Disclaimer: I came across this issue while researching a recent paper. I think it is interesting, but do not think the evidence is solid enough to prioritize this as an intervention yet.
The level of low-frequency ambient noise in the open ocean has doubled every decade since the 1950s; this increase in noise levels is primarily anthropogenic, associated with transportation, development, and resource extraction. Freshwater and estuarine noise levels have also increased in the past seventy years, perhaps even more sharply, because freshwater and estuarine habitats are closer to humans. Most anthropogenic noise is noise pollution, because it doesn’t provide any useful information to wild animals, but may cause them stress or make it more difficult to hear useful information.
A recent meta-analysis examined the effects of ambient noise on fish. It looked at 42 studies from 11 countries, which had a combined 2,354 data points. 36 studies were conducted in laboratory settings, while six studies were collected in situ. The studies included marine, estuarine, and freshwater species.
Anthropogenic noise has a significant negative effect on fish behavior and physiology. In general, behavior responses were 4.73 standard deviations worse in fish exposed to anthropogenic noise than in control fish. Physiological responses were 1.35 standard deviations worse in fish exposed to anthropogenic noise than in control fish. Conversely, there were no significant effects of non-anthropogenic noise on fish behavior or physiology.
Anthropogenic noise increases movement-related behaviors, such as directional changes and altered swimming behavior. These are likely to be predator avoidance behaviors, which are energetically costly and which may indicate that the fish is afraid of predators. Anthropogenic noise also increases the amount of time the fish spends in nest care. While increased nest care may seem to be positive, in fact, it is likely to be negative for fish; nest care is a strenuous and time-consuming activity which can exhaust some fish of some species to the point of death. Finally, anthropogenic noise decreases foraging-related behaviors, such as foraging efficiency, capacity to discriminate food, and number and proportion of food items consumed. Therefore, fish exposed to anthropogenic noise are likely to be hungrier and to have less nutritious diets.
Physiologically, anthropogenic noise increases fish’s hearing thresholds and stress levels. Increased hearing thresholds may make it more difficult for fish to hear quiet sounds, including noises made by conspecifics and predators. It is possible that high levels of anthropogenic noise may cause long-term hearing impairment in fish. The increased stress levels may indicate that anthropogenic noise causes fish distress and impairs their overall welfare.
It is important to note that the effects of noise on fish have been understudied, so just because an effect was not found does not mean it doesn’t exist. It is reasonable to predict that anthropogenic noise has many negative effects on fish which we have yet to study.
While it’s important to note that no cost-effectiveness analysis has been done on anthropogenic noise reduction and it may turn out to be cost-ineffective, nevertheless I think anthropogenic noise reduction is a good example of some heuristics I use when thinking about wild-animal welfare. Many environmentalists support reducing the level of noise pollution in the water in order to protect ecosystems and keep them in a more natural state. Anthropogenic noise reduction would, in theory, allow both environmentalists and wild-animal advocates to achieve their goals. From a purely environmentalist perspective, one might not prioritize anthropogenic noise reduction over other ways of preserving biodiversity: however, if you’re concerned both about conservation and about wild-animal welfare, reducing anthropogenic noise is more cost-effective than it would be if you only cared about one of them.