Last updated January 18, 2018 at 3:16 pm
Protecting over-exploited fish is one of the reasons that marine parks, or Marine Protected Areas, have been established all over the world. Comparing how effective they are means having a method to count the number, size and variety of fish both in and outside the protected areas.
Traditionally, this has involved methods like sending scuba divers down to count fish in visual surveys. Part of the long term monitoring project of the Great Barrier Reef involves a visual survey of 212 species. The South Australian Marine Park network uses visual surveys as well as BRUVs -baited remote underwater video systems.
These methods can be very time-consuming and relatively expensive. A team of scientists from the Scripps Institution of Oceanography at the University of California – San Diego tested hydroacoustics for comparing fish stocks inside and outside of marine areas.
Hydroacoustics uses an echo sounder to send an acoustic pulse from say a boat to the ocean floor. When the acoustic beam hits an object like a fish, it bounces back and the echo is picked up by a transducer. It’s a technique commonly used in fisheries, but that has not been applied to conservation monitoring of marine parks before.
In this study they used hydroacoustic surveys to estimate fish density and biomass within and outside the Cabo Pulmo National Park marine protected area in Mexico. Unsurprisingly they found that there were four times as many fish in the marine protected area compared to outside the zone.
The hydroacoustic survey took two people a total of 8 days to survey the whole park twice, as well as two control sites. By comparison, underwater visual surveys carried out by scuba divers took 4 divers 5 days to survey 12 reefs, or about 0.1% of the total park. Acoustic surveys also have the benefit of being able to sample the whole water column at once. Unlike surveys relying on divers, water clarity or currents don’t have an effect.
The new method does have some limitations. It only approximates fish sizes and doesn’t provide the same detail of species-specific information that a visual survey does. It also doesn’t work as well around complex habitats like overhangs and caves which can’t be monitored using acoustics.
The authors suggest an approach that uses acoustic surveys to give a broad scale estimate with then targeted visual surveys for more fine-grained detail may be a cheaper and faster way to monitor marine park fish populations.