A scientist sent sound waves into the waters of Cabo Pulmo National Park in Mexico, single-handedly surveying the entire 27-square-mile (71 square km) marine reserve twice in just eight days. By shooting sound through the water column in horizontal transects and analyzing how the sound waves behaved, the researcher gathered data that he and his colleagues later used to calculate an estimate of how many fish were swimming in the preserve off the tip of Baja California – and their size.
By doing the same thing in the ocean outside the reserve, the researchers determined that the density of fish was nearly 300 percent higher inside Cabo Pulmo. The fish were also 52 percent larger.
This new application of hydroacoustic technology could lead to much cheaper ways of keeping tabs on ocean life, enforcing prohibitions against fishing in marine protected areas (MPAs) and verifying the effectiveness of protecting vast swathes of the sea. The technology also could potentially prove a powerful tool for small island states, which have created large marine reserves in recent years but often lack the resources to monitor them.
The Cabo Pulmo project, detailed in a paper published earlier this month in the journal Scientific Reports, appears to be the first time acoustics have been used to compare marine life inside a reserve to that outside it. Over eight days, a single researcher (plus a boat operator) scanned for fish and other marine life in the park and outside it using a hydroacoustic device called an echosounder that was attached to the vessel. A previous survey by divers took four people six days to survey just 0.1 percent of the park.
“The acoustic surveys took eight days, but we did them with a higher coverage than was strictly necessary, so it could have been done faster,” said Jack Egerton, the study’s lead author, who performed the work while at Bangor University in Wales and who is now a researcher at the University of Texas at Austin’s Marine Science Institute.
Echosounders emit pulses of sound that reflect back to the boat if they hit an object like a fish. Those echoes are analyzed to determine the size and number of the objects encountered. The study used a split-beam echosounder, which can identify the locations of objects in the water in three dimensions.
“Managers and authorities in many countries spend a lot of financial resources assessing marine protected areas,” Octavio Aburto, a coauthor of the study and a marine ecologist at the Scripps Institution of Oceanography, said in a statement. “The results of this paper demonstrate that it is possible to use acoustic technologies to generate information about marine resources inside MPAs in a faster and less expensive way, reducing the costs for governments in ocean conservation.”
Besides saving time and money, acoustics surveys aren’t dependent on water clarity or limited by how far a person can dive. But those improvements come with some challenges.
Acoustics aren’t yet able to identify specific species, a critical limitation compared to surveys by divers.
If the technology can advance to the point where acoustics can identify individual species, “then the acoustic estimates can be more certain and more information on the community structure could be provided,” said Egerton. “This is, however, very tricky to do with such a mixed species assemblage, but could be attempted in the future.”
The cost savings from not having to hire a team of divers also come with a caveat. Egerton said the startup costs of the expensive hydroacoustic technology could initially be double that of a dive team.
“But once the acoustic kit is owned, then subsequent costs would obviously be much cheaper,” he said.
Ideally, he said, both acoustics – for the benefit of covering more area in less time – and divers – to be able to identify specific species – would be used.
For countries that can’t afford both, just being able to verify that there are more and bigger fish in an MPA could be a benefit. But if acoustic surveys were to get better at species identification, that might be when the real management benefits kick in.
That advancement may not be too far off.
“We’re right on the edge of doing surveys with only acoustics right now,” said Timothy Rowell, a graduate student at Scripps who was not involved in the new study but has used hydroacoustics to study grouper and Gulf corvina.
“Here, they were able to get biomass and density and so forth and to estimate sizes, but couldn’t get species,” he said. “But new technology might actually get us to the species level.” At first, he said, that data would need to be verified with divers or remotely operated vehicles or by catching fish in nets. But eventually only acoustics would be needed.
The key to that advancement would be broadband, or wideband, which produces higher-resolution data by sending out a wider spectrum of frequencies, thus gathering a larger amount of potential information.
“Broadband is a little more expensive and probably harder to analyze, but there are lots of efforts to develop it right now,” Rowell said. “The research community is really starting to grasp the potential of broadband echosounders – which use multiple frequencies and can be good for picking up different species because different species have different signals.”
As the technology continues to advance, new management applications should follow. “It would be especially interesting to be able to survey an area in this way prior to MPA establishment and then see what happens over the years,” said Egerton. He noted that hydroacoustics could be also deployed to survey fish density to identify the places most worthy of protection.