Estimating the population size and density of rare, cryptic species poses a logistical and statistical challenge
Adapting off-the-shelf GPS and camera equipment to work in concert underwater allows researchers to try out new techniques.
Knowing an endangered species’ distribution and density is key information for assessing its status.
The handfish is a curious creature. A type of anglerfish in the Brachionichthyidae family, most species of handfish live in the waters around Tasmania. But unlike many oceangoing critters, handfish often “walk” along the ocean floor instead of swimming. Handfish usually dwell on sand and silt substrates at depths ranging from two to 30 meters, where they often eat shrimp, small crustaceans and fish.
One species, known as the spotted handfish (Brachionichthys hirsutus), is in danger of extinction — it’s been disturbed by fishermen dredging for scallops, had its waters polluted by development, and its spawning areas buffeted by environmental pressures. It doesn’t help that strolling along the substrate isn’t the most efficient way for a species to find new territory.
These animals are small (a few inches long), and as with many rare and elusive animals, it can be challenging for researchers to estimate the size of a population and to track changes in it over time. So a team of Australian scientists recently tested a new technique to survey the handfish in an estuary in which handfish are known to live. By combining underwater photography with the precision of GPS technology, the new procedure makes tallying the elusive handfish easier and, statistically, far more powerful. Knowing the location and density of the fish, Lynch et al. (2015) write, is “the key data required for monitoring conservation status for this critically endangered species.”
A tedious task
To survey handfish, researchers have relied upon underwater visual censuses (UVCs). They’d swim along 40 routes, each 100 meters long by three meters wide, tracking their location with tape measure reels the endpoints of which were linked to GPS coordinates. When the swimmer would spot a fish, he or she would note the distance along the reel, and later extrapolate the fish’s location. But the swimmers were only able to conduct two 100-meter transects each dive. This, coupled with the hassle of figuring out the fishes’ locations from GPS data and reel measures, meant conducting a survey was costly and time-consuming.
Recently, scientists have begun using diver-towed GPS buoys when conducting underwater visual censuses. The technique eliminates the need to extrapolate a diver’s coordinates from his or her location along a measuring tape. Also, because there’s no need to deploy the tape reels, the target fish are less likely to be disturbed.
Working in groups of two or three SCUBA divers, the researchers swam along the ocean floor at three depths — shallow (three to six meters), medium (six to eight meters) and deep (eight to 10 meters). Each diver searched a 1.5 meter-wide band, and in each group, one diver towed a small GPS buoy on the water’s surface that recorded the team’s location every five seconds.
Each swimmer carried a camera with a clock synchronized to the GPS buoy and photographed both sides of every handfish they found. Because handfish, like some other animals, have unique markings on their sides, the researchers could later determine whether they’d counted the same fish more than once. Compared to the new technique, old surveys had required twice as many dives to conduct a UVC that covered a fraction of the area.
In an email to WildTech, Lynch laid out a few tips for conservationists or researchers interested in deploying technology under the water. “Combine GPS with Go-Pros and time-stamped digital underwater cameras (you will need to do a synchronization with the GoPro and the camera, as GoPro only has a timer, not a clock),” he wrote.
“Then use screen grabs and photographs of your species in their habitat with a classification schemes such as CATAMI and develop habitat models or micro-habitat models. Also this is all task loading so make sure your dive planning takes this increase in risk into account.”
Lynch’s team didn’t really face any technical challenges when implementing the census technique, he indicated, because they used off-the-shelf technologies.
“This is the really cool thing about the modern world,” he wrote. “When I first started using GPS for sampling in 1997, the unit cost $3500 AUS and was only accurate to 100m. Now we use a $50 AUS bike tracker that is accurate to within 10m.”
There was, of course, the occasional surprise “We did see one fish eaten by a flathead,” Lynch wrote, “a very sad day!”
Since their paper came out, the team has expanded its range and sampled all nine known populations of spotted handfish. The technique joins other technologies making their way into marine research, including UASs and surfboard-like robots working in tandem off the coast of Hawaii’s Big Isle and the OpenROV, an open-source underwater drone.
Lynch, T., M. Green, and C. Davies. “Diver-towed GPS to estimate densities of a critically endangered fish.” Biological Conservation 191 (2015): 700-706.
Schories, D., Niedzwiedz, G., 2012. Precision, accuracy, and application of diver-towed underwater GPS receivers. Environ. Monit. Assess. 184, 2359–2372. http://dx.doi. org/10.1007/s10661-011-2122-7.