Silence is golden

A study suggests that noise can be harmful for farmed salmon, as Vince McDonagh reports.

Important new research has shown that man-made noise does affect salmon – and can inhibit the fish’s growth.

The study, which was carried out by the Norwegian University of Life Sciences (NMBU), has thrown fresh light on the behaviour of fish and why they behave in a certain manner when under stress.

Fish farms can be noisy places at times and a producer had been worried for a while that this was adversely affecting the development of the stock.

The university’s Professor Marco Vindas decided to find out for himself. He has come to the conclusion that excess noise does lead to poor welfare and adversely affects the ability of salmon to grow and reproduce.

Farmed fish can hear people walking on the edge of the cage, pumps and boats, and if the facility is close to land or is land-based, there may also be cars and construction work nearby.

Low-frequency sound is particularly stressful for fish since it is an important factor in predator-prey interactions. Even though behavioural and physiological studies have been conducted to assess the effects of sound on fish species, neurobiological studies have been lacking.

In the study, the team exposed farmed salmon to low-frequency sound for five minutes a day for 30 trials and conducted behavioural observations and tissue sampling before sound exposure.

These trials included plasma cortisol, neuronal activity, monoaminergic signalling, and gene expression in four areas of the forebrain.

Salmon

The results showed that sound exposure induced an activation of the stress response by eliciting an initial startle behavioural response, together with increased plasma cortisol levels and a decrease in neuronal activity.

He adds: “Together, our results suggest that prolonged exposure to sound results in chronic stress that leads to neurological changes which suggest a reduction of life fitness traits.”

Professor Vindas has researched both sporadic sounds that the fish are not prepared for, and sounds they hear often.

He says fish are probably used to people walking around and talking, and the pump always coming at the same time, but a sound that only comes once in a while can be experienced as stressful and frightening.

He also attempted to find out what the salmon can hear, at what frequencies, what the sounds are and how they affect behaviour, stress levels and the brain.

In the first study, the researchers ran an experiment at the Matre Research Station in Vestland.

“We used low-frequency sound because we know from nature that it affects the salmon,” says Vindas.

Professor Vindas

The researchers exposed the fish to a sound that they made using a machine that moved up and down in the water to create sound waves.
Vindas explains that the waves that occur when something moves in water are actually low-frequency sound.

“When a predatory fish swims, the sound of the waves comes first. The predatory fish thus reveals that it is on its way through the sound waves it creates, and then the salmon knows that it must get away. Therefore, we know that such low-frequency sound stresses the salmon.”

The experiment lasted one and a half months and involved exposing the salmon to the same sound several times, at the same time and for the same amount of time each time.

The researchers took samples of the fish’s blood and brains before the experiment started, after they had heard the sound once and then after they had heard it 10, 20 and 30 times.

“We wanted to understand both what happens the first time they hear the sound and what happens over time,” he says.

“Can they get used to it if the same type of sound always comes at the same time? And how long does it take to get used to it?”

When it came to the fish’s behaviour, they saw that they responded strongly and tried to get away from the sound source the first time. Over time, the reactions became less.

“We do not know whether it is because they have become used to the sound or whether it is because they have learned the area in which the sound occurs, and that they therefore stay away from that place. For the fish that were close to the sound source, they reacted just as strongly every time,” explains Vindas.

To find out if the fish were experiencing stress, the researchers measured the level of the stress hormone cortisol in the blood.

The first time they heard the sound there was a strong stress response, but when they had heard the sound 10 times, there was a significant decrease. And when they had heard it 20 times, their cortisol levels were almost down to normal, as they were before the experiment started.

Sound waves

Both the salmon’s behaviour and stress level may indicate that the fish are able to get used to sounds quite quickly, but the investigations of the fish brains showed a more lasting effect.

“When we look at individual parameters, we see that the stress is more long-lasting than we thought in advance,” says Vindas.

Several of the brain areas affected by stress were quite active throughout the experiment.

“We see that the expression of the various genes that have to do with growth and reproduction is inhibited.”

He thinks it is too early to say precisely what this may mean for growth and reproductive capacity, but that these changes in the brain indicate that they are affected.

In addition, the researchers saw that several areas of the brain had an elevated level of the signalling substance serotonin.

An imbalance in the serotonin system can lead to many different diseases, and especially diseases that have to do with brain health.

Serotonin is a substance that affects, among other things, growth, sleep, regulation of emotions and appetite.

One of the areas in the brain that had elevated serotonin levels is an area that has not yet been fully researched, so the researchers cannot yet say what kind of consequences this may have.

“But we know that elevated serotonin levels in other areas of the brain lead to problems. And if the fish has it for a long time, it can lead to so-called ‘loser fish’. They are small fish with poor growth, and they have a chronically elevated serotonin level,” explains Vindas.

He believes that it is clear that sporadic noise is worse for the fish than constant noise, as they only partly get used to it.

Fish farm worker with salmon

Even constant sound at the same time, as we used in this study, affected the fish negatively, and this suggests that the fish’s welfare is impaired by being exposed to man-made sound.

He explains that the first time the fish hears a sound, the sound has a 100% negative effect on it. Over time, the impact decreases to about 50%.

“It is a big improvement, but it is still bad for the welfare of the fish. It is conceivable that the fish can get used to sounds in the long term, but it will depend on what type of sound it is and how powerful it is.”

The research is based on the aquaculture industry’s need to know how sound can affect the development and health of farmed fish, but Vindas points out that the findings are also relevant for wild fish.

Prof Vindas concludes: “When we know that man-made sound, and especially sporadic sound that comes out of the blue, can affect the fish’s growth and ability to reproduce, it is clear that it is also relevant for wild fish that are located near a farm or elsewhere where there is a lot of man-made noise.”