Story and Photos By Aaron Henderson,
Resident Lecturer, School for Field Studies

Sharks have inhabited the planet for millions of years and are one of the oldest groups of animals found on earth today. During this time, their evolutionary adaptations have allowed them to inhabit most of the ocean’s habitats, ranging from shallow coastal waters to the deepest trenches and everything else in between. Some species are even found in freshwater lakes and rivers hundreds of miles from the ocean.

Sharks are generally regarded as apex predators, meaning that they are found at the end of the food chain and have few natural predators themselves. Animals that hold such a position play a crucial role in maintaining the health of an ecosystem. However, apex predators tend to naturally have small population sizes, due mainly to the fact that evolutionary pressure has not forced them to cope with high levels of predation.

This “approach” has served sharks well for millennia. However, shark populations around the world are facing collapse, and even extinction. The reason, as you may have guessed, is because of increased exploitation and degradation of the marine habitat by man.

Unlike many fish species, sharks grow slowly and produce very few young. In fact, the life histories of sharks have more in common with mammals such as whales and dolphins than with other fish. Therefore, even at moderate levels of exploitation, sharks are unable to replenish their stocks and most shark fisheries have a history of “boom and bust,” i.e. a short period of large landings followed by complete collapse of the fishery.

So, why should we be concerned about the decimation of shark populations around the world? Aren’t they just nasty creatures that swim around feeding on lucrative fish species and attacking people when they get the chance? Well, people have certainly lost their lives and limbs to sharks, but it’s actually a very small number when compared with the annual number of deaths caused by other animals such as dogs, wasps, and even donkeys! The fact of the matter is that sharks are generally very shy creatures which normally shun human contact. Most shark bites occur because sharks are attracted to dead fish being carried along by a diver/swimmer, or because people start harassing the shark by following it or trying to pet it.

As apex predators, sharks play important ecological roles. For example, certain species of shark may feed on a particular species of fish, which may in turn feed on other fish/marine species. If the shark population is depleted, the fish species it normally feeds on increases dramatically, and then the other fish/marine species further down the food chain get eaten very quickly, resulting in those populations crashing as well. So, by removing sharks from the ecosystem there is a knock-on effect which removes other species too. When these other species are commercially important, it can have serious consequences. For example, there is one case in Australia where a local shark population was over-fished, and it had a knock-on effect of reducing local lobster numbers, eventually putting the commercial lobster fishery in jeopardy.

Over decades of marine research it has become apparent that the only successful approach to managing our marine resources is to take an ecosystem-wide approach. It is simply not effective to look at a single species in isolation; we must understand its relationships with other species and the environment in general. The School for Field Studies (SFS) on South Caicos has therefore initiated research into apex predator species such as barracuda and sharks, in the hope that it will shed some light on the importance of these fish in maintaining the health of the local ecosystem and, ultimately, the local commercial fisheries.

Shark research at SFS is dominated by the lemon shark, for the Caicos Bank plays a very important role in the life-cycle of this species. Early in summer, pregnant female lemon sharks come into the shallow coastal waters of the TCI where they give birth to a small number of shark pups. The pups then remain in the shallow waters of the sand banks and mangroves, where they are protected from larger predators, until they are large enough to move out into deeper water. The area that the shark pups inhabit is referred to as a nursery and the TCI is one of only a handful of lemon shark nursery sites in the world. We are therefore interested in why the sharks are attracted to this area in particular and how they interact with other marine species. We are also interested in finding out how long the sharks remain in the nursery, and whether they favour particular sites or just casually move around.

The research that is conducted involves setting nets at suitable sites around South Caicos. The nets have a mesh size that is large enough to allow other fish species to pass through but which entangle the sharks. Once a shark has been caught it is immediately removed from the net, measured, weighed, its condition noted, a tag is inserted just below the first dorsal fin and finally, the shark is released alive. The tag is marked with a number which allows the shark to be identified if it is caught again. We can therefore determine the movements of individual sharks, as well as calculate their growth rate.

One other component of the project involves studying the stomach contents of the sharks to determine their relationships with other marine species. This involves flushing the shark’s stomach with water — a technique which does not harm the shark — and collecting the food remains which are regurgitated.

The results of the project so far suggest that individual lemon sharks are very site specific, that is to say they “hang out” at preferred locations. Furthermore, their growth rates are faster than has been reported for the species elsewhere, perhaps indicating abundant prey. Analysis of the stomach contents has revealed that these young sharks feed on a variety of small fishes, including bonefish, as well as invertebrate species such as crabs. Although these results are very much preliminary they are already painting an interesting picture. We hope that as we continue to collect data from lemon sharks and other apex predators we will gain a better understanding of the processes at play in the TCI’s marine ecosystem.