behind it. Fish exploit this effect when they swim. The most successful swimmers are streamlined to penetrate the water, but have relatively wide sides offering strong resistance to the water.


By flexing its body in a series of regular waves, the fish glides through the water much like a snake twists through grass. Each wave of flexion goes from the head to the tail, and pushes the resistance of the water allowing the fish to move forward.


To move, fishes mainly use their fins, each one having a specific role: the caudal fin, by powerful undulation movements, has a propulsion role. It also allows the change of direction by acting like a rudder.


The dorsal fin and the anal fin have a function comparable to that of the keel of a boat. The two pectoral fins corresponding to the front limbs in vertebrates are used for maneuvering: fast displacements, changes of direction, braking… Acting as balancers, they also play, with the two pelvic fins which correspond to the lower limbs, the role of stabilizers.


The rays which present a flattening of the body do not have anal fin. On the other hand, their pectoral fins, called wings, are very developed and welded to the body. The rays use them for propulsion.

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