Debate, Biology | The fish hears the sound of trains…

Thanks for the contribution from Asbjørn Haverstad, who asked whether fish can hear, and whether increased train traffic over the Randklev Bridge has the same negative impact as white stones on the river bed.

First, a few clarifications. First, the white stones that were placed to protect the bridge's pilings were not replaced. Instead, a dark stone was placed on top of the white stone in the path in the middle of the river. These have two functions, as they break up the flow of water and create dark areas and shadows on the white stones. They are designed to create a corridor with better migration conditions for fish in the central part of the river. Along the western bank of the river (towards the tunnel) dark stone has been laid in groups to create a suitable fish passage. Similar measures are being planned on the eastern bank of the river.

Secondly, the color of the rock does not mean “life or death for the fish”, but it is known that all species of fish prefer to avoid being visible to their surroundings. Therefore a large, bright field covering an area of ​​5,000 square meters can cause problems for migratory fish species. However, dark cavities beneath light stones can serve as feeding and hiding areas for small fish. Consequently, the lowlands below the Randklevsbruen have been given a changed ecological function as a result of the conservation works.

All species of fish can perceive, locate and interpret sound. Sound sources originating in the air penetrate the water only to a small extent (0.1%), while sound coming from an underwater source can be heard well by fish over a great distance. Fish do not have normal ears, but they all have inner ear structures built slightly differently than ours. The conversion of sound waves into nerve signals occurs when vibrations in the water pass through the fish's tissues into the fluid-filled spaces that make up the fish's inner ear system. Here, small otoliths (otoliths) float attached via thin hairs to sensory cells, which collect in the auditory nerves in the fish's brain. Otoliths have a greater specific gravity of liquid and therefore vibrate more than otoliths. This causes hair and sensory cells in the inner ear to move, sending nerve signals to the brain. The individual fish is constantly analyzing all this.

The swim bladder also plays a role in hearing, as well as regulating the fish's buoyancy in the water. In some species of fish, such as roaches and goldfish, the front part of the swim bladder is attached to a group of small, mobile bones that provide good transmission of vibrations from the bladder to the structures of the inner ear using otoliths. These species therefore hear much better than trout, which do not have such an advanced connection between the swim bladder and the inner ear.

In addition, all fish have a lateral line organ along the side of the body and parts of the head. This organ does not hear sound, but rather hears the movements of water close to the fish's body. The lateral line thus helps maintain order in schools of fish, record attacking predatory fish or as a tool to get away from stationary objects in the dark.

Human-made underwater noise is known to create a number of problems for fish and marine mammals in fjords and seas, especially when it comes to communication and navigation between species. In rivers, the noise level is naturally high due to water currents. The noise from railway bridges is strong, but vibrations from rails or columns are not known to be a major obstacle to fish migration in rivers. I have verified this myself, and in many cases there are good breeding and breeding areas under these bridges. There is probably no doubt that the fish hear the trains moving on the bridge, but the sound is probably not the kind that is associated with danger.

morten krapohl, Biologist Faberge