Category: General Article
Date: Tuesday, August 29, 2017
Acoustic tracking is used to study the movements of marine species in space and time. The technology works by following the movements of animals fitted with transmitters. I have used this technology in a number of places, but most recently I have used it to study reef shark movements.
In this work, my team and I deployed acoustic listening stations around coral reefs where we wanted to track sharks. In one of these studies, we were studying how much time reef sharks spend on a single reef and how often they move between reefs. We put transmitters in three different species of shark to see what they would do: grey reef sharks, silvertip sharks and bull sharks. We had listening stations on 18 reefs in the central region of the Great Barrier Reef to track where they went.
The results of this work were predictable in some ways and surprising in others. What we found was that all three species did very different things. Bull sharks, the largest of the animals we tracked, moved large distances and frequently moved between reefs. Because all of these sharks were over 6 feet long (1.8 m), we weren’t surprised that they moved farthest and visited the highest number of reefs.
The surprising result came from the differences between silvertip sharks and grey reef sharks. These animals were all of similar sizes, so it could be expected that they would move similarly or use similar amounts of space. From previous studies we expected grey reef sharks to be mostly resident at a single reef and this was what we saw. There were few movements between reefs and the animals that did move were mostly adult males. We suspect that these movements were an attempt to find a mate since the females and young sharks didn’t move between reefs. The silvertip sharks, however, moved larger distances and visited several of the monitored reefs. This included between reef movements of many miles by young sharks only 3 feet (91 cm) long. The differences in how sharks moved was not based on size for at least two of the species tracked and suggests that different sharks have different movement patterns. This means they probably have different needs and uses for coral reefs. Grey reef sharks have home reefs that they rely on for food and shelter; silvertip sharks move more and may feed in areas between reefs; and bull sharks move widely feeding in a number of locations ranging from coral reefs to inshore bays.
The overall conclusions of this study are that different species behave differently, but more importantly it provided information about whether closed areas work for sharks. One way we manage marine areas is to restrict activities in certain areas to protect habitat and the animals that live there. The Great Barrier Reef Marine Park includes a series of areas that are closed off to human activities. This is like drawing a box around a reef, or set of reefs, and designating the area as protected. These areas are usually called Marine Protected Areas and the Great Barrier Reef Marine Park is one of the world’s largest Marine Protected Areas. Some areas allow fishing, some are closed to fishing and some are even no entry so people are not even allowed to visit those areas.
One of the big questions in marine science today is “How well to Marine Protected Areas work to protect species that move?” Closing reefs to activities like fishing is meant to help conserve shark and fish populations and protect habitat, but if individuals move between reefs that are open and closed to fishing, the protection may be less effective.
In the end, we concluded that grey reef sharks living on reefs closed to fishing get a lot of protection, while those living on open reefs receive little. Silvertip sharks get some protection because they spend a bit of their time on closed reefs, but their broader movements mean they spend more time in between reefs (where fishing is allowed) and also on reefs open to fishing. Bull sharks probably get very little benefit from closed areas because they move so often between reefs. Since they never spend large amounts of time at reefs closed to fishing their protection is low. This information has all be provided to the Great Barrier Reef Marine Park Authority to help them make future decisions about if and how to better protect sharks.