The Lusitanian toadfish, a shallow-water fish that lives along the Atlantic and Mediterannean coasts, does not have the most conventionally romantic name or features (at least from a human perspective). But with the onset of spring, males of this species become crooning suitors as they erupt into serenades that entice females to mate with them and warn other males to stay out of their turf.
These amorous songs, known as boatwhistles, emanate from the males’ swim bladders and sound like vibrating cell phones. Now, research has shown that noise pollution from humans in the form of vessel traffic or cacophonous undersea activities poses new challenges to their reproductive success.
To understand exactly how disruptive people are to the sex lives of toadfish, a team of scientists led by Daniel Alves, a PhD student in physiology at the Universidade de Lisboa, studied the species both under laboratory conditions and in the wild at Portugual’s Tagus estuary.
Unfortunately, the team’s results reveal that “boat noise can severely reduce the acoustic active space and affect the chorusing behaviour in this species, which may have consequences in breeding success for individuals and could thus affect fitness,” according to a study published on Tuesday in the Journal of Experimental Biology.
“The Lusitanian toadfish has been used for many years now in our research group as a model species in bioacoustics as it is well suited for both behavioural and physiological experiments,” said Alves in an email.
“This species is well suited for assessing the impact of man-made noise in reproduction because there is a direct correlation between sound production in males and the amount of eggs that they receive from females,” he added. “That means that any disturbance in the acoustic communication should have an impact on their reproductive success.”
Alves and his colleagues devised two experimental setups to comprehensively probe the effect of anthropogenic noise pollution on the Lusitanian toadfish. In the first, the team collected dozens of toadfish from the Tagus estuary with the help of local fisherman, and transported them to aquariums at the Universidade de Lisboa.
The researchers also used a hydrophone in the estuary to record the sounds of a small fishing boat with an outboard engine, at a distance of up to 32 feet, and the reverberations of a ferryboat at a distance of roughly 165 feet. Back at the university, the fish were then exposed to recordings of boatwhistles that featured no background noise, as well as recordings with those anthropogenic noises from their native habitat. Alves and his colleagues captured brain scans of the fish while they listened to the various sounds, which revealed that the vessel sounds clearly interfered with the ability of the species to discern their own calls.
The fishing boat noises “had a severe impact on communication range,” reducing the distance at which the fish could hear their calls by 75 percent, according to the study. While the ferry was not as disruptive, it still shortened the communication range from about 34 feet to 20 feet. After the completion of this laboratory experiment, the fish were successfully returned to the wild.
In their second experiment, the team visited a Lusitanian toadfish breeding ground in the Tagu estuary and set up nests that included underwater microphones so that they could acoustically monitor the animals.
Male toadfish are known to stagger their calls in very precise patterns as a response to each other, and these rhythmic choruses occurred in the absence of noise pollution. But when the team played recordings of vessel traffic, the toadfish songs took on a more random pattern that suggested the anthropogenic sounds threw off their regular timing. The fish also simply sang less when disrupted by the vessel noise, which has big implications for attracting females and communicating with other males.
Taken together, the experiments show that humans are clearly interrupting the sex lives of these singing fish, which did not come as a surprise to the team.
“We expected that boat noise would have a significant impact in their acoustic communication because boat noise has a frequency range that overlaps heavily with the frequency range of their hearing and the sounds they produce,” Alves said.
Fortunately, the Lusitanian toadfish is not currently endangered, so there is no immediate threat to their survival. That said, the researchers plan to follow-up on the new study with further observations of the females’ behavior, including the impact of noise pollution on interactions with males and on the survival of their eggs.
These avenues of research could help assess whether vessel traffic could threaten this species of toadfish in the future. On a broader level, though, the new study adds to the mountains of evidence that human noise pollution is having deleterious effects on animals throughout the oceans, including species that are key to human diets and livelihoods.
“Without studies directly measuring the impact of man-made noise on reproductive success or egg survival in this species, I don't want to speculate too much if boat noise is enough to endanger this species,” Alves said, referring to the Lusitanian toadfish.
“However there are species that are endangered and/or have high commercial value that also rely heavily on acoustic communication, such as cod, haddock, and meagre,” he added. “If the impact of boat noise in their communication is as high as in the toadfish, then the consequences could be very costly.”
With that in mind, it’s more important than ever that these noises are reduced through measures such as propeller redesigns and better vessel traffic management. Fish mojos are not the only thing on the line, as this is also an issue that affects our own food supply and economic health.