This is the third installment in a four-part series on exhibits, public programs, lectures and scientific research in conjunction with Shark Summer at the Birch Aquarium at Scripps.
By Lynne FriedmannAmong the sights and sounds of summer at La Jolla Shores, keep an eye peeled for a white advertising balloon; the type most often seen hovering over car dealerships. Aloft at 150 feet, tethered offshore to metal weights by two strands of monofilament fishing line, and labeled “Birch Aquarium Leopard Shark Research,” what looks deceptively simple is, in fact, sophisticated science in progress.
Mounted to a PVC-plastic housing hanging beneath the helium-filled balloon is a state-of-the-art GoPro Hero3 high-definition video camera that is recording — at 30 frames per second — the movements of scores of leopard sharks cruising the waters below. Wind-free days permitting, hundreds of thousands of images will be collected then analyzed using powerful, custom software that can isolate and map the movement of each individual shark, frame by frame. The goal is to decipher this collective animal behavior.
“Specifically, we’re looking to see if there is a social component to this aggregation behavior,” said Andrew P. Nosal, Ph.D., the Birch Aquarium’s DeLaCour Postdoctoral Fellow in Ecology and Conservation.
Nosal and his colleagues at Scripps Institution of Oceanography and Princeton University (where the analytical software was developed) will look for patterns of behavior such as sharks accelerating toward each other or trying to keep up with each other as they’re swimming. Also of interest is tracking where in the aggregation the fastest swimming occurs, whether sharks turn back or swim out of the aggregate when they reach its periphery, as well as “residency time” —that is, how long they stay in the aggregation before they leave again.
This is quite different than fish schooling behavior.
“A school indicates they are all swimming together in one direction,” Nosal said. “An aggregation is a broader definition indicating a group all milling around randomly.”
The software can also size animals and pinpoint whether larger sharks are behaving differently than smaller ones in certain circumstances and vice versa. Digitally tracking individual sharks reveals details such as a shark’s turning angle relative to its neighbors, as well as acceleration rates turning toward or away from other sharks.
“Are the fish attracted to each other? Are they trying to keep up with each other?” asked Nosal. “Those are the kinds of things you look for to indicate social behavior.”
Heretofore, this type of research-video capture was used in a controlled laboratory setting in which a camera was rigidly positioned only one to two feet above a grouping of subject animals such as ants, swarming insects, rodents, or fish in a tank. The technique has also been tried in the field using helicopters and ultra-light aircraft but at considerable expense.
“We are the first to use a tethered, helium balloon,” Nosal said. “It is the most stationary, cost-effective way to do this type of research in the wild.”
Leopard sharks are a convenient model and La Jolla Shores a unique natural laboratory for this collective animal behavior study. Each year, thousands of pregnant females spend the summer months here. While there are other locations where leopard shark aggregations are found (Humboldt Bay, San Francisco Bay, Elkhorn Slew, and several sites off Catalina Island) these locations have either murky water or a rocky bottom, making it difficult to capture images of dark-body sharks.
“What’s really nice about the La Jolla aggregation is that it occurs along the open coast, the water is relatively calm, and you have the nice sandy bottom which makes it a lot easier to see the sharks,” Nosal said.
In adapting this technique to a balloon platform outdoors, the wild card is the wind, which ideally should be 0-5 mph. Those conditions generally exist in the early mornings. The most sharks, however, are found at the observation site in the afternoon when the shallow water is warmer. But that’s also when the wind picks up. So, weather permitting, video will be shot at different times of day.
This is the first year of a multi-year study. Perfecting the technique is this summer’s goal. So far, initial tests of the system have yielded very stable video. It also determined that the camera’s optimum field of view of the water is 500 feet by 375 feet, within which more than a hundred sharks can be recorded at a time.
“It’s a nice wide field of view,” he said. “We see a lot of animals.”
Once preliminary data is generated, analyzed, and initial findings published, Nosal will apply for a larger grant to keep the project going and expand the study by asking new questions. For example, what happens to collective shark behavior when a predator appears? An answer could come by dragging a decoy sea lion through the middle of the aggregation and looking at patterns the sharks make as they swim away from it.
“Is there a transmission of information from shark to shark?” wonders Nosal. “Does one shark scattering cause the next shark to scatter creating a domino effect?”
The questions to be answered are endless. For now, just the ability to exquisitely document the movement patterns of these animals when they’re in a collective group adds to scientific knowledge.
“We have never seen this kind of fine-scale detail before,” Nosal said. “That’s why it’s really exciting.”