Large populations of sharks, manta rays, and sea turtles swim through the lagoons of a ring-shaped group of coral islands out in the Pacific called Palmyra Atoll. To get in and out of one of these lagoons, mobile marine species use a deep channel that was dredged during World War II. It’s a key highway of sorts, and according to researchers monitoring the traffic, rush hour for sharks peaks just after dusk. The findings were published in the Journal of Experimental Marine Biology and Ecology this month.
Many sharks have been classified as near-threatened, vulnerable, or endangered, and while they frequently move between reefs, lagoons, and open sea habitats, quantifying these small-scale movements has been a challenge. To find a better way of monitoring shark movement, a team led by Douglas McCauley from UC Santa Barbara turned to a high-resolution acoustic camera originally designed for the US Navy. Unlike traditional optical cameras, acoustic cameras employ sonar technology – or pulsed sound wavelengths – to create detailed, fast-frame images even in low-light settings. Using their dual-frequency identification sonar, the team created a “sound gate” that sharks would travel through.
They tested their acoustic camera by monitoring reef and coastal sharks for 443 hours in July of 2009 at the near-pristine coral reef atoll. Palmyra is a protected US National Wildlife Refuge, and its waters are “no-take” zones. An expanded channel physically connects the forereef and offshore marine habitats with the Western Lagoon basin. It measures 1.5 kilometers long (1 mile long), 80 meters wide (262 feet wide), and 8 meters (26 feet) deep. It has a sandy bottom and near-vertical walls of coral rubble.
Their acoustic camera – installed on a side wall mid-way along the channel – generated detailed images of 1,196 sightings of sharks making their way through the corridor that links together the atoll’s different environments. From these images, the team was able to calculate shark size and density. At least seven species of sharks frequent Palmyra Atoll, and the most common visitors were blacktip reef sharks. They recorded the highest density of sharks during the post-dusk period: It was about seven times higher than that observed post-dawn, during the day, and pre-dusk.
"We see this as being an important part of new technology coming online to better track the health of shark populations, better understand their behavior and help do a better job with their conservation," study co-author Paul A. DeSalles of UC Santa Barbara explained in a statement. "Before, we had no way to quantify how often and which sharks were using this space. This study fills in those gaps."