Researchers at Scripps Institution of Oceanography at the University of California San Diego were part of an international team that for the first time used hydroacoustics as a method for comparing the abundance of fishes within and outside marine protected areas (MPAs).
Cabo Pulmo has been the site of several studies by Scripps researchers since 2002. In 1995, local fishermen led the creation of a 71-square-kilometer (27-square-mile) undersea park to protect the waters they fished. The current MPA has been identified as the most successful in the world in terms of maintaining a sustainable fishery in which fleets operate just beyond the boundaries of the MPA. There, as in other parts of the world, surveys of coastal marine life are often performed through underwater visual censuses taken by scuba divers.
Researchers surveyed the waters of the MPA using sound waves produced by hydroacoustic equipment mounted on boats to image schools of fish and other marine life. They performed transects, scanning the water column in rows. They similarly surveyed waters outside the MPA. Fish density, total biomass, and the size of individuals were significantly greater inside the MPA. In comparison with waters outside the MPA, animal abundance in reefs was as much as 50 times higher, highlighting the importance of both habitat complexity and protection from fishing for fish populations.
“Both hydroacoustics and marine protected areas are well-established but it is novel to use the former to assess the latter,” said study lead author Jack Egerton now a researcher at the University of Texas who performed the work while at Bangor University in Wales, U.K. “Through this, we have been able to see how important the Cabo Pulmo National Park is for fish populations in the area.”
Although acoustic surveys can be done much faster than underwater visual censuses, the researchers acknowledge that fish sizes can only be approximated, and the method doesn't provide species-specific information. However, they concluded that the hydroacoustic method could still be useful in gauging the benefit of MPAs, as conventional survey methods are often prohibitively expensive and can be limited by issues such as diver depth limits and water clarity.