Dr. Brown’s team tagged and released the sharks, using a combination of tracking tools. Passive integrated transponder, or PIT, tags, slightly larger than a grain of rice, hold electronics that allow them to act as lifelong bar codes that can be detected and read without the animal having to be recaptured. The tags were inserted into each shark via a small incision on the animal’s underside.

An acoustic tag attached to the fin of each shark sends out a ping each time it comes within about a third of a mile of an underwater receiver, or within about 30 feet of another tagged shark. Each ping is time-stamped, meaning it is possible to detect when a particular shark was at a particular location, and whether it came into contact with any other tagged sharks there.

During the initial tagging process, Dr. Brown’s team tagged 250 sharks in Jervis Bay with both PIT and acoustic tags. Last year, 38 more sharks were tagged. From the data analyzed so far, Dr. Brown found that the Port Jackson sharks tagged four years ago consistently returned to some locations. At first, it was thought they were coming together to breed, but Dr. Brown later discovered the sharks were of mixed ages and sexes, leading to another theory: that the sharks liked to dine together.

To rule out the possibility that the animals were attracted to the location itself rather than one another, Dr. Brown needed an artificial environment in which to observe them up close. So he reached out to a former student, Jo Day, who had studied social interactions in bottlenose dolphins and now worked as a research and conservation coordinator at Taronga Zoo.