The study, published in the journal Environmental Pollution, reported that fish exposed to engine noise had elevated levels of the stress hormone cortisol and the reproductive hormones testosterone and 11-ketotestosterone, which corresponded with the behavioural changes observed by the researchers. The techniques used by the scientists to measure hormones, which offer a window into complex behaviours, could be used to gauge the effectiveness of future noise-reduction measures.18

As the COVID-19 pandemic sweeps across the globe, people are beginning to understand, at a very personal level, the ways in which infectious diseases can devastate life. But disease outbreaks are not confined to just humans or to life on land. Infectious disease-induced mass mortality events are known to afflict a variety of species, including invertebrates, birds, fish, and both land and aquatic mammals. However, these events in aquatic mammals are understudied compared to their land-dwelling counterparts. I Sanderson and Alexander discovered that infectious disease-induced mass mortality events occurred in 14 percent of marine mammal species between 1955 and 2018. Viruses were responsible for 72 percent of these events and caused 20 times the number of deaths than bacterial outbreaks. Specifically, morbillivirus and influenza A outbreaks were the most commonly recorded. Due to their life cycles, both viruses can infect multiple hosts since they have the potential to be transmitted between various species.

The Amazon river dolphin (Inia geoffrensis) or boto, already classified as “endangered” since 2018 by the IUCN, the International Union for Conservation of Nature, is now coming under even more serious threat across Brazil and Latin America. An estimated population in the low tens of thousands is thought to exist in the wild, though counting the animals accurately in the Amazon’s murky streams is challenging. Like other river dolphins, the boto communicates with variable whistle tones. Occupying the Amazon and Orinoco river basins, which cut across the northern half of the South American continent, these freshwater mammals were historically abundant, and are protected today by Brazilian law; it is illegal to kill them. But for years, poachers have targeted the dolphins, using their fatty blubber as bait to catch a carnivorous catfish called piracatinga, which is drawn to the scent of rotting flesh.