Category: General Article
Date: Wednesday, August 30, 2017
Understanding the movement patterns of sharks is crucial to defining their role in ecosystems. Sharks are often assumed to have similar habitat needs and movement patterns, especially when they live in the same area, or are of the same size. During my research into shark movements one of the questions I wanted to answer was what happened when several species of shark live in the same location, share space and possibly even compete with each other for prey? I’ve examined this in two different areas, an inshore bay and a coral reef.
In the coastal bay, we tracked six species of sharks for several years to see how their movement patterns compared. Most of the sharks were medium sized (4 to 5 feet or 1.21 to 1.52 m) including 4 of the tracked species. All 4 species used small to medium amounts of space in the bay, but appeared to separate by habitat. Pigeye sharks were found to stay near river mouth habitats, blacktip reef sharks stayed near an inshore reef, creek whaler sharks stayed in shallow seagrass areas and blacktip reef sharks used mud and seagrass areas. So although these animals were all about the same size and were eating the same type of fish, they were spreading out around the bay. It is possible they do this to avoid competing with each other for food. Their similar size and diets mean they would have to compete for food if they were in the same area. The two other species were much smaller in size, only growing to 3 to 4 feet (91 cm to 1.21 m), but they did the opposite of what was expected – they moved more than the bigger sharks and used much more space. These small sharks might move more to try to avoid predators, including the medium sized sharks.
In the case of the reef shark study, we tracked 5 species of sharks in the southern Great Barrier Reef over 5 years. In this case, we tagged 3 common reef sharks that were similar size (5 feet, 1.52 m): grey reef sharks, blacktip reef sharks and whitetip reef sharks. We also tagged a small species, the weasel shark and a large species, the tiger shark. Like the bay study we expected the 3 reef sharks to use similar amounts of space, but this was not the case. Whitetip reef sharks stayed fairly deep, near the bottom and used very little space. This is because they tend to stay in caves or under ledge areas for extended periods of the day. The grey reef sharks swam more in the middle of the water column and used at least 2-3 times as much space as the whitetip reef sharks. Finally, the blacktip reef sharks stayed mainly on the reef crest and in the lagoon and used even more space.
This distribution from deep to shallow means that these three species live in similar locations, but are separated by depth. This is a clever way to stay separate from each other when you have a limited amount of space available based on the size of the reef.
We were not surprised to learn that the large tiger sharks used all of the available reef space including deep and shallow areas. They overlapped in space with the other three species, but because of the size difference probably weren’t competing for food, and the smaller sharks might even by potential prey for tiger sharks.
The final species, the weasel shark also used a large amount of space, but in a much different way to tiger sharks. Weasel sharks showed a patchy distribution where they spent time in a location along the reef slope and bottom before moving to another area of the reef and returning to restricted movements. This is likely the result of weasel sharks feeding selectively on octopuses. Their movements were related to hunting for prey in a small area, then moving on to a new area to hunt some more. Although weasel sharks overlap in distribution with whitetip reef sharks and grey reef sharks, their specialised diet means they are not in competition with the other species.
These results show that shark populations can have complex patterns of distribution even within a small area such as a single reef. Separation of species by location or depth reduces competition for resources and helps maintain multiple populations in a single location. These patterns allow a variety of sharks to co-exist and maintains their populations in coastal and reef habitats.